1). That gypsum has been selected by almost all
Continental geologists as affording the best line of demarkation between the
Middle and Lower Tertiary, or, in other words, between the Lower Miocene and
Eocene formations.
In reference to Table 16.1 I may observe, that the correlation of the French and
English subdivisions here laid down is often a matter of great doubt and
difficulty, notwithstanding their geographical proximity. This arises from
various circumstances, partly from the former prevalence of marine conditions in
one basin simultaneously with fluviatile or lacustrine in the other, and
sometimes from the existence of land in one area causing a break or absence of
all records during a period when deposits may have been in progress in the other
basin. As bearing on this subject, it may be stated that we have unquestionable
evidence of oscillations of level shown by the superposition of salt or
brackish-water strata to fluviatile beds; and those of deep-sea origin to strata
formed in shallow water. Even if the upward and downward movements were uniform
in amount and direction, which is very improbable, their effect in producing the
conversion of sea into land or land into sea would be different, according to
the previous shape and varying elevation of the land and bottom of the sea.
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