458 Action of the Atmosphere upon newly -deepened Soil. 



The disintegration and changes brought about upon rocks and 

 minerals are due both to chemical and mechanical causes, the 

 latter being chiefly frosts, alternations of temperature, and attri- 

 tion or friction. As the most widely-dispersed and abundant 

 mineral body I shall commence with — 



Quartz, which, when it is quite pure, is silica, a compound of 

 the elements silicon and oxygen in the following proportions : — 



Silicon 48-04 



Oxygen 51*96 



100-00 



In this state it can be hardly affected by the chemical agency of 

 the atmosphere, being insoluble in acids except the fluoric; when 

 finely powdered it is partly soluble in solution of potash ; any 

 wearing down of it must therefore be principally due to attrition 

 and mechanical friction. Silica is, however, less abundant in 

 this pure state than in combination with some of the earthy or 

 alkaline bases forming silicates, in which it plays the part of a 

 weak acid ; and even when forming a mineral by itself it fre- 

 quently contains an appreciable amount of oxides of iron or 

 titanium, of lime, alumina, and other substances. Silica has been 

 estimated by De la Beche to constitute 45 per cent, of the mineral 

 crust of the globe {^Researches on Theoretical Geology, p. 8) ; it is 

 therefore worthy of some attention, the more so as it is a neces- 

 sary ingredient in many plants, and also essential to some 

 animals. Although in its pure crystalline state it is one of the 

 most refractory substances, it is yet in some of its combinations 

 clearly subject to the solvent effects of the atmosphere and rains, 

 as it is found in the stems of oar grain crops, and must therefore 

 enter their rootlets in a liquid form. It is well known as in solution 

 in some thermal springs. The Geysers of Iceland, it has been 

 seen, contain 0'50 of silica in 1000 parts of their waters, and 

 their siliceous deposits extend to half a mile in various directions, 

 with a thickness of more than 12 feet. (Sir Geo. M'Kenzie, 

 Travels in Iceland?) Professor Bunsen states that, in the waters 

 of the great Geyser the silica is dissolved in the water by alka- 

 line carbonates, and in the form of a hydrate ; is not precipitated 

 on cooling the water, and it is only on evaporation that it is 

 deposited in the form of a thin film on the moistened sides of 

 the vessel, where evaporation to dryness takes place, while the 

 fluid is not rendered turbid by hydrated silica until the concen- 

 tration is far advanced. Messrs. Paine and Way also, in their 

 paper on the silica strata of the lower chalk, have shown that 

 certain strata in England, at the base of the chalk formation, con- 

 tain a large proportion of soluble silica, varying from 5 to 70 

 per cent,, 60 per cent, being in some places quite common, asso- 



