R. T. Kith — THE TEXAS-NEW MEXICAN REGION. ( -)3 



development of * " :: " '"" Miocene deposits of the Mauvaisse-Terre formation of Ne- 

 braska [White River and Loup Fork] which have yielded such a wonderful profusion 

 of extinct mammalians and chelonians. The Texan strata consist of calcareous and 

 siliceous sandstones, and white, pinkish and grayish siliceous and calcareous marls. 

 The calcareous beds are almost wholly composed of finely comminuted and water- 

 worn shells, chiefly derived from the destruction of Cretaceous strata, and in places 

 abound in fossil hones and plants, usually in a fine slate of preservation. The 

 hones * * -' consist of genera" closely allied to or identical with Titanotheriam, 

 Rhinoceros, Equus, and Crocodilus." 



If these relations between the Llano Estacado and the Washington prairie be 

 true, the great difference in level (there is no appreciable difference in dip) must be 

 explained, and to appreciate this we must first study the large area known as the 

 Rio Grande embayment. 



The Rio Graxde Embayment. 



I have previously explained how the climatic features of all the coastal plain 

 change south of the Colorado or the Guadalupe, and how the great Balcones fault 

 escarpment becomes more arid and generally different. This region southward in- 

 cludes the continuation of the coastal prairie, the Washington prairie, the Timber 

 Belt Eocene, and the Black prairie, and includes all the Rio Grande counties as far 

 west as Val Verde, embracing all of Maverick, Encinal, Duval, Nueces, Webb, 

 Dimmit, La Salle, Starr, Zavalla, Frio, Atascoso, Karnes, Goliad, Refugio, San Pa- 

 tricio, Wilson. Aransas, ( lameron and the southern or eastern portions of Uvalde, 

 .Medina, Bexar and Guadalupe. The 97th meridian, which is accepted as tin- 

 western limit of reliable rainfall, intersects the gulf at Aransas Passand is the east- 

 ern limit of the region: and if reports he true, the lower part of the region. at least, 

 is certainly one of the arid portions of Texas, a drouth of over eighteen months' 

 duration having been recently reported from Hidalago within 100 miles of the coast. 

 The rainfall, however, is much greater toward its interior margin, from Sun An- 

 tonio to Del Rio, where the drouth has not extended. 



This region is in many respects the least studied portion geologically of Texas. 

 its predominant and topographic feature is its generally low altitude, the contour 

 or line of equal altitude of 600 feet, which marks its western margin, making a great 

 deflection westward along the Balcones escarpment and up the Rio Grande to 

 Eagle Pass, and thence hack toward the coast on the Mexican side, constituting 

 a great indentation, as if it had been a hay of the gulf covering the region in 

 comparatively recent time;* and this is, further proved by the great deposits of 

 Pleistocene gravel and conglomerate marking its interior border and indicating 

 late deposition of nt least two formations, and which remains in places over much 

 of the area, though greatly denuded by a still more recent and restricted elevation, 



as seen nearer the Rio Grande valley. I am inclined to believe these sedimenta- 

 tions to he of late Neocene and Pleistocene age ami closely connected with the 

 Llano Estacado and Basin epochs of the northwest. The oldest and furthest in- 

 land of t his debris, visible from San Antonio to Uvalde, is only a thin and incon- 

 spicuous sheet found at the ancient margin of the Edwards plateau. 



The fundamental structure underlying these surface sheets in this v*as1 region is 

 the system of rock sheets from the Eagle Ford i Ren ton i shales (bordering the fault 



* This embayment commenced ul the beginning oi the upper < ret iceous oi Dakota < poch. and was 

 i epeated many times, 



