16 University of Texas Bulletin 







the region between the Brazos and the Colorado Rivers. South and west of 

 this region the Rudistid facies invades the Fredericksburg division, begin- 

 ning with the level representing the top of the Edwards limestone (see page 

 24). The Rudistid facies is characterized by considerable thickness of 

 indurated massive limestone, so that southward as this facies vertically 

 invades the stratigraphic column the invaded formations thicken, Rough 

 estimates of the thickness of this division are: Red River (Denison, 50 

 feet ; Goodland, Oklahoma, 25 feet) ; Trinity River (Fort Worth), 242 feet; 

 Colorado River (Austin), 350 feet. And for the Washita division: Den- 

 ison, 541 feet; Fort Worth, 374 feet; Austin, 220 feet. 



In the following table of approximate thicknesses it should be noted that 

 the basal Comanchean deposits are transgressive northwards in such a 

 manner that we do not know with certainty what part if any of the Base- 

 ment Sands of Cooke, Grayson and Denton Counties is of Trinity age. The 

 lowest determined Comanchean fossils in this area indicate the level of 

 Exogyra texana and its varieties of Walnut age. In the Duck Creek forma- 

 tion of the table the "marl" is taken to be the portion above the Mineralized 

 Ledge 1 (Stratum 17) of the Fort Worth region, i. e., above the horizon of 

 abundance of Scaphites worthensis. 



LITHOLOGICAL CHANGES 



The lithology of the Washita division is diverse and each formation 

 must be considered separately. However, it is generally true in North- 

 Central Texas that the formations decrease in thickness southwards to 

 the Brazos-Colorado divide south of Waco, Texas ; these changes in thick- 

 ness and lithology may be considered as a unit, and are related to the 

 depositional conditions in the large North-Central Texas trough already re- 

 ferred to. Crossing the Brazos-Colorado Divide in which the thinned Penn- 

 sylvanian and Ellenburger and pre-Cambrian floor are structurally high, 

 there is in the Cretaceous another set of thickness changes which for cer- 

 tain formations, notably the Buda, amount to a regional increment in 

 thickness as far south as the turning point of the Washita outcrops in 

 Bexar County. Westward from this point there appears a little known set 

 of changes in thickness and lithology of these formations. In West Texas 

 again, the northern, near-shore facies (Cerro de Muleros, Finlay Moun- 

 tains, Kent, Sierra Blanca) passes quickly into the southern, massive 

 limestone, in part rudistid facies (Shafter, Terlingua, Fort Stockton, Shef- 

 field) and the Texas and Pacific Railway is approximately the boundary 

 line between these two facies. The transition zone from the Edwards 



1 Winton and Adkins: The Geology of Tarrant County, Univ. Texas Bull. 1931 

 pp. 42-43. 



