398 EF. W. Hilgard—Geological History of the Gulf of Mexico. 
upper division should have preserved no vestiges of either 
animal or vegetable life. 
0 not see how, in view of the nature, thickness (about 260 
feet) and wide distribution of this formation, the inference can 
be avoided that during the whole or a part of the interval 
tween the Vicksburg and Drift ages, the Mexican Gulf was, by 
some means, isolated from the Atlantic ocean; or that at least 
its communication, perhaps across the still submerged penin- 
sula of Florida, was so imperfect as to render the influx from 
the interior of the continent predominant over the original , 
supply of sea-water. An upheaval of the northern borders of 
ear the marks of the event recorded by the Grand Gulf rocks 
on the northern shore of the basin. The observations of Mr. 
Gabb in Sto. Domingo, and of the English geologists in Jamaica, 
seem to indicate the existence there of marine Miocene a 
Pliocene tertiaries, which are altogether unrepresented im the 
hat beds of that 
character may be covered by the Grand Gulf and later beds of the 
“ Northern Lignitic” of the older Tertiary, the Grand Gulf beds 
could be explained away as a mere littoral formation. — 
It is worthy of remark that while east of the Mississippi, the 
peculiar sand- and clay-stones of this group are confined 10 the 
northwesterly portion of its area of occurrence, in Louisiat@ 
and eastern Texas these rocks are altogether predominant, & 
cially along the northern (or landward) border, the clays 
eing subordinate. 
