310 GEOLOGY. 



rocks, or at small depths below the present surface. 

 Human bones are never found, except among those 

 of animal species now living, and in situations which 

 show that they have been, comparatively speaking, 

 recently deposited. 



TJie Chemical and Mechanical Deposits of Werner. 



By mechanical deposits are understood sand, gra- 

 vel, and whatever bears the marks of fracture and 

 attrition. 



By chemical deposits are understood, such as are 

 regularly crystallized, or which have a tendency to 

 crystallization, and in which the action of mechanical 

 causes cannot be traced. 



The primitive rocks contain none but chemical 

 deposits, and are entirely composed of them. 



The intermediate rocks (or transition) contain a 

 mixture of both, and also some vestiges of organized 

 bodies. 



The secondary rocks consist almost entirely of 

 mechanical deposits, or of the remains of such bo- 

 dies, with little of the chemical. 



The Constituents of Primitive Rocks. 



The principal constituent parts of primitive rocks 

 are quartz, felspar, mica, limestone, and hornblende, 

 which minerals compose nearly the whole mass of 

 the upper coat of the earth. Of these the quartz, 

 mica, and hornblende, occur together in various 

 states of aggregation, while the limestone occurs in 

 beds of greater or lesser extent. 



Characters of Rocks. 



The primary characters by which alone the nature 

 of a rock or mineral mass, simple or compound, can be 



