Birds, Flying Reptiles, and
Flying Mammals might also similarly find their way into aqueous
deposits; but it is to be remembered that many birds and mammals
habitually spend a great part of their time in the water, and
that these might therefore be naturally expected to present
themselves as fossils in Sedimentary Rocks. Plants, again, even
when undoubtedly such as must have grown on land, do not prove
that the bed in which they occur was formed on land. Many of the
remains of plants known to us are extraneous to the bed in which
they are now found, having reached their present site by falling
into lakes or rivers, or being carried out to sea by floods or
gales of wind. There are, however, many cases in which plants
have undoubtedly grown on the very spot where we now find them.
Thus it is now generally admitted that the great coal-fields
of the Carboniferous age are the result of the growth _in situ_
of the plants which compose coal, and that these grew on vast
marshy or partially submerged tracts of level alluvial land. We
have, however, distinct evidence of old land-surfaces, both in
the Coal-measures and in other cases (as, for instance, in the
well-known "dirt-bed" of the Purbeck series). When, for example,
we find the erect stumps of trees standing at right angles to
the surrounding strata, we know that the surface through which
these send their roots was at one time the surface of the dry
land, or, in other words, was an ancient soil (fig.
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