30 SOILS. 



by its glass-like, irregular fracture. Besides the crystalline 

 quartz rock we find close-grained and at least partly non-crys- 

 talline varieties, such as hornstone and flint. Sandstones most 

 commonly consist of grains of quartz cemented by some other 

 mineral, or by silica itself; in the latter case the siliceous sand- 

 stone frequently passes insensibly into true quartzite. The 

 loose sand so well known to common life is prevalently com- 

 posed of quartz grains, whose hardness and resistance to 

 weathering enables them to survive longest the soil-forming 

 agencies. 



Quartz and its allied rocks jasper, hornstone, siliceous 

 schist, etc., are all, as already stated, acted on with difficulty 

 by the " weathering " agencies. Crystalline quartz rock may 

 be considered as practically refractory against all but the 

 mechanical agencies, and hence remains in the form of sand 

 and gravel, more or less rounded by attrition, as a prominent 

 component of most soils ; sometimes to the extent of over 92 

 per cent, even in soils highly esteemed in cultivation, especially 

 in the arid region. Such soils are mostly the result of the dis- 

 integration of sandstones, the cement of which has been dis- 

 solved out in the course of weathering; or they may be derived 

 directly from geological deposits of more or less loose and un- 

 consolidated sand. Among crystalline rocks, granites, gneiss 

 and mica-schists are those most usually concerned in the form- 

 ation of sandy soils; since in common parlance, quartz is un- 

 derstood to be the substance of the sand unless otherwise stated. 

 The exceptions are especially important in the regions of de- 

 ficient rainfall. 



But while crystalline quartz is practically insoluble in all 

 natural solvents, the same is not true of the jaspers and horn- 

 stones. These consist of a mixture of crystalline and amor- 

 phous (non-crystalline) silica, which is more readily soluble 

 than the crystalline, and is attacked by many natural waters, 

 especially by those containing even very small amounts of the 

 carbonates of potash or soda. We thus often find that horn- 

 stone and jasper pebbles buried in the soil, while still hard in- 

 ternally, have externally been converted into a friable, almost 

 chalky substance, consisting of crystalline quartz from which 

 the cementing amorphous silex has been removed by the soil 

 water. In the course of time such pebbles may be completely 



