the Chemistry Gf>Geolog^.x s v 249 



alteration both in their mecbanica I state and chemical constitution. 

 Their tenacity Jiwwhaoiyeetirely .destroyed, that the sjiglitest 

 force, a shower, or- the breeze, HufTiced to ovtTcome thecohesioft: 

 of their particles. The alkali of the i"eU})ar was entirely washed^ 

 away, and an earthy mixture combined witli water remained* 

 The ochreous tint of decomposed basalt and greenstone, 8uffi« 

 ciently indicated that their iron luid passed into a higher stajtf* 

 of oxidation ; but felspar often left a perfectly white earth, tbc^t 

 small portions of iron and manganese contained in the origim^l) 

 rock having been removed, probably in the state of carbonate, 

 during the progress of disintegration. These changes consti- 

 tuted one of the great sources of the alkalies present in springs 

 and in the soil ; and the alkaline matter of the nitrates of pot- 

 ash 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 agricultu- 

 ral character of the soil of certain districts, and the rocks from 

 which it was derived. 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 mat-?'^ 

 t^in proportions peculiarly favourable to vegetation. i,iOij uinod 



^p Deposites Jrom Aqueous Solution of Substances commot^^, 

 ly considered insoluble, — The lecturer next discussed the sQia 

 cond branch of this subject, referring more especially to siliw 

 ceous depositions, such as flint, calcedony, and rock.crystal»'> 

 Mapy circumstances, he remarked, proved the fact that silica 

 very frequently existed in solution. Mineral waters, he said, 

 commonly contained silica. Chemists, indeed, frequently over- 

 looked} tH Will their analyses; but whea carefully sought for, ilfj 

 might in most instances be detected. It was constantly cob»£ 

 tained ip, tlje sap of certain plants, if not in all. For it wa»T 

 shown by the late Sir Humphry Davy, that silex is contained 

 in grass, and in the epidermis of reeds, corn, canes, and 

 of hollow plants in general. The existence of silex in the 

 sap of the bamboo, was not only attested by its flinty epider- 



