Evidence of the amount of denudation which the land has
undergone Sub-aerial denudation The deposition of dust Vegetable mould,
its dark colour and fine texture largely due to the action of worms The
disintegration of rocks by the humus-acids Similar acids apparently
generated within the bodies of worms The action of these acids
facilitated by the continued movement of the particles of earth A thick
bed of mould checks the disintegration of the underlying soil and rocks.
Particles of stone worn or triturated in the gizzards of worms Swallowed
stones serve as mill-stones The levigated state of the castings Fragments
of brick in the castings over ancient buildings well rounded. The
triturating power of worms not quite insignificant under a geological
point of view.
NO one doubts that our world at one time consisted of crystalline rocks,
and that it is to their disintegration through the action of air, water,
changes of temperature, rivers, waves of the sea, earthquakes and
volcanic outbursts, that we owe our sedimentary formations. These after
being consolidated and sometimes recrystallized, have often been again
disintegrated. Denudation means the removal of such disintegrated matter
to a lower level. Of the many striking results due to the modern
progress of geology there are hardly any more striking than those which
relate to denudation. It was long ago seen that there must have been an
immense amount of denudation; but until the successive formations were
carefully mapped and measured, no one fully realised how great was the
amount. One of the first and most remarkable memoirs ever published on
this subject was that by Ramsay, {210} who in 1846 showed that in Wales
from 9000 to 11,000 feet in thickness of solid rock had been stripped off
large tracks of country. Perhaps the plainest evidence of great
denudation is afforded by faults or cracks, which extend for many miles
across certain districts, with the strata on one side raised even ten
thousand feet above the corresponding strata on the opposite side; and
yet there is not a vestige of this gigantic displacement visible on the
surface of the land. A huge pile of rock has been planed away on one
side and not a remnant left.
Until the last twenty or thirty years, most geologists thought that the
waves of the sea were the chief agents in the work of denudation; but we
may now feel sure that air and rain, aided by streams and rivers, are
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