420 



GEOLOGY. 



The term basalt is sometimes used to embrace any of the very fine- 

 grained dark igneous rocks. In such cases, it covers the very fine- 

 grained dolerites, diorites, peridotites, etc. The term granite was used 

 originally for any coarse-grained crystalline rock, and there is a ten- 

 dency to revive this early use. In general descriptions, some of our 

 best petrographers call any coarsely crystalline rock (e.g., coarse-grained 

 syenites, diorites, gabbros, etc.) granite. The term granitoids may be 

 used with strict propriety to cover all rocks of this class. 



DERIVATION OF SECONDARY ROCKS. 



Rocks, though commonly made the symbol of the abiding, are sub- 

 ject to constant slow changes. Through these changes newer rocks 



Ve^-' 



Fig. 348. — Quartzitic breccia. About one-third natural size. (Photo, by Church.) 



have been derived from older ones, and still others in turn from these 

 derivatives, and so on in an endless chain. All derived rocks are con- 

 veniently termed secondary, though they may be several generations 

 removed from the primitive rocks, and even the primitive rocks, as we 

 now understand them, may be themselves derived. The ordinary 

 changes of rock are most active at or near the surface, and the processes 

 of such change have already been discussed in part under the titles 

 "Weathering'^ (pp. 54 and 110), " Erosion '' (pp. 119-123 and 342-349), 

 " Transportation " (pp. 115-119 and 354-355), and " Deposition '' (pp. 

 177-204 and 355-363). 



