PRIMARY SOILS. 239 



carbonate of lime and magnesia, forming the magnesian lime- 

 stone soil. A third variety contains feruginous limestone, 

 or iron combined with the lime, and the fourth variety con- 

 tains silex or siliceous carbonate of lime. This latter soil is 

 very fertile, and yields very sweet food for grazing. 



The primary limestones diifer from the secondary in be- 

 ing less friable and in containing no organic remains. 



The magnesian variety from both the secondary and primary 

 rocks is highly fertile, for although magnesia, in its caustic 

 state, appears to be injurious to vegetation, the rock itself, 

 when crumbled into soil, exerts no such effects ; probably 

 because it is already combined with carbonic acid. 



The best test of a limestone soil whatever be its origin, is 

 any dilute acid such as the sulphuric, in which case, the car- 

 bonic acid will escape with effervescence or with foam, when 

 the soil is put into water, and the acid poured upon it. 



The calcareous or limestone soil is of every degree of fer- 

 tility, and is best fitted for wheat, clover and the sweet gras- 

 ses. 



3. 3Iica slate soil, like the rock, is composed mostly of mi- 

 ca and quartz. It is distinguished from clay slate soil, by 

 its lighter color, yet these two rocks frequently pass into each 

 other, and of course the soils are also mingled. In some 

 cases the mica slate passes into gneiss and argillaceous slate, 

 and the soil of course will partake of the character of both 

 rocks. This soil is found in very many places in Worcester 

 county, Mass. and in all the New England States, and is 

 generally very fertile. It contains but little feldspar, and 

 hence but little potash, but the mica yields a large quantity 

 of magnesia, which gives it an adhesive and loamy character. 



4. Talcose slate soil can hardly be distinguished from the 

 mica slate by its color, although it is somewhat lighter. It 

 contains talc instead of 7nica, and these may be easily distin- 

 guished ; the former is non-elastic and of a soapy feel, the 

 latter elastic and tough. 



