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.
[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]
[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."
[214] Cambridge Phil. Trans. vi. (1835), p. 1.
[215] Ibid. p. 10.