CHOCTAAV AND GKAYSON TERRANES OF THE ARIETIXA. 4i3 

 THE GRAYSON MARLS. 



The Choctaw limestone is couformably succeeded by the 

 Grayson marls, passing up into them throujjfh a horizon of 

 sometimes quite abrupt, sometimes more i>;radual transition, 

 the marls bein<2: uncouformably overlaid by the Dakota sand- 

 stone. In their northeasterly exposures, the Grayson are 

 chiefly yellow, highly calcareous, sparingly arenaceous marls, 

 of such consistency as to crumble readily under the hand or 

 break into rough lumps under light blows of the hammer, 

 T)ut which contain, especially in the lower parts, irregular 

 concretionary tracts of cement-like hardness and texture. 

 They yield rapidly to the weather and are hence often absent 

 or reduced in thickness by erosion, both Predakota and 

 recent. Again they occur only as slopes, grass-grown or 

 mantled with debris from the overlying Dakota. In Denton 

 county the Grayson terrane consists of alternations of marl 

 and limestone,* and attains a thickness of thirty to forty 

 feet. In Travis county it contains, in the upper part, beds of 

 hard limestone charged with GrypJura mucronafa, and there, 

 as elsewhere south of the Brazos, it is overlaid conformably 

 by the Shoal creek limestone, into which it seems to graduate 

 through the basal part of the latter. 



In Northern Texas, where the thickness of the Arietina is 

 reduced, the Grayson marls have a thickness of fifteen to 

 twenty-five or thirty feet. 



A typical locality of the calcareous yellow marl phase, 

 showing its basal transition to the underlying limestone, may 

 be seen at the cut and fill of tlie once projected D. B. & N. O. 

 railway, between Pawpaw creek and a right-hand branch of 

 the same immediately southeast of the quarry above cited as 

 typical for the Choctaw limestone. 



The Grayson marls were considered by TafP to be the 

 equivalent of the Shoal creek, or Vola, limestone of the 

 Austin district, and were referred to by him as the Vola 

 limestone and marl in his second "Report on the Cretaceous 

 Area North of the Coloratlo River," which was published in 

 the Fourth Annual Report of the Geological Survey of Texas. 



* See Taff's Elm creek section, in Fourth Ann. Rep. Geol. Surv. of Texas, Pt. I, p. 280. 



