172 DR GEIKTE ON THE HISTORY OF VOLCANIC ACTION 



2. Sills or Sheets. 



Not only have the acid rocks been protruded in huge bosses, they have also been 

 injected in sheets between the bedding planes of stratified rocks, between the surfaces of 

 the basalt-beds, and between the bottom of the plateau-basalts or of the gabbros and the 

 platform of older rock on which the volcanic series has been piled up. Every gradation of 

 size may be observed, from mere partings not more than an inch or two in thickness, up 

 to massive sheets, which now, owing to the removal of their original covering of rock by 

 denudation, form minor groups and ranges of hills. Where the sheets are numerous, they 

 are usually small in size ; where, on the other hand, they are few in number, they reach 

 their greatest dimensions. 



In Mull they are profusely abundant throughout the central mountainous tract 

 between Loch na Keal and Loch Spelve. If we ascend the slopes from the Sound of 

 Mull, for instance, we have not gone far before some of these sheets make their appearance. 

 They are usually dull granular quartz-porphyries, often only two or three feet in thick- 

 ness, and interposed between the beds of basalt that form the mass of the hills. Along 

 the crest of the ridge that stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh 

 they take a prominent place among the ledges of basalt, basalt-conglomerate, and dolerite. 

 The largest sheet in Mull is probably that which has thrust itself between the base of the 

 basalts and the underlying Jurassic strata and crystalline-schists on the shore of the 

 Sound of Mull at Craignure. The porphyry of this sheet is referred to by Zirkel as 

 only a finer-grained variety of the same quartziferous rock, with hornblende and ortho- 

 clase crystals, which in Skye breaks through the Lias.* On the south coast also, at the 

 base of the thick basalt series, similar porphyries have been injected into the underlying 

 strata ; and under the great gabbro mass of Ben Buy similar protrusions occur. But as 

 we retire from the mountainous tract into the undisturbed basalts of the plateau, these 

 acid intercalations gradually disappear. 



In the islands of Eigg and Eum, excellent examples occur of the tendency which 

 the sheets of porphyry or granophyre manifest to appear at or about the base of the 

 bedded basalts. I have already alluded to the boss or sheet at the north end of the 

 former island. A still more striking illustration occurs in Rum. All along the base of 

 the great mass of gabbro, protrusions of various kinds of acid rock have taken place. 

 The great mass of Orval, already described, is one of these. Below Barkeval and round 

 the foot of the hills to the south-east of that eminence an interrupted band of quartz- 

 porphyry may be traced, from which veins proceed into the gabbros and clolerites. 



But it is in Skye and Raasay that the intrusive sheets of the acid group of rocks 

 reach their chief development. They form a band or belt which, though not continuous, 

 can be traced round the east side of the main body of granophyre at a distance of from 

 a mile and a half to about three miles. Beginning near the Point of Suisnish, this belt 

 curves through the hilly ground for some five miles until it dies out on the slopes above 



* Zeitsch, Deutsch. Geol. Geselhch,, xxiii. p. 54. 



