CHEMICAL INFLUENCE OF THE ATMOSPHERE; 275 



tion of loose materials by the infiltration of calcareous matter, or 

 by iron ; the destruction of such aggregations is no less remark* 

 able. Thus we find rocks which contain iron pyrites or sulpur- 

 et of iron, constantly crumbling to pieces^ The oxygen of the 

 atmosphere, and of the watery vapor with which it is charged., 

 combining with the sulphur, forms sulphate of iron or copperas, 

 the sulphuric acid of which, acts powerfully on the rock. Rocks 

 which contain potash and soda are also very liable to disintegra- 

 tion. Indeed, it is not a little remarkable that our supplies of 

 these valuable alkalies, particularly of the potash, are obtained 

 from the vegetable kingdom, salts of potash containing some 

 vegetable acid are of constant occurrence in plants, performing a 

 hidden but yet important part in their economy ; when these 

 plants are burned, the organic acids are destroyed* and the potash 

 remains in the state of carbonate, or united with carbonic acid. 

 The great natural depository of potash is the felspar of the gran- 

 itic and other unstratified rocks, combined with silica in an in- 

 soluble state. When these rocks disintegrate into soil, the alkali 

 acquires solubility. Rocks which readily absorb moisture are 

 liable to decay ; the red sandstone, or freestone, for example, 

 which ie destroyed very rapidly according to its porosity, by the 

 splitting of portions of the stone in consequence of the freezing 

 of the water. De la Beche, observes that, " rocks receive con- 

 siderable chemical modification by the percolation of water 

 through them. There is scarcely any spring-water, which does 

 not contain some mineral substances in solution, which it must 

 have procured in its passage through the rocks. Now, though 

 this'quantity may be small, when we regard the composition of 

 any particular spring-water, yet, when we consider the soluble 

 matter contained in the spring-waters of any given 1000 square 

 miles of country, and that this subtraction of matter from rocks 

 has been going on for ages, we may readily conceive that the 

 chemical change, may be greater than, at first sight we might 

 anticipate. We may also infer that the most soluble portions of 

 rock, have a constant tendency to be removed, when exposed not 

 only to direct atmospheric influences, but also to the percolation 

 of rain-water through them, so that most rocks would experience 



