466 Action of the Atmosphere upon neioly-decpened Soil. 



hydrate of the peroxide of iron is the result, in combination with 

 silicate of alumina and such remainder of the lime and magnesia 

 as may not have been washed away. The magnesia seems to 

 disappear faster than the lime, as will be seen by comparing the 

 analyses given of the fresh and the decomposed mineral in the 

 table, and this might have been looked for from the results of the 

 Professors Rogers' experiments, as they found that in water im- 

 pregnated with carbonic acid, such as rain-water passing through 

 the soil always is, carbonate of magnesia is much more soluble 

 than carbonate of lime. 



The changes that occur in augite seem more various, green 

 earth being frequently the result, with different degrees of decom- 

 position. Steatite, or soapstone, seems also to be occasionally a 

 product of the decay of this mineral, and also of horneblende, the 

 lime and oxide of iron giving place to an increased proportion of 

 magnesia and water. Analyses are given in the table, showing 

 different stages of decomposition of these minerals, which are 

 interesting as indicating the changes that have taken place. 



These igneous rocks, decomposing under atmospheric influences, 

 form soils generally noted for their fertility. In addition to the 

 ingredients already mentioned as entering into their composition, 

 Dr. Fownes, by careful analyses, ascertained the presence of phos- 

 phoric acid in various rocks of igneous origin. He examined the 

 porcelain-clay of Dartmoor, resulting from the disintegration of 

 the granite of that district, also lavas, basalts, and trachytes ; and 

 from his researches he inferred that phosphoric acid is a very 

 usual component part of volcanic rocks, and is a principal source 

 of the remarkable fertility possessed by soils derived from their 

 disintegration. {^Proceedings of Royal Soc, v. 508.) 



Dr. Anderson, of Glasgow, in reference to the igneous masses, 

 ■writes : — " Rocks of the greenstone type are divisible into two 

 classes. — the diorite and dolerite ; the former a mixture of albite 

 and horneblende, the latter of augite and labradorite, sometimes 

 with considerable quantities of a sort of oligoclase, containing both 

 soda and lime, and of different kinds of zeolitic minerals. Gene- 

 rally speaking, the soils produced from diorite are superior to 

 those from dolerite ; the albite which the former contains under- 

 goes a rapid decomposition, and yields abundance of soda along 

 with some potash, which is seldom altogether wanting, while the 

 horneblende supplies both lime and magnesia. Dolerite, when 

 composed entirely of augite and labrador, produces rather inferior 

 soils ; but when it contains oligoclase and zeolites, and comes 

 under the head of basalt, its disintegration is the source of soils 

 remarkable for their fertility ; for these latter substances, under- 

 going rapid decomposition, furnish the plants with abundant 

 supplies of alkalies and lime, Avhile the more slowly decomposing 



