516 GEOLOGICAL HISTORY OF THE GULF OF MEXICO. 
us to determine the equivalents in time of the marine Tertiary 
groups. 
The Vicksburg series of rocks, ends the marine Tertiary of the 
Southwest ; the transitions between its fauna and the older Eocene 
are so cogent as to render any great separation in time or space 
inadmissible. The Vicksburg deposits are rather of a deep-sea 
character, less lignitiferous than the stage next below. But the Mio- 
cene and Pleiocene deposits, observed on the Atlantic coast, are 
unrepresented on the waters of the Gulf, save by beds equally de- 
void of a marine or fresh water fauna, and with but very few and 
poorly preserved remains of plants. A single fragment of a 
turtle has been found in a clay stratum filled with calcareous con- 
eretions, possibly the remains of a fauna destroyed by maceration. 
These “ Grand Gulf” beds lack all analogy outside of the 
basin, unless it be in the interior, perhaps, in the Bad Lands of 
Nebraska, whose analogues have now been found so much farther + 
south than heretofore supposed, that a connection may have been 
possible ; the lithological resemblance is very great— at all events, 
since the Grand Gulf rocks alone represent the period between 
the Eocene and Drift, they include the equivalents in time of the 
White river beds as well as others. . 
It seems impossible fo account for the character, thickness and 
position of these beds, without assuming that after the close of 
the Eocene period, the Gulf was either cut off from the Atlantic, oF 
communication was so slight as to cause the continental waters to 
freshen the brine so much as to destroy the marine fauna, without , 
rendering it fit for fresh water life. An upheaval of the northern 
border of the Caribbean would even now readily produce such an 
isolation, were it not for the deep channels excavated by the Gulf 
stream, in the straits of Yucatan and Florida. Since on the far- 
ther Antilles Miocene and Pliocene beds have been found, it is evi- 
dent that this state of things was confined to the Gulf basin. 
The geology of Cuba and Yucatan is too little known to determine 
how far they were concerned in the same. on 
The Grand Gulf as well as the older rocks are almost every- 
where overlaid by the “Orange Sand” or stratified Drift; W ile 
on the Palæozoic territory it is more or less localized in conform- 
ity with the larger valleys. On the area under consideration, it 
forms a huge delta-shaped mass, consisting mainly of ferruginous 
and variegated sand and subordinate clay beds, and traversed by 
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