Evolution of the Moimtainous Texan Region. — Hill. 107 
After the close of the Carboniferous, and continuing through 
the undetermined period of the Red Beds epoch, this Paleozoic 
tongue was a land barrier extending south and east, an undefined 
distance; it drained westward into a great interior bt^sin, now only 
indefinitely known by the extent of the Red Beds sediments 
which are in part derived from it. There is no trace of an east- 
ward flowing drainage, for the eastern projecting edges have been 
planed off by the numerous subsidences it has since undergone, 
and the sediments of the Red Beds epoch are not present. 
It is evident that this land barrier, which kept the marine waters 
of the gulf from the Great Plains region in Permo-Triassic time, 
was gradually degraded, and the great basin between it and the 
Rocky mountain area filled in, thus preparing the way fo"' the 
free invasion of the Great Plains region by marine waters during 
the subsequent epochs of profound subsidence. 
At the beginning of Lower Cretaceous time, during the Weal- 
dan epoch, as is accurately determined by fossil plants, molluscs 
and vertebrates, a profound subsidence began, during which, the 
narrow strip of land was completely base-leveled, and the marine 
waters of the gulf swept west and north over the entire state of 
Texas to the Cordilleran continent and the first marine invasion 
of at least a portion of the Great Plains occurred. It was during 
this epoch that the calcareous sediments of the Comanche series 
were deposited, which now form the most conspicuous feature 
in the topography and structure of the Texas Mexican region. 
During the deposition of these beds, the first half of Cretaceous 
time, the oceanic waters covered all of the Texan, West Indian 
and Mexican regions, and the northwest corner of South America. 
The shore line was around tlie southern front of the Colorado 
group of the Rocky mountain nucleus, extending eastward along 
the base of the Ouachita group of southern Indian Territory 
and Arkansas to the end of the Appalachians, around which it 
deflected northward via Washington, coinciding with the known 
limits of the Potomac formations. 
At the close of the Comanche epoch another uplift of the con- 
tinent ensued, the ocean waters receded nearl}- to their present out- 
line, and the processes of land degradation were again accelerated. 
Once more the loading of the coastal plain began, and the sedi- 
ments of another great formation were laid down. This was the 
Upper Cretaceous formation or the familiar Meek and Ilayden 
section. Subsidence began again while the ocean receded interior- 
