elements, according to Mr. Harker's observations, are constantly
felspar, frequently subordinate augite, and exceptionally quartz. The
felspars have for the most part rounded outlines with a bordering
zone of glass cavities apparently of secondary origin. The augite, in
rounded composite crystal-grains, differs from that of the groundmass
and resembles the augite of the gabbros. The quartz-grains are likewise
rounded, and show sometimes a distinct corroded border.

These characters, Mr. Harker observes, are those of crystals derived
from some foreign source, and it can scarcely be doubted that this
is the explanation of their presence. He noticed that the dykes in
question frequently enclose fragments, varying up to several inches in
diameter, of gabbro, granite or granophyre, bedded lava, quartzite,
etc., which show clear evidence of having been rounded and corroded
by an enveloping magma, and recognizable crystals from some of the
fragments may be observed in the surrounding parts of the matrix
of the dykes. Most of the felspar and augite crystals disseminated
through these porphyritic basic dykes may be referred to the partial
reabsorption of enclosed fragments of gabbro. The same observer has
found that many of the dykes which rise through the basalt-plateau of
Strathaird are crowded with gabbro fragments.

Another megascopic character of the material composing the dykes is the
frequent presence of amygdales. It has sometimes been supposed that
amygdaloidal structure may be relied upon as a test to distinguish
a mass of molten rock which has reached the surface from one which
has consolidated under considerable pressure below ground. That this
supposition, however, is erroneous is demonstrated by hundreds of dykes
in the great system which I am now describing. But the amygdales of a
dyke offer certain peculiarities which serve in a general way to mark
them off from those of an outflowing lava. They are usually smaller
and more uniform in size than in the latter rock. They are also more
regularly spherical and less frequently elongated in the direction of
flow. Moreover, they are not usually distributed through the whole
breadth of a dyke, but tend to arrange themselves in lines especially
towards its centre (Fig. 236). In these central bands the cavities are
largest and depart farthest from the regular spherical form, so that
for short spaces they may equal in bulk the mass of enclosing rock.
In some rare instances, a whole dyke is composed of cellular basalt,
like one of the lava-sheets in the plateaux, as may be seen on the
north flank of Beinn Suardal, Skye. Mr. Harker has observed that an
amygdaloidal structure is more common among the earlier than among the
later dykes of that district.

[Illustration: Fig. 236.--Arrangement of lines of amygdales in a dyke,
Strathmore, Skye.]

Besides the common arrangement of fine-grained edges and a more
coarsely crystalline centre, instances are found where one of the
contrasted portions of a dyke traverses the other in the form of veins.
Of these, I think, there are two distinct kinds, probably originating
in entirely different conditions. In the first place, they may be of
coarser grain than the rest of the rock; but such a structure appears
to be of extremely rare occurrence. I have noticed some examples on the
coast of Renfrewshire, where strings of a more coarsely crystalline
texture traverse the finer-grained body of the rock. Veins of this kind
are probably of the same nature as the so-called "segregation-veins,"
to be afterwards referred to as of frequent occurrence among the
thicker Tertiary sills. They consist of the same minerals as the
rest of the rock, but in a different and more developed crystalline
arrangement, and they contain no glassy or devitrified material, except
such portions of that of the surrounding groundmass as may have been
caught between their crystalline constituents.

The second kind of veins, which, though not common, is of much more
frequent occurrence than the first, is more particularly to be met with
among the broader dykes, and is distinguished by a remarkable fineness
of grain, sometimes approaching the texture of felsite or jasper, and
occasionally taking the form of actual glass. Such veins vary from
half an inch or less, up to four or five inches in breadth. They run
sometimes parallel with the walls of the dyke, but often irregularly
in all directions, and for the most part avoid the marginal portions,
though now and then coming up to the edge. They never extend beyond
the body of the dyke itself into the surrounding rock. Though they
have obviously been injected after the solidification of the rock
which they traverse, they may quite possibly be extrusions of a deeper
unconsolidated portion of the same rock into rents of the already
stiffened overlying parts. The field-geologist cannot fail to be struck
with the much greater hardness of these fine-grained veins and strings
that ramify through the coarsely crystalline dolerite, andesite or
other variety of the broader dykes. He can readily perceive in many
cases their more siliceous composition, and the inferences he deduces
from the rough observations he can make in the field are confirmed by
the results of chemical analysis (see p. 137).

In connection with veins of finer material, that may belong to a late
stage of the consolidation of the general body of a dyke, reference may
be made here to the occasional occurrence of patches of an exceedingly
compact or homogeneous texture immersed in the usual finely crystalline
marginal material. They look like angular and subangular portions of
the more rapidly cooled outer edge, which have been broken off and
carried upward by the still moving mass in the fissure.[165]

[Footnote 165: See Mr. J. J. H. Teall, _Quart. Journ. Geol. Soc._ xl.
(1884), p. 214.]

In general, each dyke is composed of one kind of rock, and retains
its chemical and mineralogical characters with singular persistence.
The difference of texture between the fine-grained chilled margin,
with its occasional glassy coating, and the more coarsely crystalline
centre is due to cooling and crystalline segregation in what was no
doubt originally one tolerably uniform molten mass. The glassy central
bands, too, though they indicate a rupture of the dyke up the middle,
may at the same time quite conceivably be, as I have said, extrusions
from a lower portion of the dyke before the final solidification of the
whole. The ramifying veins of finer grain that now and then traverse
one of the large dykes are likewise explicable as parts of a stage
towards entire consolidation. All these vitreous portions, whether
still remaining as glass or having undergone devitrification, are
more acid than the surrounding crystalline parts of the rock. They
represent the siliceous "mother-liquor," so to speak, which was left
after the separation from it of the crystallized minerals, and which,
perhaps, entangled here and there in vesicles of the slowly cooling
and consolidating rock, was ready to be forced up into cracks of the
overlying mass during any renewal of terrestrial disturbance.

But examples occur where a dyke, instead of consisting of one rock, is
made up of two or more bands of rock which, even if they resemble each
other closely, can be shown to be the results of separate eruptions.
These, which are obviously not exceptions to the general rule of the
homogeneity of dykes, I will consider in the next Chapter.

Among the petrographical varieties observable in the field is the
occasional envelopment of portions of the surrounding rocks in the
body of a dyke. Angular fragments torn off from the fissure-walls
have been carried upwards in the ascending lava, and now appear more
or less metamorphosed, the amount of alteration seeming to depend
chiefly upon the susceptibility of the enclosed rock to change from
the effects of heat. Cases of such entanglement, however, are of less
common occurrence than those already referred to, where pieces of some
deep-seated rock, such as the gabbros of Skye, have been carried up in
the ascending magma. Occasionally, where the enclosed fragments are
oblong, they are arranged with their longer axes parallel to the walls
of the dyke, showing flow-structure on a large scale. Mr. Clough has
found some dykes near Dunoon which enclose fragments of schist nearly
three feet in length.

One of the most interesting of the megascopic features of the dykes is
the joints by which they are traversed. These divisional planes are no
doubt to be regarded as consequences of the contraction of the original
molten rock during cooling and consolidation between its fissure-walls.
They are of considerable interest and importance, inasmuch as they
furnish a ready means of tracing a dyke when it runs through rock of
the same nature as itself, and also help to throw some light on the
stages in the consolidation of the material of the dyke.

[Illustration: Fig. 237.--Systems of joints in the dykes.

_a_, parallel; _b_, transverse.]

Two distinct systems of joints are recognizable (Fig 237). Though
sometimes combined in the same dyke, they are most conspicuously
displayed when each occurs, as it generally does, by itself. The first
and less frequent system of joints (_a_) has been determined by lines
of retreat, which are parallel to the walls of the dyke. The joints
are then closest together at the margin, and may be few or altogether
absent in the centre. They are sometimes so numerous, parallel and
defined towards the borders of the dyke, as to split the rock up into
thin flags. Where transverse joints are also present these flags are
divided into irregular _tesseræ_.

In the second or transverse system of joints (_b_), which is the more
usual, the divisional lines pass across the breadth of the dyke,
either completely from side to side, or from one wall for a longer or
shorter distance towards the other. Where this series of joints is
most completely developed the dyke appears to be built up of prisms
piled horizontally, or nearly so, one above another. These prisms, in
rare instances, are as regular as the columns of a basalt-sheet (see
Fig. 166). Usually, however, they have irregularly defined faces, and
merge into each other. Where the prismatic structure is not displayed,
the joints, starting sharply at the wall of the dyke, strike inwards
in irregular curving lines. It is such transverse joints that enable
the eye, even from a distance, to distinguish readily the course of
a dyke up the face of a cliff of basalt-beds, for they belong to the
dyke itself, are often at right angles to those of the adjacent basalt,
and by their alternate projecting and re-entering angles seam the dyke
with parallel bars of light and shade (see the double dyke in Fig.
333). Where they traverse not only the general mass of a dyke, but
also the "contemporaneous veins" which cross it, it may be inferred
that these veins were injected before the final solidification and
contraction of the whole dyke.

  Fig. 238.--Section of cylindrical vein or dyke, cutting the bedded
  lavas, east side of Fuglö, Faroe Islands.
]

An interesting modification of the transverse joints may sometimes be
observed, where, as in the case of the Palæozoic "Rock and Spindle,"
at St. Andrews (Fig. 222), the molten material has solidified in a
tubular or spherical cavity. The joints then radiate inwards from the
outer curved surface. The most remarkable instance of this structure
which I have found among the Tertiary volcanic plateaux occurs on the
east side of the island Fuglö, the most north-easterly of the group of
the Faroes. It is cut in section by the face of the precipice, where
it appears as a round mass about 40 or 50 feet in diameter piercing
the plateau-basalts. A selvage of finer material round its outer edge
shows the effect of rapid chilling, while the joints diverge from the
periphery and extend in fan-shape towards the centre (Fig. 238).

[Illustration: Fig. 239.--Joint-structures in the central vitreous
portion of the Eskdale Dyke (B. N. Peach).

  A, View of a square yard of the outer wall of the vitreous central
  band, showing the polygonal arrangement of the prisms and their
  investing sheath of ribs.

  B, View of a smaller portion of the same wall to show the detailed
  structure of the ribs (_a_ _a_) and their vitreous cores (_b_ _b_).

  C, Profile of a part of the weathered face of the wall, showing the
  way in which the hard ribs or sheaths project at the surface.
]

One of the most remarkable exhibitions of joint-structure hitherto
noticed among the Tertiary dykes is that which occurs in the central
vitreous band of the Eskdale dyke already referred to. The rock is
divided into nearly horizontal prisms, each of which consists of an
inner more vitreous core and an outer more lithoid sheath. By the
coherence of their polygonal and irregular faces, and the greater
durability of their material, these sheaths project on the weathered
wall of the vitreous centre of the dyke in a curiously reticulated
grouping of prominent ribs each about two inches broad (Fig. 239, A),
while the vitreous cores, being more readily acted on by the weather,
are hollowed out into little cup-shaped depressions. Each rib is thus
composed of the sheaths or outer lithoid portions of two prisms, the
line of separation being marked by a suture along the centre (B).
Between this median suture and the inner glassy core the rib is further
cut into small segments by a set of close joints, which are placed
generally at right angles to the course of the rib (C). Examined with a
lens, the lithoid substance of these sheaths has a dull finely granular
aspect, like that of felsitic rocks, with scattered felspars. It is
obviously a more devitrified condition of the material which forms
the core of each prism. This material presents on a fresh fracture a
deep iron-black colour, dull resinous lustre and vitreous texture.
It at once recalls the aspect of many acid pitchstones, and in the
early days of petrography was naturally mistaken for one of these
rocks. Through its substance numerous kernels of more glassy lustre
are dispersed, each of which usually contains one or more amygdales
of dull white chalcedony, but sometimes only an empty black cavity.
These black glistening kernels of glass, of all sizes up to that of a
small bean, scattered through the dull resinous matrix, form with the
white amygdales the most prominent feature in the cores; but crystals
of felspars may also be observed. Some details of the microscopic
characters of this remarkable structure will be given in a subsequent
page. The relation of the cores and sheaths to the prismatic jointing
of the rock seems to show that devitrification had not been completed
when these joints were established, and that it proceeded from the
faces of each prism inwards.

(2) _Microscopic Characters._--Much information has now been obtained
regarding the microscopic structure of the basaltic, doleritic and
andesitic dykes. The crystalline characters of those in the North of
England have been studied by Mr. Teall,[166] and some of those from
the West of Scotland have been investigated by Professors Judd and
Cole.[167] Taken as a whole, the rocks composing the dykes are found,
when examined microscopically, to consist essentially of mixtures of a
plagioclase felspar, pyroxene and iron oxide, with or without olivine,
and usually with more or less interstitial matter.

[Footnote 166: _Quart. Journ. Geol. Soc._ vol. xl. (1884).]

[Footnote 167: _Op. cit._ vol. xxxix. (1883) p. 444 (basalt-glass); xlii.
(1886) p. 49, where Professor Judd discusses the gabbros, dolerites and
basalts as a whole.]

The felspar appears to be in some cases labradorite, in others
anorthite, but there may be a mingling of several species in many
of the dykes, as in the augite-andesite of the Santorin eruption in
1866, wherein Professor Fouqué found that the larger porphyritic
felspars were mainly labradorite, but partly anorthite, while those
of the groundmass were microlites of albite and oligoclase.[168] The
large felspars scattered porphyritically through the groundmass
are evidently the result of an early consolidation, unless where
they are survivals from fragments of older porphyritic rocks which
have been enveloped and partially dissolved in the dykes. They are
often cracked, penetrated by the groundmass, or even broken into
fragments, and have corroded borders. They sometimes include portions
of the groundmass, and present the zonal growth structure in great
perfection. The small felspars of the groundmass, on the other hand,
are as obviously the result of a later crystallization, for they vary
in size and crystallographic development according to their position
in the dyke. Those from the centre are often in well-formed crystals,
which sometimes pass round their borders into acicular microlites.
Those in the marginal parts of the dyke occur chiefly in the form
of these microlites, forming the felted aggregate so characteristic
of the andesites. Curious skeleton forms, composed of aggregates of
microlites, connect the latter with the more completely developed
crystals, and illustrate the mode of crystallization of the felspathic
constituents of the dykes.[169]

[Footnote 168: _Santorin et ses Éruptions_, 1879, p. 203.]

[Footnote 169: See Mr. Teall's excellent description of the Cleveland
dyke, in the paper above cited.]

The pyroxene is probably in most cases monoclinic (black or common
augite), but is sometimes rhombic (usually enstatite, less frequently
perhaps hypersthene). It occurs in (_a_) well-developed crystals,
(_b_) crystalline masses with some of the faces of the crystals
developed, (_c_) granular aggregates which polarise in one plane, (_d_)
separate granules and microscopic microlites, which may be spherical
(globulites) or oblong (longulites).

The black iron-oxide is sometimes magnetite, sometimes ilmenite, or
other titaniferous ore. Apatite not infrequently occurs among the
original constituents. Olivine is entirely absent from most of the
large solitary dykes, especially at a distance from the great volcanic
centres, and no serpentinous matter remains to indicate that it was
ever present in them. But it is to be met with in numerous basalt-dykes
in the volcanic areas, either in sparsely scattered or in tolerably
abundant crystals. Biotite occasionally appears. Among the secondary
products, calcite and pyrites are doubtless the most common. To these
must be added quartz, chalcedony and various zeolitic substances,
besides the aggregates which result from the decomposition of the
ferro-magnesian constituents and the oxidation of the ferrous oxides.

In many dykes there is little or no interstitial matter between the
crystalline constituents of the groundmass. In others this matter
amounts to a half or more of the whole composition, and from such cases
a series of gradations may be traced into a complete glass containing
only the rudimentary forms of crystals (globulites, longulites, etc.),
with scattered porphyritic crystals of an earlier consolidation. The
process of the disappearance of this original glass may be admirably
studied in many dykes. At the outer wall, the glass remains nearly
as it was when contact with the cold walls of the fissure solidified
it. From that external vitreous layer the successive devitrification
products and crystalline growths may be followed inwards until in the
central parts of a broad dyke little or no trace of the interstitial
matter may be left.

[Illustration: Fig. 240.--Microscopic structure of the vitreous part of
the Eskdale Dyke.

  This section shows a crystal of augite, enclosing magnetite and
  surrounded with microlites, each of which consists of a central
  pale yellow rod crusted with pale yellow isotropic globulites. The
  glass around this aggregation is clear, but at a little distance
  globulites (many of them elongated and dichotomous) abound, with
  here and there scattered microlites, some of which are curved and
  spiral. (800 diameters.)[170]
]

[Footnote 170: _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 255.]

The most instructive example of the process of devitrification which
has come under my observation occurs in the Eskdale dyke. The central
"cores" already referred to present a true glass, which in thin
sections is perfectly transparent and almost colourless, but by streaks
and curving lines of darker tint shows beautiful flow-structure. The
devitrification of this glass has been accomplished by the development
of crystallites and crystals, which increase in number until all
the vitreous part of the rock disappears. What seems under a low
power to be a structureless or slightly dusty glass can be resolved
with a higher objective into an aggregate of minute globules or
granules (globulites), which average perhaps 1/20,000 of an inch in
diameter. Some of these bodies are elongated and even dichotomous at
the ends. These granules are especially crowded upon clear yellow
dart-shaped rods, which in turn are especially prominent upon crystals
and crystalline grains of augite that bristle with them, while the
immediately surrounding glass has become clear. There can be little
doubt that these rudimentary bodies are stages in the arrested
development of augite crystals. There occur also opaque grains, rods
and trichites, which no doubt consist in whole of magnetite (or other
iron oxide), or are crusted over with that mineral.

At least two broad types of microscopic structure may be recognized
among the basic and intermediate dykes. (1) Holocrystalline, or with
only a trifling proportion of interstitial matter. This type includes
the dolerites and basalts, as well as rocks which German petrographers
would class as diabases or diabase-porphyrites. The rocks are very
generally characterized by ophitic structure, where the lath-shaped
felspars penetrate the augite, and are therefore of an earlier
consolidation. In such cases there is a general absence of any true
interstitial matter. The rocks of this type are often rich in olivine,
and appear to be on the whole considerably more basic than those of
the second group. It is observable that they increase in numbers from
the centre of Scotland westwards, and throughout the region of the
basalt-plateaux they form the prevailing type. (2) In this type there
is a marked proportion of interstitial substance, which is inserted in
wedge-shaped portions among the crystallised constituents ("intersertal
structure" of Rosenbusch). The ophitic structure appears to be absent,
and olivine is either extremely rare or does not occur at all. The
rocks of this group are obviously less basic than those of the other.
They form the large dykes that rise so conspicuously through the South
of Scotland and North of England, and their general characters are well
described by Mr. Teall in the paper already cited. In some instances
they enclose abundant porphyritic felspars of earlier consolidation,
and then present most of the characters of andesites. Professor
Rosenbusch has extended the name of "Tholeiites" to rocks of this group
in the North of England.[171] The vitreous condition is found in both
types, but is perhaps more frequent in the second. The glass of the
basalts, however, even in thin slices, is characteristically opaque
from its crowded inclusions; while that of the andesitic forms, though
black in hand specimens, appears perfectly transparent and sometimes
even colourless in thin slices.

[Footnote 171: _Mikroskopische Physiographie_, 3rd edit. 1071 _et seq._]

(3) _Chemical Characters._--The only one of these to which reference
will be made here is the varying proportion of silica. While the dykes
as a whole are either intermediate or basic, some of them contain so
high a percentage of silica as to link them with the acid rocks. The
average proportions of this ingredient range from less than 50 to
nearly 60 per cent. The rocks with the lower percentage of acid are
richer in the heavy bases, and have a specific gravity which sometimes
rises above 3·0. They include the true dolerites and basalts. Those,
on the other hand, with the higher ratio of silica, are poorer in
the heavy bases, and have a specific gravity from 2·76 to 2·96. They
comprise the tholeiites, andesites and other more coarsely crystalline
rocks of the great eastern and south-eastern dykes.[172]

[Footnote 172: For analyses of dykes, see Sir I. L. Bell, _Proc. Roy.
Soc._ xxiii. p. 546; Mr. J. S. Grant Wilson, _Proc. Roy. Phys. Soc.
Edin._ v. p. 253; Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209;
Professors Judd and Cole, _Quart. Jour. Geol. Soc._ xxxix. p. 444.]

Not only do the dykes differ considerably from each other in their
relative proportions of silica, but even the same dyke may sometimes be
found to present a similar diversity in different parts of its mass. It
has long been a familiar fact that the glassy parts of such rocks are
more acid than the surrounding crystalline portions. The original magma
may be regarded as a natural glass or fused silicate, in which all the
elements of the rock were dissolved, and which necessarily became more
acid as the various basic minerals crystallised out of it.[173] In the
Eskdale dyke the silica percentage of this glassy portion is 58·67,
that of the little kernels of black glass dispersed through the rock
as much as 65·49.[174] In the Dunoon dyke observed by Mr. Clough the
siliceous finer-grained veins contain no less than 68·05 per cent of
silica, while the mass of the dyke itself shows on analysis only 47·36
per cent.[175] Similar red strings have been noticed by the same careful
observer in an east and west dyke near Lochgoilhead. From Mr. Teall's
examination a large part of the felspar in these veins is probably
orthoclase. It forms a much larger percentage of the entire rock than
the felspar does in normal dolerites.

[Footnote 173: On this subject see a paper by Dr. A. Lagorio, "Über die
Natur der Glasbasis sowie der Krystallisationsvorgänge im eruptiven
Magma," Tschermak's _Mineralog. Mittheil._ viii. (1887), p. 421.]

[Footnote 174: Mr. J. S. Grant Wilson, _Proc. Roy. Soc. Phys. Edin._ v.
(1880) p. 253.]

[Footnote 175: Unpublished analyses made by the late Professor Dittmar of
Glasgow, and communicated to me by Mr. Clough.]

2. Trachyte Dykes.--In the Cowal District of Argyleshire, and in
the south of Skye, Mr. Clough has encountered a limited number
of dykes of trachyte. On a hasty inspection these are not always
readily distinguished from the basalt-dykes with which they agree in
general external aspect and in direction. Where their relation to
these dykes, however, can be determined they are found to traverse
them, and thus to be on the whole later, though one case has been
observed where a trachytic dyke is in turn traversed by one of the
basic series. Mr. Clough has supplied me with the following notes
of his observations regarding the trachytic dykes. They are all
characterized by the possession of spherulitic structures near their
margins. These features, easily perceptible to the naked eye, afford
the readiest means of distinguishing the dykes of this group. So
abundant are the spherulites that they not infrequently impinge on
each other in long parallel rows forming rod-like aggregates. Thus
in a dyke near Craigendavie, at the head of Loch Striven, numerous
planes about a quarter of an inch apart, and composed of such close-set
rods, may be observed running parallel to the marginal wall for a
distance of several inches from the edge. Most of these planes show
on their surfaces that the rods are always parallel to each other,
but may run in different directions in the different layers, being
sometimes horizontal, sometimes vertical, or at any angle between. On
examination, each rod is found to be made up of polygonal bodies, the
angles of which are quite sharp, but with their sides often slightly
curved, as if they had assumed their forms from the mutual pressure of
original spherical bulbs. Further scrutiny shows that the polygonal
bodies often exhibit an internal radiate structure.

In the central parts of the dyke the spherulitic arrangement is not
traceable. About a foot from the margin it begins to be recognizable.
At a distance of three or four inches the spherulites are about the
size of peas, and gradually diminish towards the edge until they can no
longer be seen.

Another characteristic of the trachyte dykes has been found by
Mr. Clough to be a useful guide in discriminating them from the
basalt-group. While the amygdales in the latter are generally rudely
spherical, those in the trachytes are commonly elongated in the
direction of the length of the dyke, and are frequently three quarters
of an inch, sometimes even an inch and a half, in length, though less
than a quarter of an inch in breadth.

A good example of these trachytic dykes, which occurs at Dunans,
about the head of Glendaruel, has been examined microscopically and
chemically. The central better crystallised portion was found by
Mr. Teall to be composed mainly of small lath-shaped crystals of
orthoclase, together with scales of brown biotite, a few prismatic
crystals of pale somewhat altered pyroxene and scattered granules of
magnetite. The chemical analysis of this rock by Mr. J. H. Player gave
the following composition:--

  Silica            56·4
  Alumina           19·0
  Ferric oxide       3·5
  Ferrous oxide      4·8
  Lime               2·6
  Magnesia           1·5
  Soda               4·5
  Potash             5·0
  Loss on ignition   2·6
                    ----
                    99·9
                    ====

4. HADE

In the majority of cases, especially among the great single dykes,
the intrusive rock has assumed a position nearly or quite vertical.
But occasionally, where one of these solitary examples crosses a deep
valley, a slight hade is perceptible by the deviation of the line of
the dyke from its normal course. Sedgwick long ago noticed that the
Cleveland dyke has, in places, an inclination of at least 80° to its
N.E. side.[176] In the coal-workings, also, a trifling deviation from
the vertical is sometimes perceptible, especially where a dyke has
found its way along a previously existing line of fault, as in several
examples in Stirlingshire. But in those districts where the dykes are
gregarious, departures from the vertical position are not infrequent,
more particularly near the great basalt-plateaux. It was noticed by
Necker, that even in such a dyke-filled region as Arran, almost all of
the dykes are vertical, though sometimes deviating from that position
to the extent of 20°.[177] Berger found that the angle of deviation
among those of the north of Ireland ranges from 9° to 20°, with a mean
of 13°.[178] The most oblique examples are probably those which occur
in the basalt-plateaux of the Inner Hebrides, where the same dyke in
some parts of its course runs horizontally between two beds, across
which it also descends vertically (see Figs. 251, 252, 374). But with
these minor exceptions, the verticality of the great system of dykes,
pointing to the perpendicular fissure-walls between which the molten
rock ascended, is one of the most notable features in their geological
structure. In the Strath district of Skye Mr. Harker has noticed that
while the earlier dykes have sometimes a hade of 45°, those younger
than the granophyre are generally vertical or nearly so. In the Blath
Bheinn group of hills, however, as already alluded to, he has observed
that it is the youngest dykes which are inclined in a north-westerly
direction, with a hade of as much as 40° from the horizon.

[Footnote 176: _Cambridge Phil. Trans._ ii. p. 28.]

[Footnote 177: _Trans. Roy. Soc. Edin._ xiv. p. 677.]

[Footnote 178: _Trans. Geol. Soc._ iii. p. 227.]

5. BREADTH

An obvious characteristic of most dykes is the apparent uniformity of
their breadth. Many of them, as exposed along shore-sections, vary as
little in dimensions as well-built walls of masonry do. Departures
from such uniformity may often indeed be noted, whether a dyke is
followed laterally or vertically. The largest amount of variation is,
of course, to be found among the dykes of the gregarious type, the
thinner examples of which may diminish to a width of only one inch or
less, while their average breadth is much smaller than in the case
of the great solitary dykes. In the district of Strathaird, in Skye,
Macculloch estimated that the remarkably abundant dykes there developed
vary from 5 to 20 feet in breadth, but with an average breadth of not
more than 10 feet.[179] In the isle of Arran, according to Necker's
careful measurements, most of the dykes range from 2 or 3 to 10 or 15
feet, but some diminish to a few inches, while others reach a width of
20, 30, or even 50 feet.[180] In the North of Ireland, Berger observed
that the average breadth of thirty-eight dykes traversing primitive
rocks (schist, granites, etc.) was 9 feet; and of twenty-four in
Secondary rocks, 24 feet.[181]

[Footnote 179: _Trans. Geol. Soc._ iii. p. 80.]

[Footnote 180: _Trans. Roy. Soc. Edin._ xiv. p. 690 et seq.]

[Footnote 181: _Trans. Geol. Soc._ iii. p. 226. He believed that dykes
in Secondary rocks reach a much greater thickness than in other
formations. My own observations do not confirm this generalisation.]

But when we pass to the great solitary dykes, that run so far and so
continuously across the country, we encounter much thicker masses of
igneous rock. Most of the measurements of these dykes have been made
at the surface, and the variations noted in their breadth occur along
their horizontal extension. The Cleveland dyke, which is the longest
in Britain, varies from 15 feet to more than 100 feet, with perhaps an
average width of between 70 and 90 feet.[182] Some of the great dykes
that cross Scotland are of larger dimensions. Most of them, however,
like that of Cleveland, are liable to considerable variations in
breadth when followed along their length. The dyke which runs from
the eastern coast across the Cheviot Hills and Teviotdale to the head
of the Ale Water, is in some places only 10 feet broad, but at its
widest parts is probably about 100 feet. The Eskdale and Moffat dyke
is in parts of its course 180 feet wide, but elsewhere it diminishes
to not more than 40 feet. These variations are repeated at irregular
intervals, so that the dyke alternately widens and contracts as its
course is traced across the hills. Some of the dykes further to the
north and west attain yet more gigantic proportions. That which crosses
Cantyre opposite Ardlamont Point has been measured by Mr. J. B. Hill,
of the Geological Survey, who finds it to be from 150 to 180 feet broad
on the shore of Loch Fyne, and to swell out beyond the west side of
Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo,
in Fife, is about 400 feet wide. The broadest dyke known to me is one
which I traced near Beith, in Ayrshire, traversing the Carboniferous
Limestone. Its maximum width is 640 feet.

[Footnote 182: At Cockfield, where it has long been quarried, it varies
from 15 to 66 feet; at Armathwatie, in the vale of the Eden, it is
about 54 feet (Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 211).]

Unfortunately, it is much less easy to get evidence of the width of
dykes at different levels in their vertical extension. Yet this is
obviously an important point in the theoretical discussion of their
origin. Two means are available of obtaining information on the
subject--(_a_) from mining operations, and (_b_) from observations at
precipices and between hill-crests and valley-bottoms.

(_a_) In the Central Scottish coal-field and in that of Ayrshire,
some large dykes have been cut through at depths of two or three
hundred feet beneath the surface. But there does not appear to be any
well-ascertained variation between their width so far below ground and
at the surface. In not a few cases, indeed, dykes are met with in the
lower workings of the coal-pits which do not reach the surface or even
the workings in the higher coals. Such upward terminations of dykes
will be afterwards considered, and it will be shown that towards its
upper limit a dyke may rapidly diminish in width.

(_b_) More definite information, and often from a wider vertical range,
is to be gathered on coast-cliffs and in hilly districts, where the
same dyke can be followed through a vertical range of many hundred
feet. But so far as my own observations go, no general rule can be
established that dykes sensibly vary in width as they are traced
upward. Every one who has visited the basalt-precipices of Antrim or
the Inner Hebrides, where dykes are so numerous, will remember how
uniform is their breadth as they run like ribbons up the faces of the
escarpments.[183] Now and then one of them may be observed to die out,
but in such cases (which are far from common) the normal width is
usually maintained up to within a few feet of the termination.

[Footnote 183: This point did not escape the attention of that excellent
observer, Berger, in his examination of the dykes in the North of
Ireland. We find him expressing himself thus:--"The depth to which the
dykes descend is unknown; and after having observed the sections of a
great many along the coast in cliffs from 50 to 400 feet in height, I
have not been able to ascertain (except in one or two cases) that their
sides converge or have a wedgeform tendency" (_Trans. Geol. Soc._ iii.
p. 227).]

All over the southern half of Scotland, where the dykes run along
the crests of the hills and also cross the valleys, a difference of
level amounting to several hundred feet may often be obtained between
adjacent parts of the same dyke. But the breadth of igneous rock
is not perceptibly greater in the valleys than on the ridges. The
depth of boulder clay and other superficial deposits on the valley
bottoms, however, too frequently conceals the dykes at their lowest
levels. Perhaps the best sections in the country for the study of this
interesting part of dyke-structure are to be found among the higher
hills of the Inner Hebrides, such as the quartzites of Jura and the
granophyres and gabbros of Skye. On these bare rocky declivities,
numerous dykes may be followed from almost the sea-level up to the
rugged and splintered crests, a vertical distance of between 2000 and
3000 feet. The dykes are certainly not as a rule sensibly less in width
on the hill-tops than in the glens. So far, therefore, as I have been
able to gather the evidence, there does not appear to me to be, as
a general rule, any appreciable variation in the width of dykes for
at least 2000 or 3000 feet of their descent. The fissures which they
filled must obviously have had nearly parallel walls for a long way
down.

6. INTERRUPTIONS OF LATERAL CONTINUITY

In tracing the great solitary dykes across the country, the geologist
is often surprised to meet with gaps, varying in extent from a few
hundred feet to several miles, in which no trace whatever of the
igneous rock can be detected at the surface. This disappearance
is not always explicable by the depth of the cover of superficial
accumulations; for it may be observed over ground where the naked
rocks come almost everywhere to the surface, and where, therefore, if
the conspicuous material of the dykes existed, it could not fail to
be found. No dyke supplies better illustrations of this discontinuity
than that of Cleveland. Traced north-westward across the Carboniferous
tracts that lie between the mouth of the Tees and the Yale of the Eden,
this dyke disappears sometimes for a distance of six or eight miles.
In the mining ground round the head of the South Tyne the rocks are
bare, so that the absence of the dyke among them can only be accounted
for by its not reaching the surface. Yet there can be no doubt that
the various separated exposures, which have the same distinctive
lithological characters and occur on the same persistent line, are all
portions of one dyke which is continuous at some depth below ground. We
have thus an indication of the exceedingly irregular upward limit of
the dykes, as will be more particularly discussed further on.

But there are also instances where the continuity is interrupted
and then resumed on a different line. One of the best illustrations
of this character is supplied by the large dyke which rises through
the hills about a mile south of Linlithgow and runs westward across
the coal-field. At Blackbraes it ends off in a point, and is not
found again to the westward in any of the coal-workings. But little
more than a quarter of a mile to the south a precisely similar dyke
begins, and strikes westward parallel to the line of the first one.
The two separated strips of igneous rock overlap each other for
about three-quarters of a mile. But that they are merely interrupted
portions of what is really a single dyke can hardly be questioned. A
second example is furnished by another of the great dykes of the same
district, which after running for about twelve miles in a nearly east
and west direction suddenly stops at Chryston, and begins again in the
same direction, but on a line about a third of a mile further north.
Such examples serve to mark out irregularities in the great fissures up
which the materials of the dykes rose.

7. LENGTH

In those districts where the small and crowded dykes of the gregarious
type are developed, one cannot usually trace them for more than a
short distance. The longest examples known to me are those which have
been mapped with much patience and skill by Mr. Clough in Eastern
Argyleshire. Some of them he has been able to track over hill and
valley for four or five miles, though the great majority are much
shorter. In Arran and in the Inner Hebrides, it is seldom possible to
follow what we can be sure is the same dyke for more than a few hundred
yards. This difficulty arises partly, no doubt, from the frequent
spread of peat or other superficial accumulation which conceals the
rocks, and partly also from the great number of dykes and the want of
sufficiently distinct lithological characters for the identification of
any particular one. But making every allowance for these obstacles, we
are compelled, I think, to regard the gregarious dykes as essentially
short as well as relatively irregular.

In striking contrast to these, come the great solitary dykes. In
estimating their length, as I have already remarked, we must bear
in mind the fact that they occasionally undergo interruptions of
continuity owing to the local failure of the igneous material to
rise to the level of what is now the surface of the ground. A narrow
wall-like mass of andesite or dolerite, which sinks beneath the surface
for a few hundred yards, or for several miles, and reappears on the
same line with the same petrographical characters, while there may
be no similar rock for miles to right and left, can only be one dyke
prolonged underneath in the same great line of fissure. But even if we
restrict our measurements of length to those dykes or parts of dykes
where no serious interruption of continuity takes place, we cannot
fail to be astonished at the persistence of these strips of igneous
rock through the most diverse kinds of geological structure. A few
illustrative examples of this feature may be selected. It will be
observed that the longest and broadest dykes are found furthest from
the basalt-plateaux, while the shortest and narrowest are most abundant
near these plateaux.

Not far from what I have taken provisionally as the northern boundary
of the dyke region, two dykes occur which have been mapped from the
head of Loch Goil by Arrochar across Lochs Lomond and Katrine by Ben
Ledi to Glen Artney, whence they strike into the Old Red Sandstone of
Strathmore, and run on to the Tay near Perth--a total distance of about
60 miles. If the dyke which continues in the same line on the other
side of the estuary of the Tay beyond Newburgh, is a prolongation of
one of these, then its entire length exceeds 70 miles. A few miles
further south, one of a group of dykes can be followed from the
heart of Dumbartonshire by Callander across the Braes of Doune to
Auchterarder--a distance of 47 miles, with an average breadth of more
than 100 feet. In the district between the Forth and Clyde a number
of long parallel dykes can be traced for many miles across hill and
plain, and through the coal-fields. One of these is continuous for
25 miles from the heart of Linlithgowshire into Lanarkshire. Still
longer is the dyke which runs from the Firth of Forth at Grangemouth
westward to the Clyde, opposite Greenock--a distance of about 36 miles.
Coming southward, we encounter a striking series of single dykes on
the uplands between the counties of Lanark and Ayr, whence they strike
into the Silurian hills of the southern counties. One of these runs
across the crest of the Haughshaw Hills, and can be followed for some
30 miles. But if, as is probable, it is prolonged in one of the dykes
that traverse the moorlands of the north of Ayrshire and south of
Renfrewshire to the Clyde, its actual length must be at least twice
that distance. The great Moffat and Eskdale dyke strikes for more than
50 miles across the South of Scotland and North of England. The Hawick
and Cheviot dyke runs for 26 miles in Scotland and for 32 miles in
Northumberland.

But the most remarkable instance of persistence is furnished by the
Cleveland dyke. From where it is first seen near the coast-cliffs of
Yorkshire the strip of igneous rock can be followed, with frequent
interruptions, during which for sometimes several miles no trace of it
appears at the surface, across the North of England as far as Dalston
Hall south of Carlisle, beyond which the ground onwards to the Solway
Firth is deeply covered with superficial deposits. The total distance
through which this dyke can be recognized is thus about 110 miles. But
it probably goes further still. On the opposite side of the Solway,
a dyke which runs in the same line, rises through the Permian strata
a little to the east of the mouth of the Nith. Some miles further
to the north-west, near Moniaive, Mr. J. Horne, in the progress of
the Geological Survey, traced a dark compact dyke with kernels of
basalt-glass near its margin, running in the same north-westerly
direction. Still further on in the same line, another similar rock is
found high on the flanks of the lofty hill known as Windy Standard.
And lastly, in the Ayrshire coal-field, a dyke still continuing the
same trend, runs for several miles, and strikes out to sea near
Prestwick. It cannot, of course, be proved that these detached Scottish
protrusions belong to one great dyke, or that if such a continuous dyke
exists, it is a prolongation of that from Cleveland. At the same time,
I am on the whole inclined to connect the various outcrops together as
those of one prolonged subterranean wall of igneous rock. The distance
from the last visible portion of the Cleveland dyke near Carlisle to
the dyke that runs out into the Firth of Clyde near Prestwick, is about
80 miles. If we consider this extension as a part of the great North
of England dyke, then the total length of this remarkable geological
feature will be about 190 miles.

8. PERSISTENCE OF MINERAL CHARACTERS

Not less remarkable than their length is the preservation of their
normal petrographical characters by some dykes for long distances.
In this respect the Cleveland dyke may again be cited as a typical
example. The megascopic and microscopic structures of the rock of this
dyke distinguish it among the other eruptive rocks of the North of
England. And these peculiarities it maintains throughout its course.[184]
Similar though less prominent uniformity may be traced among the long
solitary dykes of the South of Scotland, the chief variations in
these arising from the greater or less extent to which the original
glassy magma has been retained. The same dyke will at one part of its
course show abundant glassy matter even to the naked eye, while at a
short distance the vitreous groundmass has been devitrified, and its
former presence can only be detected with the aid of the microscope.
Where a dyke has caught up and absorbed abundant foreign materials its
composition naturally varies considerably from point to point. Mr.
Harker has observed some good examples of this variation in Skye.

[Footnote 184: See the careful examination of this dyke by Mr. Teall,
_Quart. Journ. Geol. Soc._ xl. p. 209.]

                             CHAPTER XXXV

                   THE SYSTEM OF DYKES--_continued_

  Direction--Termination upward--Known vertical Extension--Evidence
  as to the movement of the Molten Rock in the Fissures--Branches and
  Veins--Connection of Dykes with Intrusive Sheets--Intersection of
  Dykes--Dykes of more than one infilling--Contact Metamorphism of
  the Dykes--Relation of the Dykes to the Geological Structure of the
  Districts which they traverse--Data for estimating the Geological
  Age of the Dykes--Origin and History of the Dykes.

9. DIRECTION

Another characteristic feature of the dykes is their generally
rectilinear course. So true are the solitary dykes to their normal
trend that, in spite of varying inequalities of surface and wide
diversities of geological structure in the districts which they
traverse, they run over hill and dale almost with the straightness
of lines of Roman road. In the districts where the gregarious type
prevails, the dykes depart most widely from the character of the great
solitary series, but still tend to run in straight or approximately
straight lines, or, if wavy in their course, to preserve a general
parallelism of direction.

Yet even among the great persistent dykes instances may be cited where
the rectilinear trend is exchanged for a succession of zig-zags, though
the normal direction is on the whole maintained. In such cases, it is
evident that the fissures were not long straight dislocations, like the
larger lines of fault in the earth's crust, but were rather notched
rents or cracks which, though keeping, on the whole, one dominant
direction, were continually being deflected for short distances to
either side. As a good illustration of this character, reference may be
made to the Cheviot and Hawick dyke. In Teviotdale, this dyke can be
followed continuously among the rocky knolls, so that its deviations
can be seen and mapped. From the median line of average trend the
salient angles sometimes retire fully a quarter of a mile on either
side. Some examples of the same feature may be noticed in the Eskdale
dyke. The large dyke which runs westward from Dunoon has been observed
by Mr. Clough to change sharply in direction three times in four
miles, running occasionally for a short distance at a right angle to
its general direction (see Fig. 257).

Among these solitary dykes also, though the persistence of their trend
is so predominant, there occur instances where the general direction
undergoes great change. Some of the most remarkable cases of this
kind have been mapped by Mr. B. N. Peach and Mr. R. L. Jack, in the
course of the Geological Survey of Perthshire. Several important
dykes strike across the Old Red Sandstone plain for many miles in a
direction slightly south of west. But when they approach the rocks of
the Highland border in Glen Artney, they bend round to south-west, and
continue their course along that new line.

Many years ago I called attention to the dominant trend of the dykes
from north-west to south-east.[185] Subsequent research has shown this
to be on the whole the prevalent direction throughout the whole region
of dykes. But the detailed mapping, carried on by my colleagues and
myself in the Geological Survey, has brought to light some curious and
interesting variations from the normal trend. In the districts where
dykes of the gregarious type abound there is sometimes no one prevalent
direction, but the dykes strike to almost all points of the compass.
Of the Arran dykes, so carefully catalogued by Necker, only about a
third have a general north-westerly course. But in Eastern Argyleshire
the abundant dykes mapped by Mr. Clough trend almost without exception
towards N.N.W. In the North of Ireland, Berger found the direction
of thirty-one dykes to vary from 17° to 71° W. of N., giving a mean
of N. 36° W.[186] In Islay, Jura, Eigg, Mull, and Skye the mean of
several hundred observations has given me similar results. Among the
Inner Hebrides, however, though the general north-westerly trend is
characteristic, many of the later dykes show marked departures from it.
Thus in Strath, Skye, some of the youngest follow a nearly north and
south direction (Fig. 253). In the Blath Bhein hill-range, Mr. Harker
has found that the latest dykes cut the gabbro at right angles to the
prevalent trend and are further distinguished by their low hade.

[Footnote 185: _Trans. Roy. Soc. Edin._ xxii. (1861), p. 650.]

[Footnote 186: _Trans. Geol. Soc._ iii. p. 225.]

It appears, therefore, that though there is sometimes extraordinary
local diversity in the direction of the dykes in those districts where
they present the gregarious type, the general north-westerly trend
can usually still be recognized. But when we turn to the long massive
solitary dykes, we soon perceive a remarkable change in their direction
as we follow them northward into Scotland. I formerly pointed out how
the general north-westerly trend becomes east and west in the Lothians,
with a tendency to veer a little to the south of west and north of
east.[187] This departure from the normal direction is now seen to be
part of a remarkable radial arrangement of the dykes. Beginning at
the southern margin of the dyke-region, we have the notable example
of the Cleveland dyke, which in its course from Cleveland to Carlisle
runs nearly W. 15° N. The Eskdale dyke has an average trend of W.
32° N., and the same general direction is maintained by the group of
dykes which run from the Southern Uplands across the south-west of
Lanarkshire and north-east of Ayrshire. But proceeding northwards
we observe the trend to turn gradually round towards the west. The
dyke that runs from near the mouth of the Coquet across the Cheviot
Hills to beyond Hawick has a general course of W. 8° N. In the great
central coal-field of Scotland the average direction may be taken to
be nearly east and west, the same dyke running sometimes to the north,
and sometimes to the south of that line. But immediately to the north
a decided tendency to veer round southwards makes its appearance. Thus
the long dyke which runs from the Carse of Stirling through the Campsie
Fells to the Clyde west of Leven, has a mean direction of W. 5° S. This
continues to be the prevalent trend of the remarkable series of dykes
which crosses the Old Red Sandstone plains, though some of these revert
in whole or in part to the more usual direction by keeping a little to
the north of west. Even as far as Loch Tay and the head of Strathardle,
the course of the dykes continues to be to the south of west. Tracing
these lines upon a map of the country we perceive that they radiate
from an area lying along the eastern part of Argyleshire and the head
of the Firth of Clyde (see Map I.).

[Footnote 187: _Trans. Roy. Soc. Edin._ xxii. p. 651.]

[Illustration: Fig. 241.--Section along the line of the Cleveland Dyke
at Cliff Ridge, Guisbrough (G. Barrow).

Scale, 12 inches to 1 mile.]

10. TERMINATION UPWARDS

It was pointed out many years ago by Winch that some of the dykes
which traverse the Northumberland coal-field do not cut the overlying
Magnesian Limestone. The Hett dyke, south of Durham, is said to end
off abruptly against the floor of the limestone.[188] Here and there,
among the precipices of the Inner Hebrides, a dyke may be seen to die
out before it reaches the top of the cliff. But in the vast majority
of cases, no evidence remains as to how the dykes terminated upwards.
I have referred to the occasional interruptions of the continuity of
a dyke, where, though the rock does not reach the surface, it must be
present in the fissure underneath. Such interruptions show that, in
some places at least, there was no rise of the rock even up to the
level of what is now the surface of the ground, and that the upward
limit of the dykes must have been exceedingly irregular.

[Footnote 188: This is expressed in the Geological Survey Map, Sheet 93,
N.E.]

Excellent illustrations of this feature are supplied by sections on
the line of the Cleveland dyke. Towards its south-easterly extremity,
this great band of igneous rock ascends from the low Triassic plain of
the Tees into the high uplands of Cleveland. Its course across the
ridges and valleys there has been carefully traced for the Geological
Survey by Mr. G. Barrow, who has shown that over certain parts of its
course it does not reach the surface, but remains concealed under the
Jurassic rocks, which it never succeeded in penetrating. But that in
places it comes within a few feet of the soil is shown by the baked
shale at the surface, for the alteration which it has induced on the
surrounding rocks only extends a few feet from its margin. These
interruptions of continuity show how uneven is the upper limit of the
dyke. The characteristic porphyritic rock may be observed running up
one side of a hill to the crest, but never reaching the surface on the
other side. At Cliff Ridge, for example, about three miles south-west
of Guisbrough, Mr. Barrow has followed it up to the summit on the
west side; but has found that on the east side it does not pierce the
shales, which there form the declivity. This structure is represented
in Fig 241. The vertical distance between the summit to the left, where
the dyke (_b_) disappears, and the point to the right, where the Lias
shale (_a_) of the hillside is concealed by drift (_c_), amounts to 250
feet, the horizontal distance being a little more than 900 feet. But as
the shale when last seen at the foot of the slope is quite unaltered,
the dyke must there be still some little distance beneath the surface,
so that the vertical extension of this upward tongue of the dyke must
be more than 250 feet. Mr. Barrow, to whom I am indebted for these
particulars, has also drawn the accompanying section (Fig. 242) along
the course of the dyke for a distance of nearly 11 miles eastward from
the locality represented in Fig. 241. From this section it will be
observed that in that space there are at least three tongues or upward
projections of the upper limit of the dyke. Several additional examples
of the same structure are to be seen further east towards the last
visible outcrop of the dyke.

[Illustration: Fig. 242.--Section along the course of the Cleveland
Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the _Memoirs of
the Geol. Surrey_, Geology of Cleveland, p. 61).

_a_, Liassic shales, sandstones and ironstones; _b_, the dyke.]

Another feature connected with the upward termination of the dyke is
well seen in some parts of the ground through which the two foregoing
sections are taken. Mr. Barrow informs me that at Ayton a level course
has been driven into the hill for mining operations, at a height of 400
feet above sea-level, and the dyke has there been ascertained to be 80
feet broad. Higher on the hill, close to the 750 feet contour--line,
its breadth is only 20 feet, so that it narrows upward as much as 60
feet in a vertical height of 350 feet. Its contraction in width during
the last twenty feet is still more rapid, and in the last few yards it
diminishes to two or three feet, and has a rounded top over which the
strata are bent upward. The accompanying section (Fig. 243) across the
upper part of the dyke will make these features clear.

[Illustration: Fig. 243.--Section across the extreme upper limit of
Cleveland Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow).

_a_ _a_, Jurassic shales, etc.; _b_, Dyke.]

[Illustration: Fig. 244.--Upper limit of Cleveland Dyke in quarry near
Cockfield (after Mr. Teall).

_a_ _a_, Carboniferous shales; _b_, dyke.]

Further to the west an exposure of the upper limit of the dyke has been
described and figured by Mr. Teall. In 1882, at one of the Cockfield
quarries (Fig. 244), the dyke was "seen to terminate upwards very
abruptly in the form of a low and somewhat irregular dome, over which
the Coal-measure shales passed without any fracture, and only with a
slight upward arching."[189]

[Footnote 189: _Quart. Jour. Geol. Soc._ xl. p. 210.]

Near the other or north-western termination of this great dyke, similar
evidence is found of an uneven upper limit. After an interrupted course
through the Alston moors, the dyke reaches the ground that slopes
eastward from the edge of the Cross Fell escarpment. Its highest
visible outcrop is at a height of 1700 feet. But westwards from that
point the dyke disappears under the Carboniferous rocks, and does not
emerge along the front of the great escarpment that descends upon the
valley of the Eden, where among the naked scarps of rock it would
unquestionably be visible if it reached the surface. Its upper edge
must rapidly descend somewhere behind the face of the escarpment, for
the igneous rock crops out a little to the west of the foot of the
cliff, about 1000 feet below the point where it is last seen on the
hills above. Here the top of the dyke has a vertical drop of not less
than 1000 feet, in a horizontal distance of five miles, as shown in
Fig. 245, which has been drawn for me by Mr. J. G. Goodchild.

It will be observed that in these sections (Figs. 241, 242 and 245)
there is a curiously approximate coincidence between the inequalities
in the upper surface of the dyke and those in the form of the overlying
ground. The coincidence is too marked and too often repeated to be
merely accidental. Whether the ancient topographical features had any
influence in determining, by cooling or otherwise, the limit of the
upward rise of the lava, or whether the dyke, even though concealed,
has affected the progress of the denudation of the ground overlying it,
is a question worthy of fuller investigation.

[Illustration: Fig 245.--Section along the course of the Cleveland Dyke
across the Cross Fell escarpment. The shaded part shows the position of
the dyke, the unshaded part overlying it marks where the dyke does not
reach the surface. Scale of one inch to one mile.]

11. KNOWN VERTICAL EXTENSION

Closely connected with the determination of the upper limit reached by
the dykes, is the total vertical distance to which they can be traced.
Of course, the depth of the original reservoir of molten rock which
supplied them remains unknown, and probably undiscoverable. But it is
possible, in many cases, to determine at least the inferior limit of
the thickness of rock through which the molten material of the dykes
has ascended. Along the great basalt-escarpments of Mull and Skye,
the ascent of dykes from base to summit may often be observed. Thus,
on the cliffs of Dunvegan Head, on the west coast of Skye, which rise
out of the sea to a height of about 1000 feet, several dykes may be
observed rising through the whole series of basalts up to the crest of
the precipice. In the dark gabbro hills of the same island, numerous
dykes may be seen climbing from the glens right up the steep rugged
acclivities and over the crests, through a vertical thickness of more
than 3000 feet of rock (Fig. 333). The dykes which cross Loch Lomond,
and ascend the hills on either side of that deep depression, must rise
through at least as great a thickness. But where a knowledge of the
geological structure of the ground enables us to estimate the bulk of
the successive rock-formations which underlie the surface, it can be
shown that the lava ascended through a much greater depth of rock.
Measurements of this kind can best be made towards the eastern end
of the Cleveland dyke, where the different sedimentary groups have
not been seriously disturbed, and where, from natural sections and
artificial borings, their thicknesses are capable of satisfactory
computation. The highest bed of the Jurassic series anywhere touched
by the dyke is the Cornbrash. It is certain, therefore, that the
igneous rock rises through all the subjacent members of the Jurassic
series up to that horizon. There can be no doubt also that the Trias
and Magnesian Limestone continue in their normal thickness underneath
the Jurassic strata. To what extent the Coal-measures exist under
Cleveland has not been ascertained; possibly they have been entirely
denuded from that area, as from the ground to the west. But the
Millstone Grit and Carboniferous Limestone probably extend over the
district in full development; and below them there must lie a vast
depth of Upper and Lower Silurian strata, probably also of still older
Palæozoic rocks and beneath all the thick Archæan platform. Tabulating
these successive geological formations, and taking only the ascertained
thickness of each in the district, we find that they give the results
shown in the subjoined table.[190]

[Footnote 190: Drawn up for me by Mr. G. Barrow.]

STRATA CUT BY THE CLEVELAND DYKE

    Cornbrash--
                                                                   Feet.
  Lower Oolite and Upper Lias, as proved by bore-hole on
      Gerrick Moor,                                                  950
  Middle and Lower Lias, ascertained from measurement of
      cliff-sections and from mining operations to be more than      850
  New Red Sandstone and Marl, found by boring close to the Tees
      to exceed                                                    1,600
  Magnesian Limestone, at least                                      500
  Coal-measures, possibly absent                                       0
  Millstone Grit, not less than                                      500
  Carboniferous Limestone series at least                          3,000
  Silurian rocks, probably not less than                          10,000
                                                                 -------
                                                                  17,400

There is thus evidence that this dyke has risen through probably more
than three miles of stratified rocks. How much deeper still lay the
original reservoir of molten material that supplied the dyke, we have
at present no means of computing.

12. EVIDENCE AS TO MOVEMENT OF THE MOLTEN ROCK IN THE FISSURES

It is usual to speak of the molten material of the dykes as having
risen vertically within the fissures. And doubtless, on the whole,
the expression is sufficiently accurate. In the case of such long
dykes as those of Central Scotland and the North of England, where the
petrographical character of the material remains so uniform throughout,
it is obvious that the andesite or dolerite cannot have come from a
mere single pipe like a volcanic orifice. Nor can we easily understand
how it could have been supplied even from a series of such pipes. The
general aspect and structure of the dykes suggest that the fissures
were rent so profoundly in the crust of the earth as to reach down to
a reservoir of molten rock which straightway rose in them. The roof of
such a reservoir, however, may have been irregular and uneven, so that
a fissure need not have traversed it continuously, but may have only
touched its upward projecting vaults. Hence gaps would arise in the
continuity of the dyke-material.

The ascent of lava from a line of such separate openings along a
fissure would necessarily involve lateral as well as vertical movements
in the molten mass which would be forced along the open rent until
the several streams united and filled it up. We might therefore
expect somewhere to find instances of flow-structure in the dykes
pointing to these movements. I have already referred to the lines of
amygdales frequently noticed in dykes, especially towards the centre.
Occasionally these steam-vesicles may be observed to be drawn out in
one general direction indicative of the trend of motion of the molten
rock.

Some of the best examples of this feature which have come under my
observation occur among the trachytic dykes of the south-east coast of
Skye between Kyle Rhea and Loch na Daal, where they have been mapped
and carefully investigated by Mr. Clough, who has conducted me over the
sections. In some of these dykes, as already narrated, the marginal
portions display a finely spherulitic structure, the small pea-like
spherulites being grouped into fine ribs or rods. It is also observable
that the steam-vesicles which may retain their spherical forms in the
centre are elongated in the same direction as the rows of spherulites.
Where this lineation is developed vertically, it no doubt points to the
vertical ascent of the lava between the two walls of the fissure.

But in other examples, the elongation is nearly horizontal, and between
the two positions Mr. Clough has registered many intermediate trends.
It would thus appear that in some places the lava has certainly
flowed laterally between the fissure-walls. Moreover, the trend of
the spherulitic rods and of the amygdales is found to vary in closely
adjoining planes at different distances from the margin, as if after
the outer portions of the dyke had consolidated into position, there
was still movement enough to drag the rows of spherulites and vesicles
up or down along the trend of the fissure.

Mr. Clough has observed that in some dykes, while the amygdaloidal
vesicles are large and undeformed in the centre, they become elongated
and inclined downward in the direction of the margin, as if the central
portions had not only remained fluid longer than the rest, but had a
tendency to rise upwards in the fissure, though there was obviously
less motion after these central vesicles appeared than in the marginal
parts where the vesicles are so much drawn out.

13. BRANCHING DYKES AND VEINS

It might have been anticipated that the uprise of such abundant masses
of molten rock, in so many long and wide fissures, would generally be
attended with the intrusion of the same material into lateral rents and
irregular openings, so that each dyke would have a kind of fringe of
offshoots or processes striking from it into the surrounding ground.
It might have been expected also that dykes would often branch, and
that the arms would come together again and enclose portions of the
rocks through which they rise. But in reality such excrescences and
bifurcations are of comparatively rare occurrence. As a rule, each
dyke is a mere wall of igneous rock, with little more projection or
ramification than may be seen in a stone field-fence. Among the short,
narrow and irregular dykes of the gregarious type branchings are
occasionally seen, and in some districts are extraordinarily abundant.
But among the great single dykes such irregularities are far less
common than might have been looked for. A few characteristic examples
from each type of dyke may here be given.

[Illustration: Fig. 246.--Branching portion of the great Dyke near
Hawick (length about one mile).]

[Illustration: Fig. 247.--Branching Dyke at foot of Glen Artney (length
about four miles).]

The Cleveland dyke, which in so many respects is typical of the great
solitary dykes of the country, has been traced for many miles without
the appearance of a single offshoot of any kind. Yet here and there
along its course, it departs from its usual regularity. As it crosses
the Carboniferous tracts of Durham and Cumberland, there appear near
its course lateral masses of eruptive rock, most of which doubtless
belong to the much older "Whin Sill." But there is at least one
locality, at Bolam near Cockfield, in the county of Durham, where
the dyke, crossing the Millstone Grit, suddenly expands into a boss,
and immediately contracts to its usual dimensions. Around this knot
several short dykes or veins seem to radiate from it. The dyke has been
quarried here, and its relations to the surrounding strata have been
laid bare, as will be again referred to a little further on.[191]

[Footnote 191: This locality was well described by Sedgwick, in his early
paper on Trap-Dykes in Yorkshire and Durham, _Trans. Cambridge Phil.
Soc._ ii. p. 27.]

Among the great persistent dykes of Scotland the absence of bifurcation
and lateral offshoots offers a striking contrast to the behaviour of
the dykes in those districts where they are small in size and many in
number. But exceptions to the general rule may be gathered. Thus the
Eskdale dyke is flanked at Wat Carrick with a large lateral vein, which
is almost certainly connected with the main fissure. The Hawick and
Cheviot dyke splits up on the hill immediately to the east of the town
of Hawick, sends off some branches, and then resumes its normal course
(Fig. 246). Again, one of the two nearly parallel dykes which run from
Lochgoilhead across Ben Ledi into Glen Artney bifurcates at the foot of
that valley, its northern limb (about two miles long) speedily dying
out, and its southern branch throwing off another lateral vein, and
then continuing eastward as the main dyke (Fig. 247).

In the districts of gregarious dykes, however, abundant instances
may be found of dykes that branch, and of others that lose the
parallelism of their walls, become irregular in breadth, direction, and
inclination, so as to pass into those intrusive forms that are more
properly classed as veins. Excellent illustrations of bifurcating dykes
may be observed along the shores of the Firth of Clyde, particularly
on the eastern coast-line of the isle of Arran. The venous character
has become familiar to geologists from the sketches given by Macculloch
from the lower parts of the cliffs of Trotternish in Skye.[192] Still
more striking examples are to be seen in the breaker-beaten cliffs of
Ardnamurchan. The pale Secondary limestones and calcareous sandstones
of that locality are traversed by a series of dark basic veins, and the
contrast of tint between the two kinds of rock is so marked as even to
catch the eye of casual tourists in the passing steamboats. The veins
vary in width from less than an inch to several feet or yards. They run
in all directions and intersect each other, forming such a confused
medley as requires some patience on the part of the geologist who
would follow out each independent ribbon of injected material in its
course up the cliffs, or still more, would sketch their ramifications
in his note-book. A good, though perhaps somewhat exaggerated,
illustration of their general character was given by Macculloch.[193]
The accompanying figure (Fig. 248) is less sensational, but represents
with as much accuracy as I could reach, the network of veins near the
foot of the cliffs. One conspicuous group of veins, which, seen from
a distance, looks like a rude sketch of a lug-sail traced in black
outline upon a pale ground, is known to the boatmen as "M'Niven's
Sail." Another admirable locality for the study of dykes and tortuous
veins is the northern coast of the Sound of Soa, where an extraordinary
number of injections traverse the Torridon Sandstones on which the
plateau-basalts rest (Fig. 323).

[Footnote 192: _Western Islands_, plate xvii.]

[Footnote 193: _Op. cit._ plate xxxiii. Fig. 1.]

As a general rule, the narrower the vein the finer in grain is the
rock of which it consists. This compact dark homogeneous material has
commonly passed by the name of "basalt." Its minuteness of texture
probably in most cases arises from local rapidity of cooling, and
it is doubtless the same substance which, where in larger mass in
the immediate neighbourhood, has solidified as one of the other
pyroxene-plagioclase-magnetite rocks.

With regard to the places where such abundant tortuous veins are more
especially developed, I may remark that they are particularly prominent
under a thick overlying mass of erupted rock, such as a great intrusive
sheet, or the bedded basalts of the plateaux, or where there is good
reason to believe that such a deep cover, though now removed by
denudation, once overspread the area in which they appear. It will be
shown in the sequel that such horizons have been peculiarly liable to
intrusions of igneous material of various kinds, and at many different
intervals, during the volcanic period. A thick cake of crystalline rock
seems to have offered such resistance to the uprise of molten material
through it, that when the subterranean energy was not sufficient to
rend it open by great fissures, and thus give rise to dykes, the lavas
were either forced into such irregular cracks as were made partly in
the softer rocks underneath and partly in the cake itself, or found
escape along pre-existing divisional planes. In Ardnamurchan, round the
Cuillin Hills of Skye, and in Rum, the overlying resisting cover now
consists mainly of gabbro sheets. In the east of Skye, in Eigg, and in
Antrim, it is made up of the thick mass of the plateau-basalts.

[Illustration: Fig. 248.--Basic veins traversing secondary limestone
and sandstone on the coast cliffs, Ardnamurchan.]

14. CONNECTION OF DYKES WITH SILLS

Every field-geologist is aware how seldom he can actually find the vent
or pipe up which rose the igneous rock that supplied the material of
sills and laccolites. He might well be pardoned were he to anticipate
that, in a district much traversed by dykes, there should be many
examples of intrusive sheets and frequent opportunities of tracing the
connection of such sheets with the fissures from which their material
might be supposed to have been supplied. But such an expectation is
singularly disappointed by an actual examination of the Tertiary
volcanic region of Britain. That there are many intrusive sheets
belonging to the great volcanic period with which I am now dealing,
I shall endeavour to show in the sequel. But it is quite certain that
though these sheets have of course each had its subterranean pipe
or fissure of supply, they can only in rare instances be directly
traced to the system of dykes. On the other hand, the districts
where great single dykes are most conspicuous, are for the most part
free from intrusive sheets, except those of much older date, like
the Carboniferous Whin Sill of Durham and those of Linlithgowshire,
Stirlingshire and Fife.

Yet a few interesting examples of the relation of dykes to sheets
have been noticed among British Tertiary volcanic rocks. The earliest
observed instances were those figured and described by Macculloch.
Among them one has been familiar to geologists from having done duty
in text-books of the science for more than half a century. I allude to
the diagram of "Trap and Sandstone near Suishnish."[194] In that drawing
seven dykes are shown as rising vertically through the horizontal
sandstone, and merging into a thick overlying mass of "trap." The
author in his explanation leaves it an open question "whether the
intruding material has ascended from below and overflowed the strata,
or has descended from the mass," though from the language he uses in
his text we may infer that he was inclined to regard the overlying body
as the source of the veins below it.[195]

[Footnote 194: _Western Islands of Scotland_, pl. xiv. Fig. 4.]

[Footnote 195: _Op. cit._ vol. i. pp. 384, 385.]

[Illustration: Fig. 249.--Section showing the connection of a Dyke with
an Intrusive Sheet, Point of Suisnish, Skye.

  _g_, Granophyre of Carn Dearg; _f_, similar rock, which appears
  eastward under the "sill" (_d_); _e_, intrusive sheet of
  fine-grained "basalt"; _d_, intrusive sheet or sill of coarse
  dolerite, 200 feet thick at its maximum, and rapidly thinning out;
  _c_, dyke or pipe of finer grain than _d_; _b_, yellowish-brown
  shaly sandstones, and _a_, dark sandy shales (Lias).
]

The section given by Macculloch, however, does not quite accurately
represent the facts. The narrow dykes there drawn have no connection
with the overlying sheet, but are part of the abundant series of
basaltic dykes found all over Skye. The feeder of the gabbro sill was
presumably the broad dyke which descends the steep bank immediately
on the southern front of Carn Dearg (636 feet high). The accompanying
figure (Fig. 249) shows what seemed to me to be the structure of the
locality, but the actual junction of the dyke and sheet is concealed
under the talus of the slope.[196] I shall have occasion in a later
Chapter to refer again to this section in connection with the history
of intrusive sheets, and also to cite from the neighbouring island of
Raasay another good example of the same relation between dyke and sill.

[Footnote 196: In more recently surveying this ground, Mr. Harker
has been led to regard the coarse sill as independent of the other
intrusions, and as almost certainly later than the basalt-sheets of
the same locality. When it reaches the base of these sills it turns so
as to pass beneath them as a gabbro-sill, which is conspicuous near
the summit of Carn Dearg. It runs westward for some distance, almost
immediately breaking across the bedding so as to leave the basalt, and
rapidly tapering until it dies out.]

Sedgwick, in the paper above quoted, gave an account and figure of the
expansion of the Cleveland dyke at Bolam, to which allusion has already
been made. He showed that from a part of the dyke which is unusually
contracted a great lateral extension of the igneous rock takes place on
either side over beds of shale and coal. While in the dyke the prisms
are as usual directed horizontally inward from the two walls, those
in the connected sheet are vertical, and descend upon the surface of
highly indurated strata on which the sheet rests.

The most important examples known to me are those which occur in
the coal-field of Stirlingshire. In that part of the country, the
remarkable group of dykes already referred to, lying nearly parallel
to each other and from half a mile to about three miles apart, runs in
a general east and west direction. From one of these dykes no fewer
than four sills strike off into the surrounding Coal-measures. The
largest of them stretches southwards for three miles, but the same
rock is probably continued in a succession of detached areas which
spread westwards through the coal-field and circle round to near the
two western sheets that proceeded from the same dyke. Another thick
mass of similar rock extends on the north side of the dyke for two
and a half miles down the valley of the river Avon. These various
processes, attached to or diverging from the dyke, are unquestionably
intrusive sheets, which occupy different horizons in the Carboniferous
series. The one on the north side has inserted itself a little above
the top of the Carboniferous Limestone series. Those on the south side
lie on different levels in the Coal-measures, or, rather, they pass
transgressively from one platform to another in that group of strata.

[Illustration: Fig. 250.--Section to show the connection of a Dyke with
an Intrusive Sheet, Stirlingshire Coal-field.

_a_, Dyke in line of fault; _b_, Sill traversing and altering the coal;
_i_, Slaty-band Ironstone.]

No essential difference can be detected by the naked eye between the
material of the dyke and that of the sheets. If a series of specimens
from the different exposures were mixed up, it would be impossible to
separate those of the dyke from those of the sheets. A microscopical
examination of the specimens likewise shows that they are perfectly
identical in composition and structure, being chiefly referable to
rocks of the dolerite, but partly of the tholeiite type. I have
therefore little doubt that these remarkable appendages to this dyke
are truly offshoots from it, and are not to be classed with the general
mass of the sills of Central Scotland, which are of Carboniferous,
partly of Permian, age. The accompanying diagrammatic section (Fig.
250) explains the geological structure of the ground.

An interesting and important fact remains to be stated in connection
with these sheets. They are traversed by some of the other east
and west dykes. This is particularly observable in the case of the
sheet which extends northwards from the dyke through the parish of
Torphichen. Two well-marked dykes can be seen running westwards among
the ridges of the sheet. It is obvious, therefore that these particular
dykes are younger than the sheet. But, as will be shown in the sequel,
there is abundant evidence that all the dykes of a district are not of
one eruption. The intersection of one eruptive mass by another does
not necessarily imply any long interval of time between them. They
mark successive, but it may be rapidly successive, manifestations of
volcanic action. Hence the cutting of the sheets by other dykes does
not invalidate the identification of these sheets as extravasations
from the great dyke by which they are bounded.

15. INTERSECTION OF DYKES

[Illustration: Fig. 251.--Intersection of Dykes in bedded basalt,
Calliach Point, Mull.]

Innumerable instances may be cited, where one dyke, or one set
of dykes, cuts across another. To some of these I shall refer in
discussing the data for estimating the relative ages of dykes. In
considering the intersection from the point of view of geological
structure, we are struck with the clean sharp way in which it so
generally takes place. The rents into which the younger dykes have
been injected seem, as a rule, not to have been sensibly influenced
in width and direction by the older dykes, but go right across them.
Hence the younger dykes retain their usual breadth and trend (Fig.
251). In trying to ascertain the relative ages of such dykes we obtain
a valuable clue in studying the respective "chilled edges" of the two
intersecting masses, as has already been pointed out.

Not only do dykes cross each other, but still more is this the case
among the narrower tortuous intrusions known as Veins (Fig. 252). Among
the illustrations which the dykes of the Inner Hebrides supply of these
features one further characteristic example may be culled from the
shore of Skye, near Broadford, where the gently-inclined sheets of Lias
limestone are traversed by three systems of dykes (Fig. 253). One of
these systems runs in a N.W. or N.N.W. direction, a second follows a
more nearly easterly trend, while the third and youngest runs nearly
north and south.

  Fig. 252.--Basalt Veins traversing bedded dolerites, Kildonan, Eigg.
]

  Fig. 253.--Ground-plan of intersecting Dykes in Lias limestone,
  Shore, Harrabol, East of Broadford, Skye.
]

16. DYKES OF MORE THAN ONE IN-FILLING

The intersections of dykes prove that the process of fissuring in
the earth's crust took place at more than one period, and prepare us
for the reception of evidence that the same line of fissure might be
again re-opened, even after it had been filled with molten material.
Numerous instances have now been accumulated in which dykes are not
single or simple intrusions, but where the original dyke-fissure has
been re-opened and has been invaded by successive uprisings of lava.[197]
Compound dykes have thus been formed, consisting of two or more
parallel bands of similar or dissimilar rock.

[Footnote 197: See an example figured by Macculloch, _Western Isles_,
plate xviii. Fig. 1.]

While it is not difficult to conceive of the re-opening of a vertical
fissure during terrestrial strain, and the injection into it of later
intrusions of a volcanic magma, it is not so easy to understand the
mechanism where the line of weakness has been slightly inclined or
horizontal, and where, consequently, there has been the enormous
superincumbent pressure of the overlying part of the earth's crust to
overcome. Yet gently inclined compound dykes exhibit their parallel
bands with hardly less regularity than do those that are vertical.
The difficulty of explanation is felt most strongly in the attempt to
realize the origin of the compound sills described in Chapter xlviii.

In the re-opening of dyke-fissures the later intrusions have generally
taken place along the walls, or where the dykes were already compound,
between some of the component bands. Less frequently the first dyke has
been split open along the middle, and a second injection has forced its
way along the rent.

Of the first of these two types, numerous instances have now been
observed in the West of Scotland. If the portion of a compound dyke
exposed at the surface be limited in extent, we may be unable to
determine which is the older of two parallel bands of igneous rock,
though the fact that they present to each other the usual fine-grained
edge due to more rapid cooling, shows that they are not one but two
dykes, belonging to distinct eruptions. So far as I have noticed,
where one of the dykes can be continuously traced for a considerable
distance, the other is comparatively short. I infer that the shorter
one is the younger of the two.

In the Strath district of Skye, Mr. Harker has recently observed that
many of the basic dykes, both those older and those younger than the
granophyre protrusions, are double, triple or multiple. Thus in a
conspicuous dyke, more than 100 feet wide, to the south-east of Loch
Kilchrist, belonging to the older series, he has detected at least six
contiguous dykes which as they are traced south-eastward, in spite
of their interruption by the Beinn an Dubhaich granite, can be seen
to separate and take different courses, or successively die out. He
remarks, further, that "many cases of apparent bifurcation of dykes are
really due to the separation of distinct dykes which have run for some
distance in one fissure. Sometimes apparent variations in the width of
a dyke are to be explained by this dying out of one member of a double
dyke. These multiple dykes are less easily detected in the newer than
the older set, owing to greater uniformity of lithological type in the
prevalent kinds and to the frequent absence of chilled selvages."[198] An
example of a compound basic dyke cutting the crest of the gabbro-mass
of the Cuillin Hills is shown in Fig. 333.

[Footnote 198: MS. notes supplied by Mr. Harker.]

Instances of the second type of compound dykes are less common. Here,
instead of being re-opened along one of the walls, the fissure has
been ruptured along the centre of the dyke, and a second injection of
molten material has then taken place. This structure may be observed
where the materials of the compound dyke are on the whole similar,
such as varieties of dolerite, basalt, diabase or andesite. In these
cases the rock of the central dyke is generally rather fine-grained,
sometimes decidedly porphyritic, and often a true basalt. Where broad
enough to show the difference of texture between margin and centre, it
exhibits the usual close grain along its edges, indicative of quicker
cooling. The older dyke presenting no such change at its junction with
the younger, was obviously already cooled and consolidated before its
rupture.

Whilst the centre of a dyke has occasionally proved to be a line of
weakness which has given way under intense strains in the terrestrial
crust, this rupture and the accompanying or subsequent ascent of molten
material in the re-opened fissure may sometimes have been included as
phases of one connected volcanic episode. In those instances, for
example, which have been above described, where a central vitreous band
has risen along the heart of a dyke, the petrographical affinities of
the rocks may be so close as to suggest that although the main dyke had
consolidated and had subsequently been ruptured along its centre by
powerful earth-movements, these changes all belonged to the same period
of dyke-making, and the subsequent uprise of glassy material was merely
a later phase in the movements of the same subterranean magma.

But where, as probably happens in the large majority of compound dykes,
there is a strongly marked difference between the respective bands
of rock, we must either infer that two essentially different magmas
co-existed in the volcanic reservoirs underneath, and were successively
injected into the same fissures, or that a sufficient lapse of time
occurred to permit a total renewal of the nature of the magma, and an
uprise of this changed material into fissures which sometimes coincided
with older dykes. If any interlocking of the crystals of the several
bands of a compound dyke could be detected, we might suppose that the
first-injected material had not become consolidated and cold before
the uprise of the newer rock. But in general it would seem that so
sharp a line of demarcation can be drawn between the two rocks as to
indicate that their protrusion was due to two distinct and perhaps
widely-separated volcanic paroxysms.

Compound dykes of basic material occur not only among the ordinary
straight north-westerly series, but also among the less regular
gregarious dykes and veins, such as abundantly intersect the gabbro
bosses. Moreover they are to be found among the youngest intrusions,
for they traverse the masses of granophyre. Conspicuous examples of
such late compound dykes are displayed along the cliffs of St. Kilda,
as will be more particularly described in a later Chapter. These St.
Kilda dykes often occupy not vertical fissures but parallel rents with
a gentle inclination (see Figs. 367, 368).

The Tertiary volcanic series of Scotland furnishes many examples of
compound dykes of a much more complex character where parallel bands
of some acid (granophyre, felsite, quartz-porphyry) or intermediate
(andesite) rock is associated with others of the more usual basic
material (dolerite, basalt, diabase). As the acid intrusions belong
to a comparatively late part of the volcanic history, their modes of
occurrence will be discussed in Chapters xlvi., xlvii. and xlviii. But
no account of the general system of dykes would be complete without
some reference to these compound examples, which will therefore be
briefly described in the present section of this work.

Early in this century some striking illustrations of the association
of acid and more basic rocks within the same fissure were noticed by
Jameson in the island of Arran. He described and figured instances at
Tormore, on the west side of that island, where a group of pitchstones
and "basalts" or andesites have been successively protruded into
the same fissures in the (probably Permian) red sandstones of that
district.[199]

[Footnote 199: _Mineralogy of the Scottish Isles_, 1800.]

In some instances the more basic rock has been first injected, and has
subsequently been disrupted, by the more acid pitchstone. In other
cases the order has been the reverse. The successive ruptures have
taken place sometimes along the centre, sometimes at the margins, and
sometimes irregularly along the breadth of the dykes. Professor Judd
has recently studied these rocks, and has given descriptions of their
chemical composition and microscopic characters. He regards them as
having been successively injected into the fissures from the same
subterranean reservoir, in which two magmas of very different chemical
constitution were simultaneously present.[200]

[Footnote 200: _Quart. Jour. Geol. Soc._ vol. xlix. (1893), p. 536.
Full details of the compound dykes of Tormore and Cir Mhor in Arran,
and references to previous writers will be found in this paper. The
probable age of the youngest eruptive rocks of this island will be
discussed in Chapter xlvii. p. 418.]

Nowhere in the Tertiary volcanic regions of Britain do compound dykes
appear to be so abundant as in the centre and southern part of the
island of Skye. During the progress of the Geological Survey in that
district, Mr. Clough and Mr. Harker have mapped a large number in the
ground between the Sound of Sleat and the Red Hills. With regard to
these dykes Mr. Harker observes that the several members are generally
petrographically different, some being basic, others intermediate,
and others acid. "There is usually," he remarks, "a symmetrical
disposition, two similar and more basic dykes being divided by a more
acid one; for example, two andesites separated by a pitchstone. Thus at
the mouth of the little stream which runs from Torran into the bay east
from Dùn Beag a dyke, apparently 18 feet wide, is found on examination
to consist of a central dyke (specific gravity 2·86) flanked by two
more basic dykes (specific gravity 3·02)."

In the great majority of examples hitherto observed in Skye the two
lateral dykes consist of some basic rock (diabase or basalt), while
the central and thickest band is of some acid material (granophyre or
quartz-felsite). This triple arrangement occurs both in dykes and sills.

[Illustration: Fig. 254.--Compound dyke, Market Stance, Broadford, Skye.

_a_, Granophyre; _b_ _b_, Basalt; _c_ _c_, Torridon sandstone.]

As an illustration of the association of the two kinds of rock in
dykes I may cite an example which appears on the southern edge of the
Market Stance of Broadford (Fig. 254). Here the characteristic triple
arrangement is typically developed. A central light-coloured band,
about eight to ten feet broad, consists of a spherulitic granophyre
in which the spherulites are crowded together and project from the
weathered surface like peas, though they do not here show the curious
rod-like aggregation so marked in some other dykes. On either side of
this acid centre a narrow basalt dyke intervenes as a wall next to the
Torridon sandstone which here forms the country-rock. Such compound
dykes have sometimes a total width of 100 feet or more.

In this instance, and generally throughout the district, there is
nothing to indicate that the different bands of the dyke have any
relation to each other as connected uprises of material from the
same original magma which was either heterogeneous or was undergoing
a process of differentiation beneath the terrestrial crust. On the
contrary, the several parts of each dyke are as distinctly marked off
from each other as they could have been had they been injected at
widely separated intervals of volcanic activity.

Mr. Harker, in the course of his survey of this Skye ground, has
observed that "where evidence is available, the central acid dyke
is found to be newer than the basic ones. It has not split a single
basic dyke, but has insinuated itself between the two members of a
double dyke. This is more clearly seen when the acid magma has been
forced into a triple or multiple basic dyke; the perfect symmetry
of arrangement may in this case be lost. For instance, on the shore
north-east of Corry, Broadford, a 13 feet dyke of granophyre occurs in
a multiple dyke of basalt, but it has taken its line so as to leave
only a one-foot dyke on one side, and a group with a total width of
12 feet on the other. Also it has not accurately kept its course, but
has cut obliquely across one of the group of dykes alluded to. In some
cases it is certain that the acid magma has to some extent dissolved a
portion of the wall of a basic dyke with which it has come in contact.
This may account for the magma finding its easiest path along, and
especially between, pre-existing more basic dykes." This subject will
be again referred to in Chapter xlviii., when the phenomena of compound
sills are discussed.

Before closing this account of compound dykes, I may remark that no
examples have yet been observed among the ordinary Tertiary dykes
of Britain where, by a process of differentiation between the walls
of a fissure, successive zones have been developed in the dyke,
differing from each other in structure and composition, but becoming
progressively and insensibly more acid towards the centre, such as
have been described from the older rocks of Norway and Canada. Among
the Tertiary gabbro bosses, indeed, there occur sheets or dykes which
present a remarkably banded structure, to which full reference will be
made in later pages. But I have never seen anything at all resembling
such a structure among the dykes of andesite, dolerite, or basalt.

17. CONTACT-METAMORPHISM OF THE DYKES

A geologist might naturally expect that such abundant intrusions of
igneous rock as those of the dykes should be accompanied with plentiful
proofs of contact-metamorphism. But in actual fact, evidence of any
serious amount of alteration is singularly scarce. A slight induration
of the rocks on either side of a dyke is generally all the change that
can be detected.

[Illustration: Fig. 255.--Section of coal rendered columnar by
intrusive basalt, shore, Saltcoats, Ayrshire.

_a_, Fireclay; _b_, Coal rendered prismatic near the basalt; _c_, Dark
shale; _d_, Basalt-rock.]

Some of the larger dykes, however, show more marked metamorphism, the
nature of which appears in many cases to be chiefly determined by
the chemical composition of the rock affected. Thus a considerable
alteration has been superinduced on carbonaceous strata, particularly
on seams of coal. In the Ayrshire coal-field the alteration of the coal
extends sometimes 150 feet from the dyke, the extent of the change
depending not merely on the mass of the igneous rock, but on the nature
of the coal, and possibly on other causes. Close to a dyke, coal passes
into a kind of soot or cinder, sometimes assumes the form of a finely
columnar coke (Fig. 255), and occasionally has become vesicular after
being fused.[201] Shales are converted into a hard flinty substance that
breaks with a conchoidal fracture and rings under the hammer. Fireclay
is baked into a porcelain-like material. Limestone is changed for a few
inches into marble. As an illustration of this alteration, I may cite a
dyke ten feet broad which cuts through the chalk in the Templepatrick
Quarry, Antrim. For about six inches from the igneous rock the chalk
has passed into a finely saccharoid condition, and its organisms are
effaced. But beyond that distance the crystalline structure rapidly
dies away, the micro-organisms begin to make their appearance, and
within a space of one foot from the dyke the chalk assumes its ordinary
character.

[Footnote 201: Explanation of Sheet 22, Geological Survey of Scotland, p.
26.]

Sandstones are indurated by dykes into a kind of quartzite, sometimes
assume a columnar structure (the columns being directed away from
the dyke-walls), and for several feet or yards have their yellow or
red colours bleached out of them. The granite of Ben Cruachan where
quarried on Loch Awe, as I am informed by Mr. J. S. Grant Wilson, is
traversed by a basic dyke, and for a distance of about 20 feet is
rendered darker in colour, becomes granular, and cannot be polished and
made saleable.

Where many dykes have been crowded together, their collective effects
in the alteration of the strata traversed by them have sometimes been
strongly developed. One of the most remarkable illustrations of this
influence is presented by the district of Strathaird, which was cited
by Macculloch for the abundance of its dykes. In recently mapping
this ground for the Geological Survey, Mr. Harker has observed in
some places a score or more dykes in actual juxtaposition, while over
considerable distances he found it difficult to detect any trace of
the Jurassic strata, through which the igneous rocks have ascended.
As might be expected under these circumstances, such portions of
the strata as can be seen display an altogether exceptional amount
of contact-metamorphism. Mr. Harker has noticed some limestones at
Camasunary which have been changed into very remarkable lime-silicate
rocks, with singular bunches of diopside crystals.

These, however, are the extremes of contact-metamorphism by the
Tertiary basic dykes. A geologist visiting the Liassic shores of Strath
in Skye will not fail to be surprised at the very slight degree of
alteration in circumstances where he would have expected to find it
strongly pronounced. The dark shales, though ribbed across with dykes,
are sometimes hardly even hardened, and at the most are only indurated
from an inch or two to about two feet. These baked bands project above
the rest of the more easily denuded shales, and so adhere to the dykes
as almost to seem part of them. Again the limestones, where traversed
by dykes some distance apart, are not rendered in any appreciable
degree more crystalline even up to the very margin of the intrusive
rock. Where the igneous material has been thrust between the strata
in sills, it has produced far more general and serious metamorphism
than when it occurs in the form of single dykes. The famous rock of
Portrush, already referred to as having been once gravely cited as an
example of fossiliferous basalt, is a good illustration of the way in
which Lias shale is porcellanized when the intruded igneous material
has been thrust between the planes of bedding.

In the West of Scotland, where dykes are so abundantly developed,
considerable differences can be observed between the amount of
metamorphism superinduced by adjacent dykes which may be of the same
thickness, and cut through the same kind of strata. Such variations
have not probably arisen from differences in the temperature of the
original molten rock. Perhaps they are rather to be assigned to the
length of time occupied by the ascent of the lava in the fissure.
If, for instance, the fissure opened to the surface and discharged
lava there, the rocks of its walls would be exposed to a continuous
stream of molten rock as long as the outflow lasted. They would thus
have their temperature more highly raised, and maintained at such an
elevation for a longer time than where the magma, at once arrested
within the fissure, immediately proceeded to cool and consolidate
there. It would be an interesting and important conclusion if we could,
from the nature or amount of their contact-metamorphism, distinguish
those dykes which for some time served as channels for the discharge of
lava above ground.

Some dykes which have caught up fragments of older rocks in their
ascent have exercised a considerable solvent action on these
inclusions. Examples of this feature have already been cited from Skye,
where they have been studied by Mr. Harker (pp. 129, 163).

In connection with the metamorphism superinduced by dykes, reference
may again be made to the alteration which they themselves undergo
where they have invaded a carbonaceous shale or coal. The igneous
rock, as we have seen, loses its dark colour and obviously crystalline
structure, and becomes a pale yellow or white, dull, earthy substance,
or "white trap." The chemical changes involved in this alteration
have been described by Sir J. Lowthian Bell.[202] Dr. Stecher has also
discussed the alterations traceable by the aid of the microscope.[203]
Though most of the instances of such transformation in Britain occur
in the Carboniferous system, and have taken place in intrusive rocks
of probably, for the most part, Carboniferous or Permian age, yet they
are not unknown in the Tertiary volcanic series. Some of the "white
trap" of the Coal-measures may indeed belong to the Tertiary period,
but the coals and carbonaceous shales interstratified in the Tertiary
basalt-plateaux have reacted on both the superficial lavas and the
sills, and have given rise to the same kind of alteration as in the
Carboniferous system, as will be shown in a later Chapter.

[Footnote 202: _Proc. Roy. Soc._ xxiii. (1875), p. 543.]

[Footnote 203: Tschermak's _Mineralogische Mittheilungen_, ix. (1887), p.
145, and _Proc. Roy. Soc. Edin._ 1888.]

  Fig. 256.--Dolerite dyke with marginal bands of "white trap," in
  black shale, Lower Lias, Pabba.

  _a_, Black carbonaceous Lower Lias Shale; _b_ _b_, bands of
  indurated shale from 15 inches to 2 feet broad; _c_, dolerite dyke
  3 feet 3 inches broad; _d_ _d_, bands of altered dolerite or "white
  trap," 3 to 5 inches broad.
]

Some marked examples of this alteration of intrusive igneous material
are to be observed among the basalt dykes which cut the Lower Lias
Shales of Skye. These shales, where black and carbonaceous, as in
the island of Pabba, have exercised an unmistakable influence on the
abundant dykes which intersect them. The chilled selvage of each dyke
has assumed the dull earthy pale-grey or yellowish aspect, which
extends for a few inches from the wall into the interior, where it
rapidly passes into the ordinary black crystalline basalt. These
features will be readily understood from the accompanying diagram (Fig.
256). Where the dykes give off narrow veins a few inches broad, these
consist entirely of the "white trap." The shales are often traversed
with strong joints parallel to the walls of the dykes, and the
transverse joints of the dykes are sometimes prolonged into the bands
of indurated shale.

18. RELATION OF DYKES TO THE GEOLOGICAL STRUCTURE OF THE DISTRICTS
WHICH THEY TRAVERSE.

In no respect do the Tertiary dykes of Britain stand more distinguished
from all the other rocks of the country than in their extraordinary
independence of geological structure. The successive groups of
Palæozoic and Mesozoic strata have been so tilted as to follow each
other in approximately parallel bands, which run obliquely across the
island from south-west to north-east. The most important lines of fault
take the same general line. The contemporaneously included igneous
rocks follow, of course, the trend of the stratified deposits among
which they lie, and even the intrusive sills group themselves along the
general strike of the whole country. But the Tertiary dykes have their
own independent direction, to which they adhere amid the extremest
diversities of geological arrangement.

In the first place, the dykes intersect nearly the whole range of the
geological formations of the British Islands. In the Outer Hebrides and
north-west Highlands, they rise through the most ancient (Lewisian)
gneisses, through the red pre-Cambrian (Torridon) sandstones, and
through the oldest members of the Cambrian system. In the southern
Highlands, they pursue their course across the gnarled and twisted
schists of the younger crystalline (Dalradian) series. In the South of
Scotland and North of England, they traverse the various subdivisions
of the Lower and Upper Silurian rocks. In the basins of the Tay, Forth,
and Clyde they cross the plains and ridges of the Old Red Sandstone,
with its deep pile of intercalated volcanic material. In Central
Scotland, and the northern English counties, they occur abundantly in
the Carboniferous system, and have destroyed the seams of coal. In
Cumberland and Durham, they traverse the Permian and Trias groups.
In Yorkshire, and along the West of Scotland, they are found running
through Jurassic strata. In Antrim, they intersect the Chalk. Both in
the North of Ireland, and all through the chain of the Inner Hebrides,
they abound in the great sheets and bosses of Tertiary volcanic rocks.
These are the youngest formations through which they rise. But it is
deserving of note, that they intersect every great group of these
Tertiary volcanic products, so that they include in their number the
latest known manifestations of eruptive action in the geological
history of Britain.[204]

[Footnote 204: They have not been found cutting the pitchstone-lava of
the Scuir of Eigg.]

In the second place, in ranging across groups of rock belonging to
such widely diverse periods, the dykes must necessarily often pass
abruptly from one kind of material and geological structure to another.
But, as a rule, they do so without any sensible deviation from their
usual trend, or any alteration of their average width. Here and there,
indeed, we may observe a dyke to follow a more wavy or more rapidly
sinuous or zig-zag course in one group of rocks than in another.
Yet, so far as I have myself been able to observe, such sinuosities
may occur in almost any kind of material, and are not satisfactorily
explicable by any difference of texture or arrangement in the rocks
at the surface. No dyke traverses a greater variety of sedimentary
formations than that of Cleveland. In the eastern part of its course,
it rises through all the Mesozoic groups up to the Cornbrash. Further
west it cuts across each of the different subdivisions of the
Carboniferous system; and, of course, it must traverse all the older
formations which underlie these. But the occasional rapid changes
noticeable in its width and direction do not seem to be referable to
any corresponding structure in the surrounding rocks. The Cheviot
dyke crosses from the Carboniferous area of Northumberland into the
Upper Silurian rocks and Lower Old Red Sandstone volcanic tract of
the Cheviot Hills. It then strikes across the Upper Old Red Sandstone
of Roxburghshire, and still maintaining the same persistent trend,
sweeps westward into the intensely plicated Silurian rocks of the
Southern Uplands. Its occasional deviations have no obvious reference
to any visible change of structure in the adjacent formations. Again,
some of the great dykes at the head of Clydesdale furnish striking
illustrations of entire indifference to the nature of the rock through
which they run. Quitting the Silurian uplands, they keep their line
across Old Red Sandstone and Carboniferous rocks, and through large
masses of eruptive material.

In the third place, not only are the dykes not deflected by great
diversities in the lithological character of the rocks which they
traverse, they even cross without deviation some of the most important
geological features in the general framework of the country. Some
of the Scottish examples are singularly impressive in this respect.
Those which strike north-westward from the uplands of Clydesdale
cross without deflection the great boundary-fault which, by a throw
of several thousand feet, brings the Lower Old Red Sandstone against
Silurian rocks. They traverse some large faults in the valley of the
Douglas coal-field, pass completely across the axis of the Haughshaw
Hills, where the Upper Silurian rocks are once more brought up to the
surface, and also the long felsite ridge of Priesthill. The dykes in
the centre of the kingdom maintain their line across some of the large
masses of igneous rock that protrude through the Carboniferous system.
Further north, the dykes of Perthshire cut across the great sheets of
volcanic material that form the Ochil Hills, as well as through the
piles of sandstone and conglomerate of the Lower Old Red Sandstone, and
then go right across the boundary-fault of the Highlands, to pursue
their way in the same independent manner through grit, quartzite, or
mica-schist, and across glen and lake, moor and mountain.

No one can contemplate these repeated examples of an entire want of
connection between the dykes and the nature and arrangement of the
rocks which they traverse without being convinced that the lines of
rent up which the material of the dykes rose were not, as a rule, old
fractures in the earth's crust, but were fresh fissures, opened across
the course of the older dislocations and strike of the country by the
same series of subterranean operations to which the uprise of the
molten material of the dykes was also due.

In the fourth place, the dykes for the most part are not coincident
with visible lines of fault. After the examination of hundreds of dykes
in all parts of the country, and with all the help which bare hillsides
and well-exposed coast-sections can afford, the number of instances
which have been met with where dykes have availed themselves of lines
of fault is surprisingly small. Some of these cases will be immediately
cited. To whatever cause we may ascribe the rupture of the solid crust
of the earth, which admitted the rise of molten rock to form the dykes,
there can be no doubt that it was not generally attended with that
displacement of level on one or both sides of the dislocation, which
we associate with the idea of a fault. Nowhere can this important part
of dyke-structure be more clearly illustrated than along the Cleveland
dyke, where the igneous rock rises through almost horizontal Jurassic
strata and gently inclined Coal-measures (Figs. 241, 242, 243, 244).
Besides the localities already cited, mining operations both for coal
and for the Liassic ironstone have proved over a wide area that the
dyke has not risen along a line of fault. Again, in Skye, Raasay,
Eigg, and other parts of the west coast, where Jurassic strata and
the horizontal basalts of the plateaux are plentifully cut through by
dykes, the same beds may generally be seen at the same level on either
side of them.

In the fifth place, while complete indifference to geological structure
is the general rule among the dykes, instances do occur in which the
molten material has found its way upward along old lines of rupture.
Most of such instances are to be found in districts where previously
existing faults happened to run in the same general direction as that
followed by the dykes. These lines of fracture might naturally be
re-opened by any great earth-movements acting in their direction, and
would afford ready channels for the ascent of the lava, as we have seen
to have not infrequently happened in the case of dyke-fissures, which
are shown by compound dykes to have sometimes been re-opened several
times in succession even after having been filled up with basalt. Yet
it is curious that, even when their trend would have suited the line
of the dykes, faults have not been more largely made use of for the
purpose of relief. Some of the best examples of the coincidence of
dykes with pre-existing faults in the same direction are to be found
in the Stirlingshire coal-field. The dyke that runs from Torphichen
for 23 miles to Cadder occupies a line of fault which at Slamannan has
a down-throw of more than 70 fathoms. The next dyke further south has
also risen along an east and west fault.

But other examples may be observed where pre-existing fissures have
served to deflect dykes from their usual line of trend. Thus the
Cleveland dyke, after crossing several faults in the Coal-measures, at
last encounters one near Cockfield Fell, which lies obliquely across
its path. Instead of crossing this fault it bends sharply round a few
points south of west, and after keeping along the southern flank of
the fault for about a mile, sinks out of reach. Some of the Scottish
examples are more remarkable. One of the best of them occurs in the
Sanquhar coal-field, where a dyke runs for two miles and a half along
the large fault that here brings down the Coal-measures against the
Lower Silurian rocks. At the north-western end of the basin, this fault
makes an abrupt bend of 60° to W.S.W., and the dyke turns round with
it, keeping this altered course for a mile and a half, when it strikes
away from the fault, crosses a narrow belt of Lower Silurian rocks,
and finds its way into the parallel boundary fault which defines the
north-western margin of the Southern Uplands.

[Illustration: Fig. 257.--Map of the chief dykes between Lochs Riddon
and Striven (C. T. Clough, Geological Survey, Sheet 29). The large E.
and W. dyke is a continuation of that which reaches the shore of the
Firth of Clyde at Dunoon.]

Some of the Perthshire dykes, where they reach the great boundary-fault
of the Highlands, present specially interesting features. There can
be no doubt that this dislocation is one of the most important in the
general framework of the British Isles, though no definite estimate has
yet been formed of how much rock has been actually displaced by it. The
fact that in one place the beds of Old Red Sandstone are thrown on end
for some two miles back from it, shows that it must be a very powerful
fracture. Here, therefore, if anywhere, either an entire cessation of
the dykes, or at least a complete deflection of their course might be
anticipated. It would require, we might suppose, a singularly potent
dislocation to open a way for the ascent of the lava through such
crushed and compressed rocks, and still more to prolong the general
line of a fracture across the old fault. Two great dykes, about half a
mile apart, run in a direction a little south of west across the plain
of Strathearn. Passing to the south of the village of Crieff, they hold
on their way until they reach the highly-inclined beds of sandstone
and conglomerate which here lean against the Highland fault in Glen
Artney. They then turn round towards south-west, and run up the glen
along the strike of the beds, keeping approximately parallel to the
fault for about three miles, when they both strike across the fault,
and pursue a W.S.W. line through the contorted crystalline rocks of
the Highlands. About two miles further south, another dyke continues
its normal course across the belt of upturned Old Red Sandstone; but
when it reaches the fault it bends round and follows the line of
dislocation, sometimes coinciding with, sometimes crossing or running
parallel with that line, at a short distance (see Fig. 247).

Some remarkable examples have been mapped by Mr. Clough in Eastern
Argyleshire, where broad bands of basalt or other allied rock run in a
N. and S. direction, and are formed by the confluence of N.W and S.E.
or N.N.W. and S.S.E. dykes, where these are drawn into a line of fault
(Fig. 257). These broad bands, he has found to be not usually traceable
for more than a mile or so, for the dykes of which they are made up
will not be diverted from their regular paths for more than a certain
distance, so that one by one the dykes leave the compound band to
pursue their normal course. He has observed that the occasional great
thickness of these compound bands depends partly on the size and partly
on the number of separate dykes that are diverted into the line of
transverse fissure; for, where the fissure crosses an area with fewer
north-west dykes, the band becomes thinner or ceases altogether.

In some rare cases, the dykes have been shifted by more recent faults.
I shall have occasion to show that faults of more than 1000 feet have
taken place since the Tertiary basalt-plateaux were formed. There is
therefore no reason why here and there a fault with a low hade should
not have shifted the outcrop of a dyke. But the fact remains, that,
as a general rule, the dykes run independently of faults even where
they approach close to them. Mr. Clough has observed some interesting
cases in South-eastern Argyleshire, where the apparent shifting of
a dyke by faults proves to be deceptive, and where the dyke has for
short distances merely availed itself of old lines of fracture. One
of the most remarkable of these is presented by the large dyke which
runs westward from Dunoon. No fewer than three times, in the course of
four miles between Lochs Striven and Riddon, does this dyke make sharp
changes of trend nearly at right angles to its usual direction, where
it encounters north and south faults (Fig. 257). It would be natural to
conclude that these changes are actual dislocations due to the faults.
But the careful observer just cited has been able to trace the dyke in
a very attenuated and uncrushed form along some of these cross faults,
and thus to prove that the faults are of older date, but that they
have modified the line of the long east and west fissure up which the
material of the dyke ascended.

19. DATA FOR ESTIMATING THE GEOLOGICAL AGE OF THE DYKES

I have already assigned reasons for regarding the system of north-west
and south-east or east and west dykes as belonging to the Tertiary
volcanic period in the geographical history of the British Islands.
But I have no evidence that they were restricted to any part of that
period. On the contrary, there is every reason to consider the uprise
of the earliest and latest dykes to have been separated by a protracted
interval. That they do not all belong to one epoch has been already
indicated, and may now be more specially proved.

The intersection of one dyke by another furnishes an obvious criterion
of relative age. Macculloch drew attention to this test, and stated
that it had enabled him to make out two distinct sets of dykes in Skye
and Rum. But he confessed that it failed to afford any information as
to the length of the interval of time between them.[205] It is not always
so easy as might be thought to make sure which of two intersecting
dykes is the older. As was explained in Chapter vi. (vol. i. p. 81),
we have to look for the finer-grained marginal strip at the edge of a
dyke, which, where traceable across another dyke, marks at once their
relative age. The cross joints of the two dykes also run in different
directions. Reference may again be made to the illustration given in
Fig. 253 where three distinct groups of dykes intersect each other as
they traverse the Lias limestones of Skye. The chilled edges and the
different arrangement of joints mark these dykes out from each other,
while the order in which they cross each other furnishes a clue to
their relative age. If from such sections, repeated in different parts
of a district, certain persistent petrographical characters can be
ascertained to distinguish each particular system of dykes, a guide may
thereby be obtained for the chronological grouping of the intrusions
even where evidence of actual intersection is not visible. In the case
just cited from Skye, the later north and south dykes are characterized
by their lines of vesicular cavities and by the large porphyritic
felspars which they contain.

[Footnote 205: _Trans. Geol. Soc._ iii. p. 75.]

It is obvious, however, that although sections of this kind suffice to
prove the dykes to belong to distinct periods of intrusion, no longer
interval need have elapsed between their successive production than was
required for the solidification and assumption of a joint-structure by
an older dyke before a younger broke through it. They may both belong
to one brief period of volcanic activity. But when we pass to a series
of dykes traversing a considerable district of country, and find that
those which run in one direction are invariably cut by those which
run in another, the inference can hardly be resisted that they do not
belong to the same period of eruption, but mark successive epochs of
volcanic energy. An excellent example of this kind of evidence is
furnished by Mr. Clough from Eastern Argyleshire. The east and west
dykes in that district are undoubtedly older than those which run
in a N.N.W. direction (Fig. 257).[206] The latter are by far the most
abundant, and are on the whole much narrower, less persistent, and
finer in grain. On the opposite coast of the Clyde, a similar double
set of dykes may be traced through Renfrewshire, those in an east and
west direction being comparatively few, while the younger N.N.W. series
is well developed. The great sheets or "sills" connected with one of
the Stirlingshire dykes, already described, appear to me to furnish
similar evidence in the younger dykes which run through them. And this
evidence is peculiarly valuable, for it shows a succession even among
adjacent dykes which all run in the same general direction.

[Footnote 206: As already stated, Mr. Clough and also Mr. Gunn are
inclined to separate these older east and west dykes from the Tertiary
series and to regard them as probably of late Palæozoic age.]

But in all these cases it is obvious that we have little indication of
the length of time that intervened between the successive injections
of the dykes. In Skye, however, more definite evidence presents itself
that the interval must have been in some cases a protracted one.
As far back as the year 1857,[207] I showed that the basic dykes of
Strath in Skye are of two ages; that one set was erupted before the
appearance of the "syenite" (granophyre) of that district, and was cut