On the Origin^ S^c, of Roch. 2^9 



aspect. Limestone presents an example of a rock in every respect 

 simple ; while basalts and clay slates, althougli simple as rocks, 

 are chemical compounds and mechanical mixtures. 



Besides these distinctions, tlie 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 compound 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 dif- 

 ferences here, even in the mode of aggregation : which, in some 

 cases, results from the chemical interference of a simultaneous 

 crystallization ; in others, from the mere mechanical aggregation 

 of the parts ; and lastly, from the union of those two processes. 

 Granular limestone is an example of the first, and instances 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 presents a sort of uni- 

 formity throughout the whole, however 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 ciystalline or otherwise ; and as 

 granite presents a familiar example 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 compound, also enter into their structure. Such fragments 

 vary in size, from the most minute visible particles, to others of 

 many pounds, or even hundreds of pounds weight ; and these 

 rocks offer, in consequence, numerous varieties, which are fully 

 treated of in the author's Geological Classification of Rocks ; 



