thickness._--If no such
voluminous masses are formed on the land, it is not from the want of
solid matter in the structure of terrestrial animals and plants; but
merely because, as I have so often stated, the continents are those
parts of the globe where accessions of matter can scarcely ever take
place--where, on the contrary, the most solid parts already formed are,
each in their turn, exposed to gradual degradation. The quantity of
timber and vegetable matter which grows in a tropical forest in the
course of a century is enormous, and multitudes of animal skeletons are
scattered there during the same period, besides innumerable land shells
and other organic substances. The aggregate of these materials,
therefore, might constitute a mass greater in volume than that which is
produced in any coral-reef during the same lapse of years; but, although
this process should continue on the land for ever, no mountains of wood
or bone would be seen stretching far and wide over the country, or
pushing out bold promontories into the sea. The whole solid mass is
either devoured by animals, or decomposes, as does a portion of the rock
and soil on which the animals and plants are supported.

The waste of the strata themselves, accompanied by the decomposition of
their organic remains, and the setting free of their alkaline
ingredients, is one source from whence running water and the atmosphere
may derive the materials which are absorbed by the roots and leaves of
plants. Another source is the passage into a gaseous form of even the
hardest parts of animals and plants which die and putrefy in the air,
where they are soon resolved into the elements of which they are
composed: and while a portion of these constituents is volatilized, the
rest is taken up by rain-water, and sinks into the earth, or flows
towards the sea; so that they enter again and again into the composition
of different organic beings.

The principal elements found in plants are hydrogen, carbon, and oxygen;
so that water and the atmosphere contain all of them, either in their
own composition or in solution.[994] The constant supply of these
elements is maintained not only by the putrefaction of animal and
vegetable substances, and the decay of rocks, but also by the copious
evolution of carbonic acid and other gases from volcanoes and mineral
springs, and by the effects of ordinary evaporation, whereby aqueous
vapors are made to rise from the ocean, and to circulate