CHAP. XXXV 
DYKES AND GEOLOGICAL STRUCTURE 
69 
the Liassie 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. 
I 11 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 reopened 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 w r est 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. 
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 
