Vol. 52.] FOLIATED GRANITES IN EASTERN SUTHERLAND. 



641 



ends of a sill are sometimes seams of biotite whose flakes 

 have the peculiar oblique orientation which they possess in 

 the gneiss. 



There are no chilled edges ; the margins, indeed, are often fringed 

 with pegmatite ; while the thinnest seams are wholly pegmatite. 



In the granular gneiss of Strathy Point, the relations of the 

 granite are more intimate. The granites differ slightly in character, 

 as well, porphyritic felspars not being developed. There is an 

 absence of the parallel interbanding that is so marked at Portskerry, 

 the small sills being lenticular, like the large ones. The sills are 

 extremely irregular in form ; they sometimes run right across the 

 gneissic folia for 20 or 30 feet, inosculating, however, along the edges. 

 In and near the basic rocks of Armadale Bay there is an amazing 

 development of highly complex veining, though sills are also common. 

 But most important are the apparent passages between gneiss and 

 granite. Sometimes a sill will have distinct transgressive junctions 

 on one side, while on the other it is difficult to decide where the 

 boundary shall be drawn. 



The behaviour of the granular gneiss is singular. Throughout 

 the Point the coarse granitoid rock cuts it ; and yet here and there 

 it appears itself to give off' veins, and cut granulitic and basic rocks. 

 Indeed, this ' granular ' gneiss is the most perplexing of all the 

 types. Yery likely it includes rocks of various nature and origin. 



Eig. 2. — Part of base of great sill, Strathy Bay. 



A = Gneiss. B = Granite. 



There is a faint foliation in the granite, rudely parallel to that of the gneiss. 



On the cliffs of Glas Eilean and Boursa Cove, magnificent sections, 

 250 feet high, expose the internal structure of the great sills. They 

 are highly complex synthetic gneisses, consisting of foliated granite, 



