102 PLANT STRUCTURES 
disintegration, we can conceive that over each kind of 
rock a soil would be formed corresponding closely to 
the materials of which that rock is composed. In 
sections formed by quarrying, by the cutting action 
of rapid streams, and so on, we may often see this. 
Below is the solid rock. Its upper layers tend to be 
loose and rotten owing to the action of percolating 
water, etc. They merge into a layer of stony débris, 
where the harder portions still retain their rock 
character, while the softer are disintegrating into clay 
or sand. Above this the rock is wholly disintegrated 
into a soil, the upper layers of which, mixed with plant 
débris, and consequently of darker colour, are full of 
the roots of living plants descending from the sward 
which covers the surface of the ground. In practice, 
however, such close conformity of soil to underlying 
rock is not always found. 
Various distributing agents are ever at work— 
wind, water in an especial degree, and on sloping 
ground the action of gravity. In northern countries, 
besides, the ice of the Glacial Period has in its passage 
caught up all the loose surface material, added 
immensely to its volume by grinding down the rocks, 
and flung the products broadcast over the country, so 
that old sea bottoms may be strewn over coastal lands, 
sands and gravels over clayey rocks, and limy soils 
over areas where no limestone exists. The soil over 
much of the British Isles is formed from the surface- 
layer of these glacial deposits, which—tough, intract- 
able, sterile—underlie the soil often to a great depth, 
where they rest on rock. In southern England 
.the covering of glacial deposits is absent, since the 
ice-cap did not extend beyond the Thames valley; beds 
