30 



SOILS AND MANUEES 



corresponding to the difference in origin and in mineralogi- 

 cal and chemical composition, though, it should be noted, 

 there is no sharp line of demarcation between the two 

 groups. 



Granite exhibits a coarsely crystalline texture, probably 

 because it has been formed beneath the surface and has 

 cooled slowly under great pressure. It is composed of 

 quartz, orthoclase and white mica. It decomposes slowly, 

 but ultimately weathers white. 



Basalt is simply the lava poured forth from volcanoes 

 in a molten condition. It has cooled quickly, and the 

 crystals of the minerals are, therefore, often so small as 

 to give it, superficially, an almost homogeneous appear- 

 ance. It is composed of augite, anorthite and black mica, 

 and often contains also olivine, enstatite and magnetite, 

 which cause it to weather red, but it contains no quartz. 

 It is comparatively easily decomposed. 



In consequence of the difference in mineralogical com- 

 position granite always contains more potash and silica, 

 and is of lower specific gravity than basalt. The soils 

 formed from it are stiff, cold and relatively unproductive. 

 Basalt is richer in lime and decomposes more readily, 

 yielding a lighter and more fertile class of soils. The 

 difference in chemical composition will be seen from the 

 following analysis : 



AVERAGE CHEMICAL COMPOSITION OF GRANITE AND BASALT. 



