174 



DRGEIKIE ON THE HISTORY OF VOLCANIC ACTION 



gianophyre sills thus represents, in my opinion, the terrestrial surface before the beginning 

 of the volcanic period. 



But there is abundant proof that though the intruded granophyre sills followed 

 generally this plane of separation, they did not rigidly adhere to it, but burrowed, as it 

 were, along lower horizons. Thus on the south-east front of Beinn a' Chairn, which forms 

 so fine an escarpment above the valley of Heast, the base of the granophyre, after creeping 

 upward across successive beds of limestone, sends out a narrow tongue into these strata, 



FlG. 58. — Section across tlie Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye. a, Jurassic sandstones and 

 shales; b, Jurassic dark brown sandy shales; c, sills of basalt, some bands highly cellular; c 1 , basalt-sill with veins of 

 felsite rising into it from the granophyre below; dd, intrusive sheets or sills of granophyre. 



and. continues its course a little higher up in the Lias. The same rock, after spreading 

 out into the broad flat tableland of Beinn a' Chairn (983 feet), rapidly contracts north- 

 eastwards into a narrow strip which forms the crest of the ridge, and at once suggests a 

 much weathered lava-stream. The resemblance to a coulee is heightened by the curious 

 thinning off of the rocks where the two streams emerge from the Heast lochs ; it looks as if 

 the igneous mass were a mere superficial ridge which had been cut down by erosion, so as 

 to expose the shales beneath it. But that the granophyre is really a sill becomes abun- 



Manse 



Tur Ruagh 



Millstream 



Fig. 59. — Section to show the connection of a Sill of Granophyre with its funnel of supply, Raasay. aa, Jurassic 



sandstones; b, granophyre. 



dantly clear at its eastern end, where we find that it consists of two separate sheets with 

 intervening Liassic shales. The structure of this interesting locality is shown in fig. 58. 

 In this instance also, there is evidence that the acid sills are younger than the basic, for 

 the upper sheet of granophyre sends up into the overlying dark basaltic rock narrow 

 vertical felsitic veins, a quarter of an inch to an inch in width, which being more 

 durable, stand out above the decomposable surface of the containing rock, and show 

 their quartz-blebs and felspar crystals on the weathered surface. 



It is not easy to determine where lay the vent or vents from which these granophyre 



