THE SOIL 345 



stated^ that glaciation of the surface of British America has 

 been almost universal in the regions east of the Eocky Moun- 

 tains, and all over the Palaeozoic districts west and south of 

 Hudson and James Bay the average depth of the till is 100 feet, 

 and perhaps 200 feet in Manitoba and the northwest territories. 

 The following section is given by James Geikie^ as showing 

 the varying character of the glacial drift and its interstratified 

 interglaeial lacustrine deposits : 



Feet Inches 



Sandy clay 5 



Brown clay and stones (till) .... 17 



Mud 15 



Sandy mud 31 



Sand and gravel 28 



Sandy clay and gravel 17 



Sand 5 



Mud 6 



Sand 14: 



Gravel 30 



Brown sandy clay and stones (till) . , 30 



Hard red gravel 4 6 



Light mud and sand 1 8 



Light clay and stones 6 6 



Light clay and whin block 26 



Pine sandy mud 36 



Brown clay, gravel, and stones ... 14 4 



Park clay and stones (till) 68 



355 

 3. THE SOIL 



There remains now to be summarized a few of the character- 

 istics of those superficial portions of the regolith to which the 

 name soil is commonly applied" and these, too, only in direct 

 relation to their properties as soils, since as integral portions 

 of the regolith they have already been sufficiently touched upon. 



(1) The Chemical Nature of Soils. — The prevailing con- 

 stituents of any soil, whatever its source, is nearly always silica, 

 with varying amounts of alumina, oxides of iron, lime, magnesia, 

 and the alkalies.^ A small amount of organic matter, from 

 extraneous source, is usually present. This prevalence of silica 

 and alumina as may be readily understood, is an essential conse- 



^ Bull. Geol. Soc. of America, Yol. I, 1890, p. 289. 



2 The Great Ice Age, 3d ed., 1894, p. 120. 



® The peat deposits furnish almost the only exception to this rule. 



