37G 



B. N. PEACH AND J HORNE ON THE 



by referring to the table of chemical analyses appended to this paper. Nay 

 farther, it may be noted that the rhyolites are, so far as their chemical com- 

 position is concerned, closely allied to the great intrusive sheets. Taking this 

 important point into consideration, as well as their behaviour in the field, it is 

 evident that the former are merely offshoots from the latter. We are inclined 

 to believe that the rhyolites formed ducts leading in all probability to the sur- 

 face, and that in these subterranean fissures the banded structure was originally 

 developed. Had the molten matter reached the surface, it would have been 

 ejected as highly acidic and glassy lava. When we come to describe the 

 microscopic characters of these rocks, it will be seen that they have undergone 

 considerable devitrification. 



But there is another series of dykes of a highly basic type, which have 

 been injected through the great sheets of granite and quartz-felsite, and are 

 therefore of a later elate. This material is also met with in the form of bosses 



Fig. 13. — .Diabase dykes traversing pink granite, north shore of Kooeness Voe, Northmavine. 



piercing the granitoid masses, as in the case of Skeld Hill in Sandsting. These 

 rocks consist of diabase of a fine-grained character when occurring in the form 

 of dykes, but very granular and coarsely crystalline when developed in bosses. 

 The dykes are prominently developed on the Rooeness plateau, where they 

 have been noted by Hibbert and Dr Heddle, on the shores of Kooeness Voe, 

 and on the cliffs of Meikle Rooe. At these localities they have a north an<J 

 south trend ; and, owing to their dark colour, they form a striking contrast to 

 the pink granite in which they occur. Sometimes, owing to rapid decay, they 

 leave great clefts, indicating their course ; sometimes they project above 

 the general level of the acidic rock on either side. In the Sandsting granite 

 mass we likewise noted several dykes of the same rock. In the burn near 

 Garder House, close to Selie Voe, several examples occur. The granite and 



