1893.] NATURAL SCIENCES OF PHILADELPHIA. 329 



remained at least until the close of the Eocene, possibly to the begin- 

 ning of the Quaternary. Such a lapse of time must have been 

 represented by erosion amounting almost if not quite to base-level- 

 ling. It is moreover probable that the interior of Texas was very 

 different from the present as regards geography. Portions of it 

 were beneath the sea or in an inland sea condition. This applies par- 

 ticularly to the Trans-Pecos region and the Staked Plains. It was 

 also, in all probability, much lower than at present, even more than 

 seven hundred feet lower, for as yet the growth of the Rocky Mount- 

 ains had hardly begun. It was probably this growth which raised 

 the Tertiary above the sea, this elevation being represented by 

 seven hundred feet or more at the margin of the Tertiary sea. Since 

 the elevating force was to the west the elevation consisted in a tilt- 

 ing, the elevation increasing in the interior. This is shown by the 

 dip of the beds of the Cretaceous and Tertiary. 



If this line of argument is correct the interior of Texas was lower 

 than at present by more than a thousand feet, hence the land to be 

 base-leveled was less both in thickness and in areal extent. This 

 renders base-levelling of Tertiary times more probable. An exami- 

 nation of the field with this possibility in mind ought to show evi- 

 dence of this condition. The only field evidence corroborative of 

 this hypothesis which I am able to give is a point which is very 

 noticeable in the region which I have studied in some detail, namely,, 

 the Carboniferous. The rivers in this section, though young at 

 present and consequently very rapid in flow, instead of pursuing 

 direct courses, follow rock walled channels remarkable for their 

 meandering. This is strikingly shown in the Brazos and the Colo- 

 rado in the region under consideration. It seems difficult to explain 

 these meandering courses upon any other supposition than an 

 inheritance from a former plain. 



I believe that further physiographic studies of the Texas region 

 will show that at the close of the Cretaceous there was an elevation 

 due to a recurrence of the mountain growth of the old buried 

 mountainous areas of central Texas, which raised a portion of the 

 Cretaceous above the sea. This portion, now known as the Cen- 

 tral Denuded area was a moderately low land, which, during Ter- 

 tiary times, suffered extensive denudation, becoming a peneplain. The 

 Rocky Mountain elevation added to this area the plateau to the 

 west and the Tertiary to the east, and by rejuvenation gave the 

 streams new life and new tasks with which they are now wrestling. 



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