QUARTZ ROCK. 177 



yards are rare. They are frequently divided, like the 

 argillaceous schists, by joints; thus breaking into 

 rectangular and rhomboidal fragments. Hence, where 

 exposed to the force of the sea, they are often under- 

 ruined, so as to form caverns; of which Jura and Isla 

 present magnificent examples: yet eclipsed by the 

 superior splendour and number of those at Loch Eri- 

 bol in Sutherland. The total thickness of the deposits 

 may easily be estimated in various parts of Scotland; 

 as in Ben Gloe, in Sutherland, and in the islands already 

 mentioned. As it occupies the whole depth of Jura, 

 from the sea to the summit of Ben-an-oir, the thick- 

 ness of this collection of strata cannot be less than 

 2200 feet, while it may be much more: nor is it much 

 less in Canasp, on the west coat of Sutherland. If it 

 is thus a very important member of the primary strata, 

 in depth, it is no less so in extent ; occupying very 

 considerable spaces in Shetland, Sutherland, Ross, 

 Banff, Perthshire, and in many other districts of Scot- 

 land besides these Argyllshire islands- 



Quartz rock alternates with every one of the pri- 

 mary strata, and, like all of these, has therefore no 

 fixed place in the series. I know not that it has even 

 a predominant one ; but it is most abundant in the 

 gneiss districts, though the most continuously exten- 

 sive mass, that of Isla, Jura, and Scarba, is chiefly 

 associated with argillaceous schist. Its alternations 

 with gneiss are sometimes on a large scale ; at others, 

 the beds are so thin and so rare in proportion to those 

 of the accompanying rock, that they would be called 

 subordinate by those who delight in that term. In 

 this case, the variety is generally that which contains 

 felspar ; often passing into the gneiss by acquiring the 

 other necessary ingredients of that rock. Its alterna- 

 tions with micaceous schist are the next in frequency ; 

 VOL. ij, *j 



