REPORT OF THE CHEMIST. 15d 



WEATHERING OF THE ROCKS AND FORLIATION OF THE SOIL. 



All exposed rocks break up in course of time under tbe continued ac- 

 tion of atmospheric agencies, however hard and refractory the}' may be; 

 these agencies act both chemically and mechanically. The rain, owing 

 to the absorption of carbonic acid from the atmosphere, acts chemically 

 on the rocks by its solvent action, and also from its oxygen combining 

 with substances not yet fully oxidized. Its mechanical action appears 

 in its washing away the finer jiortions of the disintegrated rock or soil 

 from higher to lower ground. The changes in temperature have a loos- 

 ening influence by causing alternate expansion and contraction. The 

 atmosphere itself acts chemically upon the rocks by the slow oxidation 

 of those minerals which can absorb more oxygen, and the production of 

 carbonates and bicarbonates whose solubility still further aids disinte- 

 gration. These disintegrating agencies are still further aided by the 

 root-growths of plants, by the burrowing of worms and other earth- 

 dwelling creatures, and in no small degree by the generation of organic 

 acids, humic, crenic, &c., by organic decay. From the hardest granites, 

 basalts, and lavas to the softest limestones and marls, all are undergo- 

 ing this disintegration ; and the soils to which they give rise will vary 

 in depth, composition, and texture, according to the softness and min- 

 eral character of the rocks and the length of time they have been sub- 

 jected to these agencies. 



According to Darwin the solid rocks disintegrate even in countries 

 where it seldom rains and where there is no frost. De Koninick, a Bel- 

 gian geologist, is of opinion that such disintegration may be attributed 

 to the carbonic and nitric acids, together withi the nitrates of ammonia, 

 which are dissolved in the dew.* 



The rocks which weather most easily and rapidly do not always ex- 

 hibit most soil, very often the reverse. A pure limestone would hardly 

 exhibit any weathered band or soil, because the carbonic acid of the rain 

 would almost at once dissolve and remove the particles it acts upon. 

 Even in the case of igneous rocks, their composition may be such that 

 those which weather the most rapidly would likewise show little of a 

 weathered band, owing to the same solvent action. 



THE SOELS FORMED BY THE DIFFERENT GEOLOGICAE FORMATIONS. 



The rocks of which feldspar is one of the constituents, are the origins 

 of the clays and potash which are met with in all arable soils; feldsjjar 

 is a silicate of aluminium and potassium, which on disintegration forms 

 a clay, a silicate of aluminium, and a silicate of potassium. 



The primitive and igneous ro(jks yield sods ricli in potash, and tlie 

 fossdiferous rocks those rich in phosphoric acid. 



THE DENUDATION OF THE SOIL. 



The same agencies which form the soils are also wasting and carrying 

 them away. During every rain storm transportation of soil goes on, as 

 the brooks and rivers show, after heavy, long-continued rains, by tlio 

 yellow muddy color of their waters, that they are carrying a vast quan- 

 tity of sediment towards the sea. The running streams bear along the 

 transported matter, and gradually deposit it as the current diminishes 

 in velocity , the very finest particles being carried as long as the stream 



• Darwin's Vegetable Mold and Earth Worma, 1882, p. 235. 



