of Devonshire a?id Cornwall. Ill 



em skirts, while they form a great part of the moimtains to the 

 south. The phenomenon of twisted and arched strata is much 

 more common on the northern side, where the marine fossil re-, 

 mains are generally calcareous, as on the southern they are siliceous. 



Among the small number of general facts, to which few excep- 

 tions have yet been found, in the structure of mountains, as stated by 

 Saussure, the folio win;^ appear very applicable to the mountains of 

 Cornwall. 



1. Mountain chains are generally directed from east to west, or 

 from north-east to south-west. 



2. The strata of mountains of newer formation generally 

 incline towards, and rest upon the niass of those which are more 

 ancient. 



3. Great quantities of debris^ in the form of detached blocks, of 

 breccias, of pudding-stones, of sandstones, and of sand, are found 

 either accumulated in the form of mountains, or scattered over the 

 borders of the Alps, and even extend into the plains. This obser- 

 vation applies equally to all primitive countries. 



General Observations on Grauwacke. 



I give the name grauwacke to a rock composed of separate siliceous 

 particles united by an argillaceous cement, with a little magnesia and 

 iron. It might be called a sandstone, having a base so fine that the 

 rock appears homogeneous; but the name grauwacke, made use of 

 by the school of Werner, has been generally adopted. 



Though formed by a mechanical precipitation, grauwacke is one 

 of the oldest of the secondary rocks. I have pever found in it any 



