QUARTZ ROCK. 327 



C. An uniform rock, more or less laminar, with 

 a fracture intermediate between the granular and splin- 

 tery. 



This rock is apparently homogeneous, but is 

 proved to contain felspar by the white siliceo- 

 argillaceous crust which it assumes on weather- 

 ing. A similar rock occurs with the primary 

 sandstone, where it is also enumerated. 



D. The quartz and felspar both in distinct grains 

 and intermixed in various proportions* 



a. Highly compacted, and cemented by silica, 



or quartz, 



b. Gravelly, or sandy, sometimes loose. 



There are many different aspects of these va- 

 rieties, arising from the varying proportion of 

 the two ingredients. In some cases, the felspar 

 bears a very small one to the quartz ; in others, 

 but more rarely, it is the reverse. In some spe- 

 cimens, the felspar being white, it can scarcely 

 be detected but on weathering. Varieties of a 

 large grain are occasionally found, in which the 

 particles are large and have suffered little attrition 

 before their consolidation, and these have been 

 sometimes mistaken for granite. 



