It is, however, with the Carboniferous age that we have to do at
present, and I will not anticipate the coming chapters of my story by
dwelling now on the aspect of the later periods. To return, then, to the
period of the coal, it would seem that extensive freshets frequently
overflowed the marshes, and that even after many successive forests
had sprung up and decayed upon their soil, they were still subject to
submergence by heavy floods. These freshets, at certain intervals,
are not difficult to understand, when we remember, that, beside the
occasional influx of violent rains, the earth was constantly undergoing
changes of level, and that a subsidence or upheaval in the neighborhood
would disturb the equilibrium of the waters, causing them to overflow
and pour over the surface of the country, thus inundating the marshes
anew.
That such was the case we can hardly doubt, after the facts revealed
by recent investigations of the Carboniferous deposits. In some of the
deeper coal-beds there is a regular alternation between layers of coal
and layers of sand or clay; in certain localities, as many as ten,
twelve, and even fifteen coal-beds have been found alternating with as
many deposits of clay or mud or sand; and in some instances, where the
trunks of the trees are hollow and have been left standing erect, they
are filled to the brim, or to the height of the next layer of deposits,
with the materials that have been swept over them.
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