734 TRANS-PECOS TEXAS. 



LOWER CROSS TIMBER OR DAKOTA DIVISION. 



The resemblance of the rocks of this division to those of the Fish Bed 

 (which is the same as the Eagle Ford shale or Benton division in North 

 Texas) division of the Upper Cretaceous south of the Brazos River is very 

 marked. In each fissile yellow clays and flaggy, calcareous, argillaceous 

 sands prevail. They often become dark and decidedly sandy, highly charged 

 with organic matter. The clay shale and flaggy siliceous limestone is consid- 

 ered to belong to this division. There is no stratigraphic break between this 

 and the shale overlying the siliceous limestone, and there was nothing found 

 that makes a gap wider than exists between the Lower Cross Timber sands 

 and Eagle Ford shale in the Trinity or Brazos River section. 



In the Brazos River section, very near the division line between the Lower 

 Cross Timber sand and the Eagle Ford shale, there occurs a horizon of sili- 

 ceous limestone carrying an oyster closely resembling the specimen found 

 here in the siliceous limestone horizon. 



The Dakota division separates into four horizons, as follows: 



1. Calcareous flaggy sandstone, with oyster shell fragments 60 feet. 



2. Fissile dark shale, grading upward into arenaceous shale, with bands of argil- 



laceous sand 220 feet. 



3. .Fissile brown shale 50 feet. 



4. Cream-colored to brown fissile calcareous shale 30 feet. 



Total 360 feet. 



EAGLE FORD OR BENTON SHALE DIVISION. 



The lower portion of this division above the arenaceous limestone is mate- 

 rially the same as No. 1 of the Dakota division below the same limestone. 

 Small Inocerami occur in the upper portion of the bed. The upper bed of 

 the division includes the most extensive development of the series. There is 

 four hundred feet of homogeneous fissile black clay shale. Numerous indi- 

 viduals of a small Inoceramus and a small fragile G-ryphea were observed. 



Lithologically, the Eagle Ford or Benton Shales separates into three beds: 



1. Very fissile, black, slightly arenaceous clay shale, with numerous individuals 



of a small Inoceramus and a fragile oyster or anomia 300 feet. 



2. Flaggy, fissile, calcareous, argillaceous sandstone, with numerous oyster shell 



fragments and an Inoceramus at the upper edge 430 feet. 



3. Siliceous limestone with oyster 30 feet. 



Total thickness 760 feet. 



Higher rocks have been removed by eruptive porphyry of Eagle Moun- 

 tain. 



On account of the isolated areas into which the stratified rocks of El Paso 



