THE SOIL 83 
The relation between the various sedimentary rocks 
may be illustrated by considering a river flowing into the 
sea. When the river meets the sea, the speed of its current 
is checked, and the sediment which it bears or rolls along 
is deposited upon its bed or along its banks. The heaviest 
particles come to rest as soon as the current can move 
them along no farther. As the velocity of the stream 
decreases, the lighter particles fall—first, the coarse sand ; 
then the finer sand and silt ; and, last of all, an almost 
impalpable mud, which may be borne miles out to sea, 
and which, when deposited, forms clay. 
Humus.—This is one of the most important constituents 
of fertile soil ; when it is absent, the ground is sterile and 
unproductive. More or less of it is present in every soil 
which supports vegetation, and sometimes the soil con- 
sists of little or nothing else. Where plants grow, humus 
accumulates. The conversion of dead vegetation or 
animal remains into humus is brought about by germs 
(bacteria) and fungi living in the soil. Where air is abun- 
dant and the ground not too cold, the organic matter in 
the soil is completely destroyed, the final products of its 
decomposition being carbonic acid gas, water, and mineral 
salts—substances valuable to plants as sources of food. 
When, through any cause, decomposition is checked, 
dark-coloured ‘“‘ earth ’-acids are formed, which have a 
souring effect upon the soil. If lime, however, is present, 
it combines with these acids, rendering the humus mild 
and alkaline, and consequently fertile (p. 94). Enor- 
mous masses of humus in the form of peat have accumu- 
lated upon the rock-surfaces of the land, where, through 
lack of air, abundance of water, or extreme cold, decom- 
position has been arrested. In the form of coal, we dig 
up and burn as fuel the compacted and mineralized humus 
of past ages ; in the form of manure, the agriculturalist 
renews the fertility of the fields with “artificial ’’ humus. 
At the present day peaty matter is constantly being 
formed on wet, cold moors, on dry, cold heaths, in mossy 
bogs, and in forests from rotting leaves and wood. 
