ORIGIN, MATERIALS, &C. OF ROCKS. 198 



Besides these distinctions, the earths are sometimes 

 formed into separate minute bodies, or minerals, 

 which are again united so as to constitute rocks ; and 

 these may be, in themselves, either simple or com- 

 pound minerals. Sandstone offers an example of a 

 simple rock of this kind; simple in its chemical 

 nature, but an aggregate as to its general character. 

 Hornblende rock is an example of an analogous 

 aggregate, but one in which the integrant minerals 

 are chemical compounds. But there are differences 

 here, even in the mode of aggregation; which, in 

 some cases, result from the chemical interference of 

 a simultaneous crystallization, in others, from the 

 mere mechanical approximation of the parts, and 

 lastly, from the union of those two processes. Gra- 

 nular limestone is an example of the first; and in- 

 stances of the last are to be found in different varieties 

 of sandstone. 



In compound rocks, different kinds of minerals are 

 visibly united into a common mass; which thus pre- 

 sents a sort of uniformity throughout the whole, how- 

 ever the separate parts may differ. Such compounds 

 may consist of two or more minerals; and, within 

 certain limits, they seem to be ruled by laws as 

 general as the simpler rocks. These compounded 

 rocks vary, like the former, in being purely crystalline, 

 or otherwise; and as granite presents a familiar ex- 

 ample of the first, so quartz rock, and some of the 

 compound argillaceous schists, afford instances of the 

 other two. 



There is still another description of compound 

 rocks, to which the term conglomerate has been 

 applied. In these,, not only different minerals are 

 united in a mechanical, a mixed, or a chemical manner, 

 but fragments of former rocks, either simple or com- 

 VOL. i. o 



