192 



CHAP. XII. 



On the Origin, Materials, Composition, and 



Analogies, of Rocks. 







IF it is the first error of the observer to see, like 

 the miner, but a very limited number of rocks in the 

 system of nature, it is not long before he falls into 

 one the very reverse; creating for himself permanent 

 distinctions from every incidental variety which comes 

 under his notice. Time, however, speedily corrects 

 this error, and teaches him, that however the aspects 

 of rocks may be multiplied, Nature has limited these 

 productions by a very confined set of general and 

 constant characters. 



Of the Constituents of Rocks. 



A small number only of the Earths which Che- 

 mistry has discovered, forms the materials of all the 

 rocks; united, in some cases, with alkalies and with 

 certain metallic oxydes. In some, a single earth is 

 found; in others, two or more exist; and these are 

 either mechanically mixed, or united by the laws of 

 chemical affinity. Thus are formed those rocks 

 which are considered simple; simplicity, as applied 

 to rocks, meaning simplicity of aspect. Limestone 

 presents an example of a rock, in every respect sim- 

 ple; while basalts and clay slates, though simple as 

 rocks, are chemical compounds or mechanical mix- 

 tures. 



