THE FORMATION OF SOIL 11 



collected in the hollows to form seas and 

 lakes, or flowed off the surface by whatever 

 channels offered the opportunity of movement. 

 Such flowing water eroded the solid rock, 

 as streams do at the present time, and the 

 particles thus involved in the current were 

 ultimately deposited in lakes and seas. 

 These deposits in the course of time hardened 

 to form what are called sedimentary rocks, 

 which now constitute a large part of the 

 earth's crust. But it is well to remember that 

 the inorganic constituents of such rocks have 

 all been derived directly or indirectly from 

 igneous rocks. Sedimentary rocks, formed 

 directly from igneous rocks, have often in 

 their turn been attacked by running water, and 

 their constituents, re-sorted in the shape 

 of sand, grit, and silt, have been dropped 

 in the estuaries of rivers and streams, again 

 to form sedimentary rocks. 



If we dig into the earth's surface at any 

 point it is not long till we pass through the 

 superficial soil and reach harder material. 

 The transition from soil to the harder material 

 underneath is generally more or less gradual ; 

 and, having reached the subsoil, we become 

 aware, as we go deeper, that even the subsoil 

 is softer above than below, until at a depth of, 

 it may be, a foot or two we reach practically 



