1893.] NATURAL SCIENCES OF PHILADELPHIA. 319 



(e) Post Cretaceous Changes. — With the close of the Cretaceous 

 period of deposition there began the mountain building work which 

 culminated in the formation of the Rocky Mountains and to which 

 the present form of Texas is in large measure due. In the 

 the East it caused the bodily uplift of the great Cretaceous plain, 

 the remnant of which forms the Grand Prairie. An extensive fault, 

 the Balcones fault of Hill, occurred possibly at this time, and is at 

 present an important feature in the topography of eastern Texas. 

 It is, however, such a striking feature and so well marked that it more 

 probably occurred during the Tertiary or possibly even during the 

 Quaternary uplift, to be mentioned presently. 



The Rocky Mountain uplift which formed the plateaus and plains 

 of central Texas, by a bodily uplift more pronounced in the west, 

 took, in the Trans-Pecos region, the form of mountain building. 

 Little is known of these mountains, but the Guadalupe Mountains 

 which the author has studied, are monoclinal in structure, faulted on 

 the western side. Others are more complicated, and some are asso- 

 ciated with igneous eruptions or intrusions. Little is known about 

 this region other than that it is a mountain range and basin region 

 in which Tertiary lakes lingered until the period of Quaternary des- 

 si cation caused them to disappear. 



A period of sedimentation following the Cretaceous uplift admit- 

 ted of the deposition of the Tertiary clays and sands, which, by their 

 elevation, added another strip to the Texas region, that which Prof. 

 Hill has designated the Eocene Lignite or Forest region. Again, 

 in Quaternary time, another strip was added, and the Coastal Prairie 

 is so recent that shells now living in the Gulf are enclosed in the 

 strata as fossils. A slight submergence of verv recent date is indi- 

 cated by the fjorded character of the streams on the Texas coast. 



The Llano Estacado plateau is said by Prof. Hill 15 to be capped 

 by lacustrine sediments of late Tertiary or Quaternary age and this 

 forms another chapter in the history of Texas, but we have few 

 details in regard to this most interesting episode. 



(/) Post Cretaceous Drainage. — It has been my effort in the pre- 

 ceding pages to trace as briefly as possible the geological history 

 of the development of the Texas region — the steps by which it has 

 come to assume the present form. The minor details of the land 

 sculpturing as revealed by the river histories registered in the topo- 

 graphy remain to be considered. A few general points on the sub- 



15 Am. Geo!., V, 1890, pp. 27-2U. 



