142 THE ART OF CULTURE. 



Arable land is originally formed by the crum- 

 bling of rocks, and its properties depend on the 

 nature of their principal component parts. Sand, 

 clay, and lime, are the names given to the 

 principal constituents of the different kinds of 

 soil. 



Pure sand and pure limestone, in which there 

 are no other inorganic substances except siliceous 

 earth, carbonate or silicate of lime, form absolutely 

 barren soils. But argillaceous earths form always 

 a part of fertile soils. Now from whence come the 

 argillaceous earths in arable land; what are their 

 constituents, and what part do they play in favour- 

 ing vegetation ? They are produced by the disin- 

 tegration of aluminous minerals by the action of 

 the weather ; the common potash and soda fel- 

 spars, Labrador spar, mica, and the zeolites, are 

 the most common aluminous earths, which undergo 

 this change. These minerals are found mixed 

 with other substances in granite, gneiss, mica-slate, 

 porphyry, clay-slate, grauwacke, and the volcanic 

 rocks, basalt, clinkstone, and lava. In the grau- 

 wacke, we have pure quartz, clay-slate, and lime ; 

 in the sandstones, quartz and loam. The transition 

 limestone and the dolomites contain an intermix- 

 ture of clay, felspar, porphyry, and clay-slate ; and 

 the mountain limestone is remarkable for the 

 quantity of argillaceous earths which it contains. 

 Jura limestone contains 3 20, that of the Wur- 

 temberg Alps 4550 per cent, of these earths. 



