It. T. HILL — THE TEXAS-NEW MEXICAN REGION. 89 



of well-known Rocky mountain formations. Others consist of coarrse water-worn 

 quartz sand, loosely cemented by a lime matrix, so that they are literally coarse 

 mortar beds. The silt is usually pinkish or light chocolate brown, and Conns a rich 

 agricultural soil when watered. Another typical aspect is known to the Mexicans 

 as the tierra blanca, or white earth. This occurs as strata of white calcareous chalky 

 matter possessing strong hydraulic or setting properties, and usually forms the pro- 

 tecting or cap layers of escarpments. It is composed of sulphate and carbonate of 

 lime derived from the sediments of chalk and gypsum. The tierra blanca is well 

 shown north of Toscosa in the bluffs of the Canadian, in the bluffs of the Palo Douro 

 canyon, and in the railway cuts of the Texas Pacific west of Sweetwater, Nolan 

 county. The surface sheet extends south of the Texas Pacific an indefinite distance 

 on the Edwards plateau. It readies the Rio Grande in Val Verde county, north of 

 Del Rio, and I am inclined to believe that it once covered the whole of the Edwards 

 plateau, and has since been largely eroded. There are closely related features in the 

 neighboring coast regions of Texas and in the Rio Grande embayment. 



The floor of the Llano Estacado, or that portion underlying the above-described 

 cap-rock formation and outcropping as the basal portions of its escarpments, is more 

 complicated hut of great interest in the geologic history of the region, inasmuch 

 as it represents a great baseleveled land which existed prior to the plains deposi- 

 tion. Its conditions and structure can best be conceived, however, by considering 

 the present diversity of formations constituting the earth's surface, sands, clays, etc., 

 and imagining a great subsidence which would reduce these to a common haselevel 

 and spread over the various rocks a sheet of sediments similar to the Llano Estacado 

 formation and the Lafayette of southeastern United States. 



South of the 32d parallel this floor, which becomes the surface by the still Later 

 denudation of the Llano formation, is composed of the rocks of the Comanche 

 series, from the Trinity sands to the ( Japrina limestone mostly, the latter formation 

 constituting by far the greatest area, extending over thousands of square miles in 

 the counties of Midland, Ector, Tom Green, Pecos, Coke, Glasscock, Crane, Upton, 

 Irion, .Menard. Crickett, Sutton, Kimble, Edwards, Val Verde, and Kinney. Toward 

 the northwest this floor was eroded down to the Trinity sands, and even these 

 were worn away over the greater portion of the vast area previous to the plains 

 deposition. 



Tin' remnantal Trinity sands occur beneath the escarpment of the plains along 

 the eastern slope of the Pecos valley, at the Headquarters ranch, east of Eddy, 

 New Mexico, where the limestone and clay beds have completely disappeared. 

 The sand hills of Texas and New Mexico, at the foot of the western escarpment of 

 the plains, are probably in large part remnants of the formation. These sand hills 

 cover hundreds of square miles along the western (or Pecos) escarpmenl of the plains 

 in various counties of Texas and eastern New Mexico. 



Along the northwestern escarpmenl of the plains and along many of the buttes 



and mesas of the Red Liver valleys there is another outcrop of what may also he 

 considered the Trinity sand. There is no evidence of its presence along the entire 

 northeastern quarter in the canyons of the lied and Canadian rivers. Wherever 

 this sand is found it indicates the greal degradation which the pre-Llano Esta- 

 cado deposits have under-one and the important place they occupy in the geologic 

 history of this country. This degradation 18 worthy of especial attention, for 

 it was even greater than that which has taken place since the Llano Estacado 

 deposition. By the Neocene baseleveling an inestimable amount of the Red beds 



XII Rum.. Gi "i . 8qi \ M . Vol.. 5. L891 . 



