MODE III. LIMESTONE. 447 



structure. It is rarely metalliferous, but in Si- 

 beria it presents rich mines of copper, and in 

 South America veins of gold and silver. It is 

 remarkable, that in limestone the shells retain 

 their original form, while in clay slate they are 

 compressed ; a circumstance ascribed to the 

 great subsidence of the latter. Caverns are sel- 

 dom found except in limestone, the rock being 

 commonly eroded by a stream of water. 



The Wernerians regard limestone as of three Formations, 

 formations; the primary, the transitive, and the 

 floetz, flat or horizontal. The second often 

 contains coralites and madrepores; but Faujas 

 showed a madrepore in Carrara marble, which 

 is esteemed primitive. 



Limestone seldom or never occurs pure, there 

 being generally a small admixture of argil, 

 sometimes of silex, sometimes of iron. When 

 there is manganese it forms a more tenacious 

 mortar. 



STRUCTURE I. GRANULAR. 



Aspect 1. Common. The characters mostly 

 Correspond with those of marble ; but the mode 

 of combination must vary, as it is not capable of 

 so fine a polish. 



The colours are white, grey, black, reddish, and 



