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]
[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]).
[192] Western Islands, plate xvii.
[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.
