Geograghic Features of the Texas Region. — Hill. 75 
and the Plateau Gravel respectively — and prevented their 
extent northward into Kansas, thereby differentiating the 
Texas region from all the rest of the United States. 
The main portion of the system, however, has remained dry 
land throughout the incalculable period of time which has 
elapsed since its uplift, in common, we will assume, with the 
Appalachian uplift, gradually yielding to the wear and tear of 
atmospheric erosion, including the numerous climatic changes. 
When we consider the enormous denuding effect of these 
agencies acting through even short periods of time, it is justly a 
subject of wonder that any portion of theOuachita system should 
have survived it. The atmosphere, however, has not been the 
only agent in accomplishing the obliteration of this once con- 
spicuous mountain system, for, as above mentioned, it has 
been the shore line, during all this time, of numerous oceanic 
invasions, accompanying the subsidences and elevations, as 
recorded in the sediments deposited against it. The evidence 
of the excessive destruction of the southern front of the sys- 
tem from shore action is plainly recorded. In a previous 
paper* I have shown beneath the Tertiary or Quaternary 
overlap near the Arkansas-Choctaw line, the record of many 
miles of this planing away of the mountain folds by shore 
action, and if I may be excused for advancing a hypothesis, 
I will state that it is m}^ belief, founded upon reasonable 
evidence, that in the Eo-Lignitic epoch, the whole eastward 
continuation of the system was destroyed by combined atmos- 
pheric and shore action, producing the topographic area now 
known to geologists as the Mississippi embayment. 
The chief agent, however, in destroying the topographic integ- 
rity of the original Ouachita system has been neither erosion 
nor shore action, but subsidence, for alarge and important south- 
western extension of the system, occupying the central por- 
tion of Texas was virtually buried beneath the profound sub- 
sidences of early Mesozoic time, and especially during that 
important epoch of the Comanche series which has been so 
little appreciated in American geology. Evidence of this 
burial is afforded in the Central Denuded region of Texas, 
where the imbedding strata have been sufficiently destroyed 
to afford some testimony upon the subject as will be shown 
later. 
* See American Journal of Science, April, 1887. 
