WITH AGRICULTURE AND PLANTING. 475 



sometimes a solitary shrub or tree. Examples, Bennevis, 

 Paps of Jura, and Morven Hills. Of all rocks, vitreous 

 volcanic productions are the least capable of contributing 

 to the formation of productive soil. Their dark coloured 

 tracts descend from volcanic mountains to the valleys in 

 frightful sterility, the chinks of their rugged masses 

 scarcely affording sufficient water for the roots of mosses*. 

 To the second class we refer compact limestone, a rock 

 which contributes extensively to the formation of the 

 sol d crust of the globe. In so far as regards its princi- 

 pal constituent parts, it is not affected by atmospheric 

 water or air ; but, as its parts have but comparatively 

 little cohesion, and are usually separated in a consider- 

 able degree by minute fissures, they are more liable to 

 be broken down and crumbled by mechanical powers, 

 than those of the rocks belonging to the first class. In 

 districts where the fundamental rock is limestone, the 

 layers of loose original soil or subsoil are thin, and filled 

 with numerous fragments. As the soil arising from the 

 disintegration of limestone contains a great proportion of 

 calcareous matter, it is neither favourable to the growth 

 of plants in general, nor to that of the greater number of 

 vegetables which are the object of cultivation. Soil of 

 this kind is too hot, dry and stony ; hence the reason 

 why districts, in which pure limestone rocks predominate, 

 are often sterile. The case is different, however, where 

 a portion of clay enters as an ingredient into the compo- 

 sition of calcareous rocks, for here the soil is usually very 



" The Streams of Obsidian in Iceland, Lipari, Peak of TenerifFe, 

 Ascension, and Mexico, afford striking examples of the fact stated 

 above. 



