474 



Mr. Hinds on Climate, tyc. — Sot/. 



been divided by Professor Jameson into eight classes or va- 

 rieties^ and whilst we borrow his ideas on this subject it will 

 be convenient to reverse his arrangement, by commencing 

 with those which are found most conducive to their formation. 



The first class contains those substances which separate 

 with facility into an earthy mass ; as marl, slate-clay, basaltic 

 and volcanic tuffa. The two latter are capable of producing 

 a very fertile soil ; on the declivities of Mount /Etna the vege- 

 tation is luxuriant, and also in the vicinity of Vesuvius. 



To the second class belong conglomerate rocks of grey- 

 wacke, old red sandstone, and sandstones of various kinds, 

 easily separable by mechanical forces and convertible into 

 gravelly, sandy, or earthy soils. 



Third class. Slaty rocks, by their natural structure easily 

 divided and reduced to a mass, which, mixed with water, forms 

 a paste. 



In the fourth class are those rocks having a crystalline or 

 granular texture ; their cohesion not being considerable, they 

 are easily reduced. Granite and gneiss belong here, rocks 

 which often form a good soil, as the materials forming them 

 are loose in their aggregation and retain a proper allowance of 

 moisture. 



Fifth class. Basalt ; not very favourable to vegetation. The 

 flora of a basalt country is usually meagre. 



Sixth class. Chalk and gypsum, though without much 

 cohesion, are not likely to produce a good soil, nor do they 

 readily retain moisture. 



The seventh class comprises compact limestone ; even when 

 much comminuted this will not develope any good qualities, 

 owing to the abundance of calcareous matter. A mixture with 

 aluminous earth in some measure counteracts this excess. 



In the eighth class are assembled those substances, which, 

 by exposure to the atmosphere for long periods, undergo, if 

 any, very trivial changes. They consist of vitreous lava, pure 

 quartz, compact quartz, flinty slate, and porphyry with a sili- 

 ceous base. No soil properly so called is formed by them, 

 and the only vegetation likely to be met with in their neigh- 

 bourhood are lichens, which attach themselves to the surface. 



These are some of the principal rocks which lend their con- 

 stituent parts towards the generation of Soil. In nature the 

 number, by every possible admixture, becomes multiplied to 

 infinity ; but whatever may be the mineralogical condition of 

 its structure, vegetation will not thrive in a soil having its 

 sole origin in this source, w 7 hich an experiment by Giobert 

 admirably illustrates. The four earths, silica, alumina, lime 

 and magnesia, were mixed together in such proportions as 



