30 
TRANSACTIONS OF TIIE TEXAS ACADEMY OF SCIENCE. 
The rocks of the Lower Cretaceous are sands and limestones with some 
clay, and their present surface exposure occupies a very large area known 
as the Grand Prairies, stretching from Red River to the Rio Grande. 
The Upper Cretaceous beds are, for the most part, composed of lime 
and clay (although some beds more or less sandy are found both in the 
Red River and Rio Grande sections), and may be generally classed as 
marls, passing into a pure chalk limestone on the one hand, and into a 
clay with comparatively little lime on the other. They were deposited 
in quiet waters of moderate depth, containing vast numbers of minute 
organisms, the remains of which have contributed largely to the forma¬ 
tion of the chalk and glauconitic beds. These beds occupy a belt of 
variable width, lying between the Grand Prairies and the coast, the 
present line of which it roughly parallels. 
So far as present surface exposures indicate, the Cretaceous sea seems 
to have finally receded to the south and west, for the upper deposits of 
the Rio Grande region appear to be later in time than any of those of the 
Red River section. 
The earliest deposits of the succeeding Tertiary were derived imme¬ 
diately from the upper beds of the Cretaceous, and so closely do they 
resemble them that it is often difficult to say where the one ends and the 
other begins. A great coal-making period followed, during which were 
formed the earlier of our great deposits of brown coal and lignite. Then 
a period of marine or brackish water deposits, accompanied by or altern¬ 
ating with lagoons and peat bogs, in which were formed other beds of 
brown coal and the iron ores of East Texas. The action of Foraminifera 
in the production of glauconite is repeated in this period, as is evidenced 
by the beds of green sand marl, and the resulting red hills of East Texas. 
Succeeding these are belts of clays with still other large deposits of 
brown coal, and followed by sands. These are followed by still other 
clays and sands, forming altogether a series many hundred feet in thick¬ 
ness and of broad areal extent. 
The upper portion of the Gulf Tertiary beds contains a considerable 
amount of lime, although no compact limestones like those of the Creta¬ 
ceous are found, except in the southwestern portion of the State. 
These Tertiary deposits occupy a belt of country larger even than that 
of the Cretaceous, and lying between it and the Gulf, in whose waters, 
at that time so much more extended than at the present, they were 
deposited. 
During the middle portion of Tertiary time, there occurred another 
interval of dry land in the Texas region, and this was succeeded in the 
northwest by great lakes, the site of one of which is now indicated by 
the deposits of the Llano Estacado. These lakes probably extended into 
the mountain region of the west also. 
