224 ON THE ORIGIN, MATERIALS, COMPOSITION, 



known materials. In the intermediate cases, it has 

 produced gneiss and hornblende schist, or micaceous 

 schist ; differences similarly resulting from a nearly 

 common action on materials differently apportioned. 

 And further, as far as the aqueous or chemical rocks 

 contain the minerals of granite, or the original ma- 

 terials not in the simple forms of clay and sand, they 

 are indebted for them to that very igneous source, 

 however circuitously and remotely. Thus the laws 

 and the materials have been, throughout, simple, and, 

 consequently, the constancy of these variations could 

 not have been otherwise. I have already shown that 

 the rocks themselves are all connected in leading ana- 

 logies, or resemblances, throughout the whole series, 

 and are, virtually, reducible to a few, instead of pre- 

 senting that great variety which is popularly imagined. 

 The merely simple rocks require no remarks; as their 

 resemblance through the whole series, is such that 

 we often cannot distinguish them, except by position. 

 Now, with respect to the difficulties supposed to 

 consist in their alternation, I have equally shown that 

 this has been regulated by one leading law, and that 

 what remains may be accounted for by the operation 

 of another, equally simple. Hence the results ought 

 to be simple; and this they actually are, when justly 

 viewed. The separation of clay and sand, in conse- 

 quence of their different powers of gravitating in 

 water, is the leading law; and its modified effects are 

 the binary, but differently proportioned mixtures 

 which exist. This also applies to the mixtures of an- 

 tecedent minerals found in the strata. The case of 

 limestone from organic bodies is too obvious to re- 

 quire repetition. The second law, is the influence of 

 heat in changing the characters of different simple 

 deposits, or of differently compounded ones, whether 



