238 The Realm of Nature CHAP. 



or silicates, the soil is pure sand, which allows water to drain 

 away so rapidly that in a dry region no moisture is retained. 

 When only the finely divided silicate of alumina results from 

 weathering, the soil is a pure clay, forming when wet a 

 sticky paste through which water does not easily pass. In 

 rainy places clay land is consequently always wet and stiff. 

 Sand and clay are both produced from the decay of most 

 rocks, and the mixture of these constituents forms loams, 

 which, according to the proportion of sand and clay, are 

 either moderately porous or moderately retentive of moisture. 

 Almost all rocks contain smaller or larger quantities of car- 

 bonate of lime, iron, and sulphates or phosphates of the 

 alkalies potash and soda, all of which form part of the 

 resulting soil. Rain contributes salts of ammonia ( 152), 

 partly derived from the air, partly from decomposing animal 

 matter, and these are ultimately oxidised ( 401) to nitric 

 acid, which forms nitrates. Plants pulverise the rock frag- 

 ments of the lower layers or sub-soil by their roots pene- 

 trating the crevices and acting as wedges. The decay of 

 vegetation finally produces vegetable mould. Earth-worms 

 have been shown by Darwin to assist in the formation of 

 soil by dragging decaying vegetation into their burrows 

 and by swallowing the earth, which is thrown out again on 

 the surface as extremely finely -powdered worm -castings. 

 Professor Henry Drummond points out that a similar ser- 

 vice is rendered by the termites or white ants of tropical 

 Africa. 



312. Work of Rain. Rain is the chief agent engaged 

 in the slow but continuous moving on of particles of broken- 

 up rock-crust and soil from high ground to low ground, and 

 from low ground to the sea. When rain falls on beds of 

 clay or soft rock mixed up with harder pebbles or boulders 

 it washes away the softer material, except where it happens 

 to be protected by a stone, which in course of time remains 

 capping a pedestal. The largest examples of such earth 

 pillars are those of the Sawatch region of North America, 

 which attain a height of 400 feet. Mount Roraima, in 

 north-eastern South America, a nearly perpendicular moun- 

 tain of soft sandstone capped with hard conglomerate, and 



