468 Action of the Atmosphere ujmn newly -deepened Soil. 



anhydrous, is scarcely affected when so treated with cold muriatic 

 acid for twenty-four hours, but is unable to resist the conti- 

 nued action of water saturated with carbonic acid, much more 

 must these zeolitic minerals be dissolved by the same agent. 

 (Liebig.) They are all therefore rapidly decomposable bodies, 

 and, like the felspars, yield a clay, while alkalis are separated. 

 They are also noticeable for the absence of magnesia and oxide 

 of iron in their composition, and potash is a less frequent consti- 

 tuent than either lime or soda. They occur chiefly in the 

 amygdaloidal cavities of rocks, and although they do not, like the 

 felspar or horneblende, constitute the great mass of a rock, yet 

 they are often pretty freely dispersed. Analyses are given of 

 some of the most common species. 



Green-earth, as has been I'emarked, is sometimes merely a stage 

 in the decomposition of augite ; it is, however, quite a common 

 mineral in this country, occurring chiefly in trap rocks. Its 

 chemical composition is very variable, as will be seen even from 

 the two examples given in the table. 



Olivine is a very frequent ingredient in many igneous rocks, 

 although it does not form a large proportion of their mass, being 

 disseminated through them mostly in the form of small grains. 

 It is believed to be seldom absent in l)asalt, and its chemical 

 composition is a silicate of magnesia and of protoxide of iron. 



Glauconite resembles green-earth in colour, and in other re- 

 spects. It is referred to chiefly on account of its being a com- 

 mon body in the greensand strata, the specimen of which the 

 analysis by Dr. Turner is given being from that formation in 

 England. It also varies much in composition, some containing 

 mucli magnesia or potash ; it generally, however, has a large 

 amount of silica, protoxide of iron, and water. 



Epidote is a mineral of frequent occurrence in many crystalline 

 trap rocks and also in amygdaloids, not however forming a great 

 part of the mass. Its chemical composition is silicate of lime 

 and alumina, these bases being replaced by protoxide of iron or 

 of manganese, and also by magnesia. 



Carbonate of Lime. — This mineral is so abundant in the sedi- 

 mentary rocks that in all formations from the newest pliocene to 

 the oldest silurian almost every other bed contains more or less 

 of it, some of the great groups of rocks being merely alternations 

 of shales and limestones. It varies of course infinitely in purity, 

 the carbonate of lime being mixed often with carbonate of mag- 

 nesia, forming dolomite, or with clay, forming marl, and witfi 

 quartz grains constituting calcareous sandstone, and occasionally 

 with sulphate of lime, giving rise to gypsum. Oxides of iron may 

 also be interspersed throughout the mass ; also phosphoric or 

 fluoric acids, and bitumen and carbon, as in the coal measures. 



