On this view, the rank and long-continued vegetation
which gave rise to each coal-bed was ultimately terminated by
a slow depression of the surface on which the plants grew. The
land-surface then became covered by the water, and aqueous sediments
were accumulated to a greater or less thickness upon the dense
mass of decaying vegetation below, enveloping any trunks of trees
which might still be in an erect position, and preserving between
their layers the leaves and branches of plants brought down from
the neighbouring land by streams, or blown into the wafer by the
wind. Finally, there set in a slow movement of elevation,--the
old land again reappeared above the water; a new and equally
luxuriant vegetation flourished upon the new land-surface; and
another coal-bed was accumulated, to be preserved ultimately in
a similar fashion. Some few beds of coal may have been formed by
drifted vegetable matter brought down into the ocean by rivers, and
deposited directly on the bottom of the sea; but in the majority
of cases the coal is undeniably the result of the slow growth and
decay of plants _in situ_: and as the plants of the coal are
not _marine_ plants, it is necessary to adopt some such theory
as the above to account for the formation of coal-seams. By this
theory, as is obvious, we are compelled to suppose that the vast
alluvial and marshy flats upon which the coal-plants grew were
liable to constantly-recurring oscillations of level, the successive
land-surfaces represented by the successive coal-beds of any
coal-field being thus successively buried beneath accumulations
of mud or sand.
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