18 University of Texas Bulletin 



limestone which may be either Grayson or Buda, probably the latter ; this 

 underlies the Woodbine (Dakota) sandstone. The Kent section contains 

 equivalents of Weno and Pawpaw in division 3 of Durable and Cummins 1 

 section, which however does not permit of more precise definition. 



A north-south line drawn two miles east of Roanoke, Denton County, 

 therefore delimits the marl facies of the Grayson from the transitional beds 

 to the limestone facies, since east of this line the Grayson becomes calca- 

 reous. This line however probably turns east since on going down the Red 

 River valley it leaves the marl facies of the Grayson to the north at seen at 

 Denison, Bennington and numerous other places in Southern Oklahoma. 

 What relations exist east and south of these points depends on well records 

 not yet investigated. South of the Brazos River near Waco there is at the 

 Grayson outcrop a transitional zone to the southern clay facies (zone of 

 Gryphea mucronata, upper half of the Del Rio Clay) which continues west- 

 ward to the El Paso region. The sand facies is unknown but is to be ex- 

 pected in the removed areas of North Central Texas, in the isolated Upper 

 Washita remnants of the Panhandle not yet investigated, and in the Tu- 

 cumcari region. 



Limestone facies: 



The limestone facies, on the other hand, lying southwards in Sonora and 

 Chihuahua is still unidentified in Texas, except at one point, the Mariscal 

 Mountains, at the south tip of the Big Bend region. 2 



MAINSTREET 

 Sand facie*: 



At Cerro de Muleros, near El Paso, Texas ; apparently near Santa Rosa, 

 New Mexico (Dr. Bb'se) . 



Clay facies (Del Dio Clay, in part) : The Del Rio Clay is poorly known, 

 and therefore its correlation is tentative. It is equivalent to the Grayson 

 and the upper part of the Mainstreet, and contains the following fossil 

 levels : 



(1) Nodosaria texana Conrad. A zone of abundance above that of 

 Exogyra cartledgei Bose 3 in the upper 30 feet of the formation, which at 

 Shafter reaches a thickness of 190 feet. Below this zone of abundance 

 Nodosaria is scattering. 



(2) Gryphea mucronata Gabb. Upper half of the Del Rio Clay. 



(3) Exogyra anetina Roemer. Rare near the middle of the clay in 



tumble and Cummins: Amer. Geol., xii, 1893, 309. 

 2 Udden, Baker and Bose: Univ. Texas Bull. 44, 1919, p. 76. 

 3 B6se, Univ, Texas Bull, 1902, 



