382 METAMORPHIC ROCKS OF SCOTLAND. 



micaceous minerals along the planes of stratification, has been exten- 

 sively developed in the volcanic group of St. Davids (as we have 

 observed in ashes in the Eifel) and in the overlying sedimentary 

 rocks. It has affected many fine tuffs, the paste of some coarse 

 tuffs, and some shales. Where it occurs the rock is usually pale 

 apple green to pearl grey, sometimes pink, with a lustre like silk 

 and a soapy surface. Under the microscope the base of the schist 

 is a felted aggregate of minute scales of nearly colourless mica. 

 Chlorite and some other minerals occur with it. The schists which 

 are inters tratified with the sandstones and shales above the conglome- 

 rate present almost identical characters. 1 



Gneiss in Scotland. Gneiss is abundant in Scotland, particularly 

 in the northern and western parts, and being exceedingly variable 

 in composition, is very often indistinguishable from mica schist, under 

 which head M. Bou6 preferred to class many of its varieties. 



Gneiss constitutes almost the whole mass of lona, Tiree, Coll, 

 Rona, and the Long Island of the Hebrides, and enters largely into 

 the composition of the Shetland Isles, which are in some measure to 

 be viewed as a prolongation of the Hebridean group, just as the 

 Orkneys appear to be an extension of the eastern rocks of Caithness. 

 Housa, Burra, Whalsay, Out Skerries, and Yell, and the western parts 

 of Fetlar and Unst, and part of the mainland of Shetland, are gneiss. 

 The remainder of the mainland is principally mica slate, and the tw<? 

 rocks are partially separated from each other by an interrupted deposit 

 of limestone. The gneiss is often porphyritic, as in Unst ; at Hagra- 

 sattervoe 2 it appears to contain masses of granite, as well as to be 

 traversed by veins of syenite and talcose granite. Kaolin is derived 

 from it in the Mainland and in Fetlar. Gneiss exists in the Orkneys, 

 around the granite of Stromness. 3 



In the Hebrides this rock changes often from the typical mixture 

 of quartz, felspar, and mica, by the substitution of talcose minerals 

 and hornblende for mica, by the omission of the quartz, and by the 

 inter-lamination of argillaceous schist. Some varieties are extremely 

 slaty, and suffer rapid decomposition; others approach nearer to 

 granite, and present rude and naked surfaces and precipitous faces, 

 with few brooks and little alluvium. The direction of the strike of 

 the strata in the Hebrides is north-west and south-east, but the 

 inclination of the beds is obscured by frequent contortions. These are 

 most frequent in the vicinity of the granitic veins which divide all the 

 gneiss rocks, except those which are associated with clay slate. The 

 drawing which MacCulloch gave of the contorted laminae of gneiss 

 and hornblende slate in connection with ramifying granite veins 

 near Cape Wrath seems to justify his views, for the laminae of gneiss 

 are often peculiarly bent or apparently dislocated along the line of 

 the veins, and sometimes masses of this rock are curiously enveloped 

 in their substance. 



1 A. Geikie, Q. J. G. S., vol. xxxix. p. 310. Where also see Prof. Geikie's 

 account of the metamorphism produced by contact of granite with stratified rocks. 



2 Hibbert, Edin. Phil. Jour., vol. ii. 3 Boue, " Gdologie de 1'Ecosse." 



