ALTERNATIONS OF MARINE AND FRESH-WATER STRATA.
It has been shown in the third chapter that there is such a difference between
land, fresh-water, and marine fossils as to enable the geologist to determine
whether particular groups of strata were formed at the bottom of the ocean or in
estuaries, rivers, or lakes. If surprise was at first created by the discovery
of marine corals and shells at the height of several miles above the sea-level,
the imagination was afterwards not less startled by observing that in the
successive strata composing the earth's crust, especially if their total
thickness amounted to thousands of feet, they comprised in some parts formations
of shallow-sea as well as of deep-sea origin; also beds of brackish or even of
purely fresh-water formation, as well as vegetable matter or coal accumulated on
ancient land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the reverse. Thus,
if we bore an artesian well below London, we pass through a marine clay, and
there reach, at the depth of several hundred feet, a shallow-water and
fluviatile sand, beneath which comes the white chalk originally formed in a deep
sea.
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