56 GEOLOGY OF AGRICULTURE. 



in various forms, as fine marble, common lime-stone, 

 and chalk. It can be distinguished from almost any 

 other mineral by its effervescence (bubbling), if an 

 acid (vinegar, for instance) be poured upon it. 



Talc is a magnesian mineral, consisting of broad, 

 smooth laminae, or plates# It is soapy to the touch ; 

 admits light through it ; and is sometimes even trans- 

 parent. 



Chlorite and Soap-stone are little else than varieties 

 of the same mineral. 



ISerpentine is also a magnesian mineral, of a greenish 

 color, with spots resembling a serpent's skin — from 

 which its name. 



88. I have just quoted the opinion of a very emi- 

 nent geologist, that these seven minerals " constitute 

 the great mass of all known rocks," as also his opinion 

 that silica, alumina, lime, magnesia, potash, soda, 

 oxide of manganese, oxide of iron, water, and car- 

 bonic acid " constitute nearly all the binary compounds 

 of the accessible parts of the globe." I will now in- 

 vite attention to the opinions of the same writer with 

 regard to the proportions in which these last-mentioned 

 substances exist. 



89. " It has been calculated that oxygen constitutes 

 50 per cent, of the ponderable matter of the globe ; 

 and that its crust contains 45 pei cent, of silica, and at 

 least 10 per cent, of alumina. Potassa constitutes 

 nearly 7 per cent, of the unstratified rocks ; and enters 

 largely into the composition of some of the stratified 

 class. Soda forms nearly 6 per cent, of soAie basalts, 



