CONGLOMERATE ROCKS. 575 



the general and the local ; and these are com- 

 monly very distinct, both in their geological and 

 mineral characters. 



The general conglomerates form portions of 

 those mixed rocks, of which the origin is chiefly 

 mechanical ; being constituted from the larger 

 fragments of those substances, which, by a more 

 minute attrition, have furnished the materials of 

 the finer strata. Thus they necessarily contain 

 various rocks, simple or compound ; all of these 

 being obviously of a more antient date than 

 themselves. The red sandstones, and the argil- 

 laceous schists, contain examples of this division. 

 The materials are commonly united without any 

 distinct intervening cement of a crystalline nature, 

 particularly in the secondary class ; and the frag- 

 ments are almost always more or less rounded by 

 attrition. Though they sometimes form mere 

 portions of the finer rocks which they accom- 

 pany, they are also frequently found in extensive 

 strata of great depth. 



The local conglomerates are comparatively of 

 very limited extent. They generally constitute 

 superficial portions of some simple rock, and are 



