I.] WEATHERING 9 



solidation of the weathered material. A grain of sand, 

 for example, is practically indestructible ; it may have 

 become cemented to the other grains on the sea beach 

 where it was lying, and give rise to the rock we term 

 sandstone ; the rock thus formed may have been elevated 

 into dry land, broken up into loose grains, and washed 

 down to the sea to form a new beach, over and over again 

 in the world's history; so long a time has elapsed since 

 water first began to work on the earliest rocks. For this 

 reason, if we want to trace out the origin of a soil in 

 detail, we must in most cases go beyond the sedimen- 

 tary rock from which it immediately derives, back to the 

 so-called primitive or crystalline rocks, which represent 

 in a sense the original materials of the earth's crust 



Here we shall find certain fundamental minerals, 

 which in a weathered state, altered both mechanically 

 and chemically, go to form both the sedimentary rocks 

 and the soil which is our immediate study. Though 

 the number of distinct minerals is immense, practically 

 the mass of the earth's crust is made up of a few only ; 

 silica, various complex silicates of alumina, iron, lime, 

 magnesia, potash, and soda, together with carbonate of 

 lime, which is generally of organic origin, are all that 

 need be considered in relation to soils. 



The various agencies which reduce rocks to soil, 

 grouped under the general term of weathering, may be 

 distinguished as mechanical — including the work of alter- 

 nations of temperature, frost, wind, rain, and glacial ice — 

 and chemical, the complex effects of solution and oxida- 

 tion that are brought about by water, especially when 

 charged with carbonic acid. 



In dry climates the alternations of temperature 

 between day and night set up sufficient strain to fracture 

 even large rocks, and eventually reduce them to dust 

 The dust and sand of the deserts of Central Asia, the 



