136 HOW CROPS FEED. 
the lava is incapable of bearing any vegetation save some 
almost microscopic forms, During these years the surface 
of the rock suffers gradual disintegration by the agencies 
of air and water, and so in time acquires the power to 
support some lichens that appear at first as mere stains 
upon its surface. These, by their decay, increase the 
film of soil from which they sprung. The growth of 
new generations of these plants is more and more vigor- 
ous, and other superior kinds take rvot among them, 
After another period of years, there has accumulated a 
tangible soil, supporting herbaceous plants and dwarf 
shrubs. Henceforward the increase proceeds more rapid- 
ly; shrubs gradually give place to trees, and in a century, 
more or less, the once hard, barren rock has weathered to 
a soil fit for vineyards and gardens. 
Those lowest orders of plants, the lichens and mosses, 
which prepare the way for forests and for agricultural 
vegetation, are able to extract nourishment from the most 
various and the most insoluble rocks. They occur abund- 
antly on all our granitic and schistose rocks. Even on 
quartz they do not refuse to grow. The white quartz. 
hills of Berkshire, Massachusetts, are covered on their 
moister northern slopes with large patches of a leathery 
lichen, which adheres so firmly to the rock that, on being 
forced off, particles of the stone itself are detached. Many 
of the old marbles of Greece are incrusted with oxalate. 
of lime left by the decay of lichens which have grown 
upon their surface. 
Humus,—By the decay of successive generations of 
plants the soil gradually acquires a certain content of dead. 
organic matter. The falling leaves, seeds and stems of 
vegetation do not in general waste from the surface as 
rapidly as they are renewed. In forests, pastures, prai- 
ries, and marshes, there accumulates on the surface a brown 
or black mass, termed humus, of which leaf-mold, swamp- 
muck, and peat are varieties, differing in appearance as in 
