Lastly, Miocene deposits are found in _North America_, in New
Jersey, Maryland, Virginia, Missouri, California, Oregon, &c.,
attaining a thickness of 1500 feet or more. They consist principally
of clays, sands, and sandstones, sometimes of marine and sometimes
of fresh-water origin. Near Richmond, in Virginia, there occurs a
remarkable stratum, wrongly called "Infusorial Earth," which is
occasionally 30 feet in thickness, and consists almost wholly of
the siliceous envelopes of certain low forms of plants (Diatoms),
along with the spicules of Sponges and other siliceous organisms
(see fig. 16). The _White River Group_ of Hayden occurs in the
Upper Missouri region, and is largely exposed over the barren
and desolate district known as the "Mauvaises Terres." They have
a thickness of 1000 feet or more, and contain numerous remains
of Mammals. They are of lacustrine origin, and are believed to
be of the age of the Lower Miocene. Upon the whole, about from
15 to 30 per cent of the _Mollusca_ of the American Miocene are
identical with existing species.
In addition to the regions previously enumerated, Miocene strata
are known to be developed in _Greenland, Iceland, Spitzbergen_,
and in other areas of less importance.
The _life_ of the Miocene period is extremely abundant, and, from
the nature of the deposits of this age, also extremely varied
in its character.
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