464 Action of the Atmosphej^e upon newly-deepened Soil. 



amygdaloids, sometimes replaclnfj mica. Chlorite-slate forms 

 many mountains in Scotland. The chemical composition of 

 chlorite is somewhat various, but may be considered a combina- 

 tion of silicate of alumina and magnesia, with a variable amount 

 of oxide of iron. 



Talc, in combination with quartz and felspar, and also as talc- 

 slate, is a rock of frequent occurrence in the primary districts of 

 this country. Steatite is a different form of the same mineral, and 

 is also thought to be occasionally a result of the decomposition of 

 chlorite. In talc, as associated, witli other minerals in granite, 

 &c., it would seem as if the magnesia, once disseminated through 

 the mass, had been segregated from the mixture into a condition 

 by itself. Both talc and steatite are mostly silicates of magnesia, 

 the latter being sometimes in part replaced by oxide of iron. 



Serpentine is a well-known and abundant mineral, occurring 

 apparently both as a sedimentary deposit and as an igneous pro- 

 duction ; great dykes of it occur in many districts, sometimes 

 running for leagues in one direction. By itself it often forms 

 mountains, and also occurs in combination with other minerals ; it 

 is found both in England and in Scotland. It is also a silicate of 

 magnesia, containing much water, and having some of the magnesia 

 occasionally replaced by oxide of iron. The soil derived from the 

 decomposition of these minerals is rather noticeable for its bar- 

 renness, frequently being tenacious of water, so that swamps and 

 marshy places are common. This has been remarked by Sir H. 

 De la Beche of the soil on the serpentine of Cornwall, which 

 contrasts unfavourably with that derived from diallage rock, a 

 mineral mass containing a good deal of lime and less magnesia. 

 This wet nature of the serpentine soil is doubtless owing to the 

 remarkable tenacity of the magnesia for moisture, Schubler having 

 found that this substance far exceeded all the other earths in this 

 property. 



The Professors Rogers, in their experiments on the solvent, 

 action of water, found that when it contained carbonic acid, the 

 magnesian silicates dissolved readily. 



The oxide of iron is also another element of decay, as will be 

 more fully shown when treating of horneblende. Serpentine, as 

 well as many other rocks, also occasionally contains iron pyrites, 

 or bisulplmretof iron — a mineral very apt to suffer decomposition 

 by the action of the atmosphere, the sulphur separating while 

 the iron is converted into the hydrated peroxide. Sulphuretted 

 hydrogen is also an occasional production from the sulphur. It 

 is likewise found that another change sometimes occurs in the 

 pyrites, by the sulphur combining with oxygen to form sulphurous 

 acid — a sulphate of iron being the consequence. This pyrites is 



