280 SILICEOUS SCHIST. 



through which they pass into the ordinary schists of 

 which they form a part. The connexions of this kind 

 of siliceous schist being regulated by those of the ori- 

 ginal substance, require no further notice. But, in 

 similar circumstances, the finer varieties of micaceous 

 schist become also occasionally indurated ; and as some 

 geologists have given to these the name of siliceous 

 schist, it must be recollected that such varieties will be 

 found in the same associations as the unaltered rock 

 from which they have been derived. 



As I have, on every occasion, preferred the foun- 

 tain head of all authority to the quoting of authors, I* 

 shall here refer to some examples in Scotland, in con- 

 firmation of these views. They may be seen in Arran, 

 on the skirts of Cruachan, and in Kirkcudbright and 

 Aberdeenshires. But the promontory of Busta in 

 Shetland presents the most perfect illustration which 

 can be desired. In this spot, a body of slate is tra- 

 versed by granite veins originating in a neighbouring 

 mass ; and the progress of the change, with the rela- 

 tion which that bears to the proximity of the granite, 

 is every where distinctly visible. In many places, this 

 has insinuated itself in a direction parallel to the la- 

 rninse of the schist, so that detached specimens appear 

 to consist of alternate layers of each rock ; while 

 there is also a gradation from the common argilla- 

 ceous schist, through the siliceous, to a hornblende 

 schist, as formerly noticed under that head. 



The siliceous schists which occur among the secon- 

 dary rocks, present somewhat greater varieties of cha- 

 racter; with certain essential differences, bespeaking 

 their origin even more forcibly than their connexions 

 in the case of the primary. They accompany beds of 

 shale which may alternate with limestone or sand- 

 stone, and can easily be traced into them through a 



