GEOLOGY. 575 



a dry method, as in the craters of volcanoes or the chhn- 

 neys of smelting furnaces; or in a state of chemical precipi- 

 tation by a ivet method, as in the solutions of om^ labora- 

 tories." 



This hypothesis, as the author tells us, meets all objec- 

 tions, explaining at the same time the deposit and the 

 formation of the matrix which envelops it. 



Granite and porph3Ty must be classed among the 

 richest metalliferous rocks, but beds of ore are also met 

 with in the old transition rocks. It is in these that gold 

 and silver are found. ^ Ti\Q placers of California are often 

 formed merely by the detritus of granite rocks and schists, 

 filled Avith particles of gold, deposited on the beds of 

 ancient rivers Avhicli had borne them away. 



The rich family of precious stones, the diamond, ruby, 

 sapphire, and emerald, seem to owe their formation to the 

 same cause as the masses of metal. Volatilized in clefts of 

 the igneous rocks, these stones there turned into brilliant 

 crystalhzations — tears of nature, as M. Simonin calls them. 



CHAPTER III, 



PERIOD OF TRANSITION. 



It was at the period of transition that the dawn of life 

 l^egan to show itself. No animal could have lived upon 



' Mr. David Forbes divides the gold epochs into two; the older or grunite out- 

 burst, which occurred between the silurian and carboniferous periods ; and the 

 younger or diorite outburst, which passed through fossils of the post-oolitic forms, 

 and was possibly as late as the early chalk. — Tr. - ^ - - • 



73 



