94 PROCEEDINGS OF WASHINGTON MEETING. 



escarpment from San Antonio t<> Uvalde) on its interior margin to the coastal 

 prairies and clays at the coast, with slight variations from the same beds seen in 

 Texas north of the Guadalupe. This includes a great thickness of unconsolidated 

 beds. Succeeding the chalks and clays which overlie them, there is a great de- 

 velopment of sand and sandstones in the glauconitic divisions of the upper Cre- 

 taceous which here is quite different paleontologically (owing to the different con- 

 ditions of original sedimentation in this Rio Grande embayment) from the Arkan- 

 sas-New Jersey development. These uppermost Cretaceous beds, for which I have 

 proposed the name of Eagle 1'ass beds, outcrop from west of Eagle Pass to the 

 Webb county line along the Rio Grande, and occur all over the embayment as far 

 southward as the Santa Rosa mountains in Coahuila, constituting its predominant 

 formation. Succeeding these are various beds of the Eolignitic, Fayette (Neocene) 

 and coast prairies; the Fayette corresponding at least in part to the Lafayette, and 

 the coast prairies to the ( olumbia, of Mc< ree.* 



This embayment is a structural feature and primarily the product of an otogenic 

 event associated with the Rocky mountain uplift, which began in the late Cre- 

 taceous time and reached its culmination after the close of the Mesozoic, and is 

 distinctly recorded in the conspicuous features of the Balcones fault and the 

 mountains of northern Mexico. Its further development is a record of subsidence 

 and elevation from the above-mentioned epoch to the present time, during which 

 the shore line projected and retracted toward the present coast, with changes of 

 baselevel, to interpret which will require much study. 



This orogenic movement was the faulting and folding of the great floor of hori- 

 zontal chalky limestones of the Comanche series, which extended as an almost 

 uniform dip plain (like the present portion between Red and Colorado rivers) from 

 the Ouachita mountains of Indian Territory to central New Mexico. The move- 

 ment resulted in the folding, metamorphism and consolidation of the rocks of the 

 southwestern portion of this plain in Coahuila and trans-Pecos Texas, and pro- 

 duced lines of weakness which, by the loading down of the Tertiary and Quater- 

 nary plains, developed into the great Balcones fault, extending at right angles to 

 the axes of the Coahuila mountain blocks from Del Rio via Uvalde, San Antonio, 

 Austin and Round Rock, a distance of 200 miles. This fault was fust pointed out 

 by Professor E. 1'. Cope, and is one of the most conspicuous features of Texas. It 

 was the downthrow of this fault that constitutes the Texas margin of the Rio 

 Grande embayment, and along the escarpment line are great deposits of littoral 

 and estuarine gravel and river terraces, which are the records of the late Tertiary 

 and Pleistocene baselevels. The summit or plateau west of this fault line has been 

 already mentioned as the Edwards plateau. 



Upon the opposite or Mexican side, beyond the valley of the Rio Grande, an 

 analogous condition exists, the great difference being that the plateau, which in 

 Texas extends inward from the interior margin, is there broken up into mountain 

 blocks and is completely surrounded in some cases by the Pleistocene deposits. 



Around the margin of the interior of the embayment there are evidences of 

 igneous activity, consisting of volcanic necks on the Texas side, the flows from 

 which, if they ever existed, having been destroyed by post-Tertiary erosion. In 

 the Sabinas valley of Mexico fragments of the flows are preserved, but show 

 Pleistocene degradation on every side. 



It is my present opinion that the great fault separating the Edwards plateau 



*12th Ann. Rep. U. S. Geol. Survey, 1892, i*. 502. 



