In some instances we find that,
after the parasitic serpulae were full grown, they had become incrusted over
with a bryozoan, called Diastopora diluviana (see b, Figure 331); and many
generations of these molluscoids had succeeded each other in the pure water
before they became fossil.
We may, therefore, perceive distinctly that, as the pines and cycadeous plants
of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by
submergence under fresh water, and soon buried beneath muddy sediment, so an
invasion of argillaceous matter put a sudden stop to the growth of the Bradford
Encrinites, and led to their preservation in marine strata.
Such differences in the fossils as distinguish the calcareous and argillaceous
deposits from each other, would be described by naturalists as arising out of a
difference in the STATIONS of species; but besides these, there are variations
in the fossils of the higher, middle, and lower part of the oolitic series,
which must be ascribed to that great law of change in organic life by which
distinct assemblages of species have been adapted, at successive geological
periods, to the varying conditions of the habitable surface.
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