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.
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.