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GEOLOGICAL HISTORY OF THE GULF OF MEXICO. 517 
several pebble streams, the largest of which occupies the axis of 
the embayment. Its beds disappear beneath those of the Port 
Hudson era almost concurrently with the Grand Gulf rocks. 
The phenomena offered by these deposits, as heretofore shown, 
require the assumption that prior to its deposition the Gulf coast 
suffered an elevation of at least four hundred and fifty feet above 
its present level, accompanied by a much greater one at the head 
of the waters. Then there occurred a slow depression to about 
twice that amount, and finally, during the Terrace epoch, a reëleva- 
tion to at least the extent of four hundred and fifty feet. The 
northern derivation of the pebbles, their size, and the extensive 
plowing-up of older beds, prove a southward flow of waters, of 
considerable violence. 
These events were of no local character; they are intimately 
connected’ with, and the complement of, the Drift phenomena of 
the Northwest. It is time that the facts of the case were generally 
understood and taken into account by American geologists, and 
that the Ohio should cease to be proclaimed the southern limit of 
the Drift. Its southern representative has mostly, heretofore, 
been erroneously associated with contiguous formations of every 
age. 
An understanding should be come to as to what is meant by the 
word “ Drift.” In New England it means chiefly moraine mate- 
rial; in the West, what is presumed to be iceberg drift; in the 
South, materials clearly stratified and transported almost exclu- 
sively by water. All are properly included in the Drift Epoch, 
defined as embracing the time between the termination of the 
Tertiary, and the beginning of the Champlain era of quiet deposi- 
tion and slow depression. 
The next succeeding formation is the Port Hudson series of 
swamp, lagoon, fluviatile, estuarian and littoral deposits, formed 
during the slow depression of the continent. It underlies not 
only a wide littoral belt, now partially covered by iia waters of 
the Gulf, but also the entire alluvial area of the lower Mississippi, 
Red, and other larger rivers, then constituting extensive fresh 
water estuaries; their general valleys areni evidently, already 
been impressed upon the surface during the later peri 
though not always coincident with their sat ones. Late s. 
servations made in the Yazoo and Tensas bottoms confirm the 
statement, made by Gen. Humphreys in 1860, that the Mississippi 
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