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.
[Illustration:
Fig. 252.--Basalt Veins traversing bedded dolerites, Kildonan, Eigg.
]
[Illustration:
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.]
[Illustration:
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
off by the latter rock; and that the other arose after the "syenite"
which it intersected. Recent re-examination has enabled me to confirm
and extend this observation. The younger series which traverses the
granophyre is much less numerous than the older series in the same
districts. In Chapter xlvi., where the relations of the granophyres
to other members of the volcanic series will be discussed, further
details will be given from that region of Skye to demonstrate that
there is a pre-granophyre and a post-granophyre series of basic dykes.
As a good illustration of the younger series I may refer to the way
in which these rocks make their appearance in the island group of St.
Kilda, where both the gabbros and granophyres of the Tertiary volcanic
series are characteristically developed. Numerous dykes traverse both
these rocks. Those in the gabbro are more abundant than those in the
granophyre--a circumstance which is exactly paralleled among the basic
and acid bosses of Skye. It is not improbable that in these remote
islands a similar difference in age and in petrographical character
may be made out between two series of dykes, one older and the other
younger than the granophyre. There is ample proof, at all events, of a
post-granophyre series.
[Footnote 207: _Quart. Jour. Geol. Soc._ vol. xiv. p. 16.]
[Illustration: Fig. 258.--Basalt-veins traversing granophyre, St.
Kilda.]
The pale colour of the precipices in which the St. Kilda granophyre
plunges into the sea gives special prominence to the dark ribbon-like
streaks which mark the course of basalt-dykes through that rock.
Moreover the greater liability of the material of the dykes to decay
causes them to weather into long lines of notch or recess. Four or five
such dykes follow each other in nearly parallel bands, which slant
upward from the sea-level on the eastern face of the hill Conacher to a
height of several hundred feet.[208] (Fig. 258, see also Fig. 367.)
[Footnote 208: This relation of the later dykes to the granophyre was
observed here by Macculloch (_Western Isles_, vol. ii. p. 55).]
The acid eruptions of the Inner Hebrides are marked by so varied a
series of rocks, and so complex a geological structure, that they may,
with some confidence, be regarded as having occupied a considerable
interval of geological time. Yet we find that this prolonged episode
in the volcanic history was both preceded and followed by the
extravasation of basic dykes.
Reference has already been made to recent observations by Mr. Harker,
who, in mapping the Strath district of Skye for the Geological Survey,
has not only confirmed the generalization as to the existence of a
series of dykes earlier, and another later, than the great granophyre
protrusions of the Inner Hebrides, but has made some progress towards
the detection of a means of distinguishing the two series even where no
direct test of their relative age may be available. He thinks that the
general habit and petrographical characters of the dykes may on further
investigation be found to afford a sufficiently reliable basis for
discrimination. He finds that where the relative ages of the dykes with
reference to the granophyre can be fixed, the earlier or pre-granophyre
series is without exception basic. It consists of fine-textured basalts
or diabases, without any conspicuous porphyritic crystals. Its dykes
are less regular and persistent in their bearing than those of the
later series; have frequently a considerable hade, even as much as 45°,
and often show chilled edges with tachylitic selvages. In Skye many of
these earlier dykes may be connected with the gabbro. They appear to
be more basic and to have a higher specific gravity than those of the
later series which most resemble them.
The later or post-granophyre dykes include several types, the relative
ages of which are not yet definitely fixed. They run in straight
parallel lines, and thus seldom intersect each other. They are
generally vertical or highly inclined, and are much more frequently
characterized by amygdaloiclal structure than the earlier series.
Mr. Harker distinguishes the following varieties among them: (_a_)
Quartz-felsites and other acid rocks; these are not very common.
(_b_) Pitchstones and various spherulitic and variolitic rocks: the
actual pitchstones observed are comparatively few in number, but it is
certain that some of spherulitic varieties are devitrified pitchstones.
(_c_) Basic rocks, not conspicuously porphyritic and less decidedly
basic than the dykes of the pre-granophyre series; most of the later
groups come into this or the next group, (_d_) Porphyritic basic dykes
not infrequently carrying inclusions of gabbro, granophyre or other
rocks. The porphyritic felspars seem to be in great part of foreign
derivation, and the same is certainly true of the augite which
occasionally accompanies them and of the quartz that appears in some
examples.[209]
[Footnote 209: Annual Report of the Director-General of the Geological
Survey in Report of Science and Art Department for 1895.]
In the Carlingford district of the North-east of Ireland, similar
evidence has been obtained that one series of dykes preceded and
another followed the protrusion of the granites and granophyre which
are in all probability geologically coeval with the acid bosses of the
Inner Hebrides. The distinction was observed and mapped by Mr. Traill
for the Geological Survey. Professor Sollas in recently confirming
these observations has not noticed any striking difference between the
pre-granite and post-granite dykes, the whole appearing to consist of
the same coarsely porphyritic material.[210]
[Footnote 210: See Sheets 59, 60, and 71 of the Geological Survey Map of
Ireland; Professor Sollas, _Trans. Roy. Irish Acad._ vol. xxx. (1894),
p. 477; and Annual Report of the Director-General of the Geological
Survey for 1895.]
While the eruption of the granophyre bosses furnishes proof that the
dykes are not all of the same age, other evidence may be gathered
to show how much older some of the dykes are than the youngest
lava-streams in the volcanic history of Tertiary time in Britain.
The Scuir of Eigg, to which fuller reference will be made in Chapter
xxxviii., is formed of a mass of pitchstone, which has filled up an
ancient valley eroded out of the terraced basalts of the plateaux. At
both ends of the ridge, these basalts are seen to be traversed by dykes
that are abruptly cut off by the shingle of the old river-bed which
the pitchstone has occupied (Figs. 279, 282). It is thus evident that,
though these dykes are younger than the plateau-basalts, they are much
older than the excavation of the valley out of these basalts, and still
older than the eruption of pitchstone. The latter rock probably belongs
to the close of the period of lava-eruptions. The enormous denudation
of the basalt-plateaux after the injection of the dykes and before the
outflow of the pitchstone affords a convincing proof of the vastness of
the interval between the eruption of the two kinds of rock.[211]
[Footnote 211: _Quart. Jour. Geol. Soc._ xiv. p. 1.]
It is thus demonstrable that the dykes which in Britain form part of
the great Tertiary volcanic series, were not all produced at one epoch,
but belong to at least two (and probably to many more) episodes in one
long volcanic history. As they rise through every member of that series
of rocks (save the pitchstones), some of them must be among the latest
records of the prolonged volcanic activity. But, on the other hand,
some probably go back to the very beginning of the Tertiary volcanic
period.
20. ORIGIN AND HISTORY OF THE DYKES
Reference has already been made to the doubt expressed by Macculloch
whether the dykes in Skye had been filled in from above or from below.
That the dykes of the country as a whole were supplied from above,
was the view entertained and enforced by Boué. He introduces the
subject with the following remarks:--"Scotland is renowned for the
number of its basaltic veins, which gave Hutton his ideas regarding
the injection of lava from below; but, as the greatest genius is not
infallible, and as volcanic countries present us with examples of such
veins arising evidently from accidental fissures that were filled
up by currents of lava which moved over them, and as the Scottish
instances are of the same kind, we regard it as infinitely probable
that all these veins have been formed in the same way notwithstanding
the enormous denudation which this supposition involves; and that only
rarely do cases occur where they have been filled laterally or in some
other irregular manner."[212] I need not say that this view, which,
except among Wernerians, had never many supporters, has long ago been
abandoned and forgotten. There is no further question that the molten
material came from below.
[Footnote 212: _Essai Géologique sur l'Écosse_, p. 272.]
1. In discussing the history of the dykes, we are first confronted
with the problem of the formation of the fissures up which the molten
material rose. From what has been said above regarding the usual want
of relation between dykes and the nature and arrangements of the rocks
which they traverse, it is, I think, manifest that the fissures could
not have been caused by any superficial action, such as that which
produces cracks of the ground during earthquake-shocks. The fact that
they traverse rocks of the most extreme diversities of elasticity,
structure, and resistance, and yet maintain the same persistent trend
through them all, shows that they originated far below the limits to
which the known rocks of the surface descend. We have seen that in the
case of the Cleveland dyke, the fissure can be proved to be at least
some three miles deep. But the seat of the origin of the rents no doubt
lay much deeper down within the earth's crust.
It is also evident that the cause which gave rise to these abundant
fissures must have been quite distinct from the movements that produced
the prevalent strike and the main faults of this country. From early
geological time, as is well known, the movements of the earth's crust
beneath the area of Britain, have been directed in such a manner as
to give the different stratified formations a general north-east and
south-west strike, and to dislocate them by great faults with the same
average trend. But the fissures of the Tertiary dykes run obliquely
and even at a right angle across this prevalent older series of lines
and are distinct from any other architectonic feature in the geology
of the country. They did not arise therefore by a mere renewal of some
previous order of disturbances, but were brought about by a new set of
movements to which it is difficult to find any parallel in the earlier
records of the region.[213]
[Footnote 213: The only other known example of such a dyke-structure in
Britain is that of the Pre-Cambrian series of dykes in the Lewisian
gneiss of Sutherland, described in Chapter viii.]
We have further to remember that the fissures were not produced merely
by one great disturbance. The evidence of the dykes proves beyond
question that some of them are earlier than others, and hence that
the cause to which the fissures owed their origin came into operation
repeatedly during the protracted Tertiary volcanic period. One of
the most instructive lessons in this respect is furnished by the huge
eruptive masses of gabbro and granitoid rocks in Skye. These materials
have been erupted through the plateau-basalts. The granitoid bosses
are the younger protrusions, for they send veins into the gabbros;
but their appearance was later than that of some of the dykes and
older than that of others. Nevertheless, the youngest dykes generally
maintain the usual north-westerly trend across the thickest masses of
the granophyre. Thus we perceive that, even after the extrusion of
thousands of feet of such solid crystalline igneous rocks, covering
areas of many square miles, the fissuring of the ground was renewed,
and rents were opened through these new piles of material. From the
evidence of the dykes also we learn that some fissures were repeatedly
re-opened and admitted a new ascent of molten magma between their
walls. The general direction of the fissures remained from first to
last tolerably uniform. Here and there indeed, where one set of dykes
traverses another, as in Skye and the basin of the Clyde, we meet with
proofs of a deviation from the normal trend. But it is remarkable that
dykes which pierce the latest eruptive bosses of the Inner Hebrides
rose in fissures that were opened in the normal north-westerly line
through these great protrusions of basic and acid rock.
Such a gigantic system of parallel fissures points to great horizontal
tension of the terrestrial crust over the area in which they are
developed. Hopkins, many years ago, discussed from the mathematical
side the cause of the production of such fissures.[214] He assumed the
existence of some elevatory force acting under considerable areas
of the earth's crust at any assignable depth, either with uniform
intensity at every point or with a somewhat greater intensity at
particular points. He did not assign to this force any definite origin,
but supposed it "to act upon the lower surface of the uplifted mass
through the medium of some fluid, which may be conceived to be an
elastic vapour, or, in other cases, a mass of matter in a state of
fusion from heat."[215] He showed that such an upheaving force would
produce in the affected territory a system of parallel longitudinal
fissures, which, when not far distant from each other, could only have
been formed simultaneously, and not successively; that each fissure
would begin not at the surface but at some depth below it, and would
be propagated with great velocity; that there would be more fissures
at greater than at lesser depths, many of them never reaching the
surface; that they would be of approximately uniform width, the mean
width tending to increase downwards; that continued elevation might
increase these fissures, but that new fissures in the same direction
would not arise in the separated blocks which would now be more or less
independent of each other; that subsequent subsidences would give rise
to transverse fissures, and by allowing the separated blocks to settle
down would cause irregularities in the width of the great parallel
fissures. He considered also the problem presented by those cases where
the ruptures of the terrestrial crust have been filled with igneous
matter, and now appear as dykes. "The results above obtained," he
says, "will manifestly hold equally, whether we suppose the uplifted
mass acted upon immediately through the medium of an elastic vapour
or by matter in a state of fusion in immediate contact with its
lower surface. In the latter case, however, this fused matter will
necessarily ascend into the fissures, and if maintained there till it
cools and solidifies, will present such phenomena as we now recognize
in dykes and veins of trap."
[Footnote 214: _Cambridge Phil. Trans._ vi. (1835), p. 1.]
[Footnote 215: _Ibid._ p. 10.]
The existence of a vast lake or reservoir of molten rock under the
fissure-region of Britain is demonstrated by the dykes. But, if we
inquire further what terrestrial operation led to the uprise of so vast
a body of lava towards the surface in older Tertiary time, we find that
as yet no satisfactory answer can be given.
2. In some districts the dykes can be connected with the gabbros which
occur as intrusive sills and irregular bosses in the basalt-plateaux
and among older rocks. The gabbros, however, are traversed by still
later dykes, which must then be independent of any visible mass of
these rocks. The connection of dykes with the gabbros is what we
might naturally expect to find, if the more coarsely crystalline rock
represents portions of the basic magma which consolidated at some
depth below the surface. If we could penetrate deep enough, it is not
improbable that the dykes might be found in large measure to shade
downward into vast bodies of gabbro. Such a relation has been observed
in the Yellowstone district, where Mr. Iddings has noticed that the
centre toward which the dykes of the Old Crandale volcano converge is a
large mass of granular gabbro, passing into diorite, the dykes becoming
rapidly coarser in grain as they approach the gabbro-core.[216]
[Footnote 216: _Journ. Geol._ i. (1893), p. 608.]
3. The rise of molten rock in thousands of fissures over so wide a
region is to my mind by far the most wonderful feature in the history
of volcanic action in Britain. The great plateaux of basalt, and the
mountainous bosses of rock by which they have been disrupted, are
undoubtedly the most obvious memorials of Tertiary volcanism. But,
after all, they are merely fragments restricted to limited districts.
The dykes, however, reveal to us the extraordinary fact that, at a
period so recent as older Tertiary time, there lay underneath the area
of Britain a reservoir or series of reservoirs of lava, the united
extent of which must have exceeded 40,000 square miles.
That the material of the dykes rose in general directly from below, and
was not, except locally, injected laterally along the open fissures,
may be inferred, although proof of such lateral injection on a small
scale may here and there be detected. The narrowness of the rents, and
their enormous relative length, make it physically impossible that
molten rock could have moved along them for more than short distances.
The usual homogeneous character of the dyke-rocks, the remarkable
scarcity of any broken-up consolidated fragments of them immersed in
a matrix of different grain, the general uniformity of composition
and structure from one end of a long dyke to another, the spherical
form of the amygdales, the usual paucity of fragments from the fissure
walls--all point to a quiet welling of the lava upward. Over the whole
of the region traversed by the dykes, from the hills of Yorkshire and
Lancashire to the remotest Hebrides, molten rock must have lain at a
depth, which, in one case, we know to have exceeded three miles, and
which was probably everywhere considerably greater than that limit.
Forced upwards, partly perhaps by pressure due to terrestrial
contraction and partly by the enormous expansive force of the gases
and vapours absorbed within it, the lava rose in thousands of fissures
that had been opened for it in the solid overlying crust. That in
most cases its ascent terminated short of the surface of the ground
may reasonably be inferred. At least, we know, that many dykes do
not reach the present surface, and that those which do have shared
in the enormous denudation of the surrounding country. That even in
the same dyke the lava rose hundreds of feet higher at one place than
at another is abundantly proved. When, however, we consider the vast
number of dykes that now come to the light of day, and reflect that
the visible portions of some of them differ more than 3000 feet from
each other in altitude, we can hardly escape the conviction that it
would be incredible that nowhere should the lava have flowed out at
the surface. Subsequent denudation has undoubtedly removed a great
thickness of rock from what was the surface of the ground during older
Tertiary time, and hundreds of dykes are now exposed that doubtless
originally lay deeply buried beneath the overlying part of the earth's
crust through which they failed to rise. But some relics, at least, of
the outflow of lava might be expected to have survived. I believe that
such relics remain to us in the great basalt-plateaux of Antrim and
the Inner Hebrides. These deep piles of almost horizontal sheets of
basalt, emanating from no great central volcanoes, but with evidence
of many local vents, appear to me to have proceeded in large measure
from dykes which, communicating with the surface of the ground, allowed
the molten material to flow out in successive streams with occasional
accompaniments of fragmentary ejections.[217] The structure of the
basalt-plateaux, and their mode of origin, will form the subject of the
next division of this volume.
[Footnote 217: It is interesting to note that in the great paper on
Physical Geology already cited, Hopkins considered the question of the
outflow of lava from the fissures which he discussed. "If the quantity
of fluid matter forced into these fissures," he says, "be more than
they can contain, it will, of course, be ejected over the surface; and
if this ejection take place from a considerable number of fissures, and
over a tolerably even surface, it is easy to conceive the formation
of a bed of the ejected matter of moderate and tolerably uniform
thickness, and of any extent" (_op. cit._ p. 71).]
We can hardly suppose that the lava flowed out only in the western
region of the existing plateaux. Probably it was most frequently
emitted and accumulated to the greatest depth in that area. But over
the centre of Scotland and North of England there may well have been
many places where dykes actually communicated with the outer air, and
allowed their molten material to stream over the surrounding country,
either from open fissures or from vents that rose along these. The
disappearance of such outflows need cause no surprise, when we consider
the extent of the denudation which many dykes demonstrate. I have
elsewhere shown that all over Scotland there is abundant proof that
hundreds and even thousands of feet of rock have been removed from
parts of the surface of the land since the time of the uprise of the
dykes.[218] The evidence of this denudation is singularly striking in
such districts as that of Loch Lomond, where the difference of level
between the outcrop of the dykes on the crest of the ridges and in
the bottom of the valleys exceeds 3000 feet. It is quite obvious, for
example, that had the deep hollow of Loch Lomond lain, as it now does,
in the pathway of these dykes, the molten rock, instead of ascending
to the summits of the hills, would have burst out on the floor of
the valley. We are, therefore, forced to admit that a deep glen and
lake-basin have been in great measure hollowed out since the time of
the dykes. If a depth of many hundreds of feet of hard crystalline
schists could have been removed in the interval, there need be no
difficulty in understanding that by the same process of waste, many
sheets of solid basalt may have been gradually stripped off the face of
Central Scotland and Northern England.
[Footnote 218: _Scenery of Scotland_, 2nd edit. (1887), p. 149. But see
the remarks already made (p. 150) on the curious coincidence sometimes
observable between the upper limit of a dyke and the overlying
inequalities of surface.]
The association of fissures and dykes with the accumulation of thick
and extensive volcanic plateaux, over so wide a region of North-western
Europe as from Antrim to the North of Iceland, finds its parallel in
different parts of the world. One of the closest analogies presents
itself among the Ghauts of the Bombay Presidency, where vast basaltic
sheets, probably of Cretaceous age, display topographical and
structural features closely similar to those of the Tertiary volcanic
plateaux of the British Isles. The dykes connected with these Indian
basaltic outflows correspond almost exactly in their general character
and stratigraphical relations to those of this country. They occur in
great numbers, rising through every rock in the district up to the
crests of the Ghauts, 4000 feet above the sea. They vary from 1 or 2
to 10, 20, 40, and even occasionally 100 or 150 feet in width, and are
often many miles in length. They observe a general parallelism in one
average direction, and show no perceptible difference in character even
when traced up to elevations of 3000 and 4000 feet.[219]
[Footnote 219: Mr. G. T. Clark, _Quart. Journ. Geol. Soc._ xxv. (1869) p.
163. For remarks on the connection of dykes with superficial lavas, see
_postea_, p. 268.]
Thousands of square miles in the Western States and Territories of the
American Union have been similarly flooded with basic lavas. Denudation
has not yet advanced far enough to lay bare much of the platform on
which these lavas rest. But the dykes that traverse the rocks outside
of the lava-deserts afford an example of the structure which will
ultimately be revealed when the wide and continuous basalt-plains shall
have been trenched by innumerable valleys and reduced to fragmentary
plateaux with lofty escarpments (p. 267).
It is to the modern eruptions of Iceland, however, that we turn for
the completest illustration of the phenomena connected with dykes
and fissures. An account of these eruptions will therefore be given
in Chapter xl. as an explanation of the history of the Tertiary
basalt-plateaux of Britain.