on the Chemistry of Geology. 23 



of moist carbonic acid gas on granite, as exemplified in the 

 volcanic districts of A uvergne. Basaltic rocks were likewise 

 prone to decomposition, partly in consequence of containing 

 felspar, and partly from the protoxide of iron of the augite or 

 hornblende which enters into their composition. The pas- 

 sage of the iron into a higher degree of oxidation was due 

 to atmospheric oxygen applied in a liquid state to the rock 

 through the medium of water. It was probable that carbonic 

 acid likewise co-operated ; — that, as in the rusting of iron, a 

 carbonate of the protoxide was first generated, which subse- 

 quently passed into the hydrated peroxide of iron. 



The rocks in which these changes occurred, underwent a 

 total alteration both in their mechanical state and in chemical 

 constitution. Their tenacity was so entirely destroyed, that 

 the slightest force, a shower or the breeze, sufficed to over- 

 come the cohesion of their particles. The alkali of the fel- 

 spar was entirely washed away, and an earthy mixture, com- 

 bined with water, remained. The ochreous tint of decom- 

 posed basalt and greenstone sufficiently indicated that their 

 iron had passed into a higher state of oxidation ; but felspar 

 often left a perfectly white earth, the small portions of iron 

 and manganese contained in the original rock having been 

 removed, probably in the state of carbonate, during the pro- 

 gress of disintegration. These changes constituted one of the 

 great sources of the alkalies present in springs and in the soil; 

 and the alkaline matter of the nitrates of potash and soda, 

 generated so abundantly in parts of India and America, had 

 probably the same origin. They likewise accounted for the 

 connexion observed between the agricultural character of the 

 soil of certain districts, and the rocks from which it was de- 

 rived. The decomposition of granitic rocks led to deposites of 

 clay and sand, which were too entirely free from each other 

 and from lime to be suitable for the growth of plants ; while 

 the earth derived from most basaltic rocks was an intimate 

 mixture of argillaceous, siliceous, and calcareous matter, in 

 proportions peculiarly favourable to vegetation. 



II. Deposites from Aqueous Solution of Substances commonly 

 considered insoluble. — The lecturer next discussed the second 

 branch of his subject, referring more especially to siliceous de- 

 positions; such as flint, calcedony, and rock crystal. Many 

 circumstances, he remarked, proved the fact that silica very 

 frequently existed in solution. Mineral waters, he said, com- 

 monly contained silica: — chemists, indeed, frequently over- 

 looked it in their analyses ; but when carefully sought for, it 

 might in most instances be detected. It was constantly con- 



