THE PHYSICAL PROCESSES OF SOIL FORMATION. 3 



changed to ice, the latter's expansion in freezing (see below) 

 has still farther enlarged them and caused a scaling-off, which 

 threatens to obliterate the hieroglyphic inscriptions. Thus 

 temperature-changes and a rain followed by freezing may 

 in a few days produce a greater effect than a thousand years 

 of Egyptian climate. 



Cleavage of rocks. Many kinds of rocks have definite direc- 

 tions of ready cleavage. The most common and obvious cases 

 of this kind are schists, slates and shales, cleaving readily into 

 plates or irregular flat or lens-shaped fragments. Such struc- 

 ture greatly favors disintegration, especially when the layers 

 are on edge at steep angles. But there are other apparently 

 structureless, massive rocks, particularly basalts and other 

 eruptive rocks related to them, as well as many sandstones and 

 claystones, that have a strong tendency to cleave into more or 

 less definite forms when struck ; such as columns or prisms, 

 square, six-sided or diamond-shaped blocks, etc. Similar forms 

 are naturally produced in them under the influence of changes 

 of temperature; by the formation of minute cracks at first, then 

 enlargement of these by the several agencies already mentioned. 



Effects of freezing water. The irresistible force exerted 

 by the expansion of water in freezing, amounting to about 9 

 per cent of its bulk, is a powerful factor in widening and deep- 

 ening fissures and cracks of rocks ; not uncommonly, whole 

 masses of rock are rent into fragments by this agency, which 

 is one of the most common causes of " rock falls '" on the 

 brink of precipices. By the freezing process cracks and crevices 

 are enlarged, and the surfaces exposed to weathering are still 

 farther increased ; and the rock fragments or soil particles are 

 loosened and rendered more liable to be removed from the 

 original site, whether by gravity, wind or water. 



Glaciers. Ice in the form of the glaciers that descend from 

 mountain chains (see figure i), and of the moving ice sheets 

 that have covered large portions of North America and Europe 

 in past ages and now cover Greenland and the South Polar con- 

 tinent, exerts a most potent action in abrading and grinding 

 even the hardest rocks ; not so much by the direct friction of 

 the moving ice itself, as by the cutting, scoring, grinding and 

 crushing action which the stones imbedded in the ice, or carried 



