62 E. W. Hilgard—Later Tertiary of the Gulf of Mexico. 
a different aspect when viewed by the light now afforded by 
our knowledge of the configuration of the bottom of the Gulf, 
and of the oscillations of level to which at least its northern 
shore, and especially the central portion of the Mississippi 
Valley, have been subject in Tertiary and Quaternary times. 
I cannot but express my regret that the latter portion of 
these data should thus far rest almost alone upon my personal 
observations and conclusions. It seems to me that as the only 
portant additional data. 
he state of the evidence regarding these oscillations may 
be thus summarized: A comparatively rapid upward movement 
of the bottom of the Mississippi trough during early Tertiary 
time, is conclusively shown by the rapid decrease of the depth 
of the Mississippi embayment, which from its head near Cairo 
to about the mouth of the Arkansas, is filled with lignitiferous 
clays with only here and there a small marine estuarian 
deposit ; except that in the State of Arkansas, a residuary 
basin of the old (Cretaceous) trough retained deep-sea features 
until the beginning of the “Jackson” epoch. The latter, with 
its abundant marine fauna, headed by the great Zeuglodon, 
was evidently deposited on a comparatively steep slope forming 
the southern edge of the plateau that existed in the upper por- 
tion of the embayment; yet it also consists, in the main, of 
Louisiana speaks of a short duration of the epoch, at t 
of which the lignito-gypseous feature again appears. 
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