soil: its origin and nature. 133 



Alluvial Soils. — Superficial alluvial deposits consist of dis- 

 integrated and decomposed rock-materials which have been 

 transported and spread out by water. Some of these formations 

 have been accumulated in fresh water, others have been laid 

 down in estuaries and upon the sea-floor. They and their soil- 

 caps naturally vary much in character. The coarser deposits 

 consist of water-worn shingle and gravel, which, as they rapidly 

 absorb rain, are usually barren. When the interstices amongst 

 the stones are filled with finer grained materials — grit and sand, 

 — a light porous soil is formed. Between such coarse accumula- 

 tions and the finest mud and silt, all gradations occur. Although 

 some alluvial sands are infertile, yet such is not often the case, 

 for the finer arenaceous alluvia usually contain larger or smaller 

 proportions of clay, and not infrequently form loamy soils of 

 good quality. From loams capable of high cultivation, we pass 

 onto clays,. many of which are tenacious — although no alluvial 

 clays are so stiff and tenacious as the tills and stoneless clays of 

 glacial origin. Moreover, alluvial clays, muds, and silts often 

 contain much organic matter, and are frequently rich in soluble 

 mineral salts. The chief distinction, however, between alluvial 

 and glacial clays is simply this, that the former consist 

 essentially of mineral matter in an advanced state of decom- 

 position, and, therefore, readily assimilated by plants, while 

 the latter are composed almost exclusively of undecomposed 

 mineral matter — of crushed and pulverised rock, which had 

 previously experienced little or no chemical alteration. 



.^olian Soils. — In this country the only important accumula- 

 tions of agolian origin are the sand-dunes of our low-lying 

 maritime districts. As the dominant ingredient of these sands 

 is quartz, they can hardly be said to yield a soil. Nevertheless, 

 certain sand-loving plants find sustenance upon them, and 

 succeed in binding the loose particles together, so that eventually 

 some humus is accumulated, and a thin soil is formed. In other 

 regions, however, the fine dust swept by the wind from dry 

 desiccated regions, and distributed over adjacent tracts, has 

 not only added to the fertility of such tracts, but in some cases 

 has accumulated so as to form thick sheets of the finest loam, 

 which cover and conceal the native soil-caps and bed-rocks of 

 extensive areas. The fine loams of the Rhine Valley and the 

 low grounds of the Danube, and the far-spreading " black earth " 

 of Southern Russia, are believed to be of wind-blown origin. 



