00 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



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 would naturally be reopened by any great 

 earth-movements acting in their direction, and would afford ready channels for the ascent 

 of the lava. Yet it is curious that, even when their trend would have suited the line of 

 the dykes, they 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 E. and W. 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 Silurian rocks of the Southern Uplands. 



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. 

 We have not yet been able to ascertain definitely how much rock has been actually 

 displaced by it. But 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, we might confidently anticipate either an entire 

 cessation of the dykes, or at least a complete deflection of their course. 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 fracture on either side of the old fault. Two great dykes, about half a mile apart, 

 run in a direction a little S. of W. across the plain of Strathcarn. 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 S.W., 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. 



