GEOLOGY. 543 



tides of gold, deposited on the beds of ancient rivers which 

 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 

 crystallizations, tears of nature, as M. Sirnonin calls them. 



CHAPTER III. 



TRANSITION PERIOD. 



IT was at the transition period that the dawn of life be- 

 gan to show itself. No animal could have lived upon the 

 burning surface of the globe during the plutonic period. 

 But so soon as it was sufficiently cooled down to admit of 

 living creatures appearing on it, we see them at once enter 

 upon the scene. This is characteristic of this epoch. 



The earth, imperfectly cooled down, still maintained a 

 very high temperature, and this temperature was the same 

 from one pole to another ; the sun only brought with it 

 useless supplementary heat. There were neither seasons 

 nor climates ; the torrid zone and the polar regions were 

 peopled with the same plants and animals ; their fossilized 

 remains are identical, whether found beneath the ice of 

 Spitzbergen or in the rocks of burning tropical countries. 



SILURIAN PERIOD. This name is derived from that of a 

 part of England inhabited by the ancient Silures, and is 

 given to the strata of this epoch because they have been 

 chiefly studied there. 



