COMPARISON OF SECTIONS. 319 



in that region. They appear in the Cordilleran region of trans-Pecos, 

 Texas, and Mexico, as I have previously shown. 



COMPARISON OF THE A US TIN AND DENISON SECTIONS. 



Austin Section. — In order to understand the variation of the Washita 



division seven typical sections, at widely distributed localities, are shown 



upon plate 13. As a basis of comparison with the Red river section 



there may be taken the Austin section, which is as follows : 



f Shoal creek limestone. 

 I Exogyra arietina beds. 

 Washita division \ C Kingena wacnensis bed. 



I Fort Worth limestone < Gryphnea pitcheri bed. 

 t (old usage) ( Fort Worth limestone proper. 



Here the beds of the Washita division constitute the three simple, con- 

 spicuous lithologic and paleontologic members; the lowest of them are 

 alternations of slightly arenaceous limestones and chalky, laminated 

 marls -occurring in alternations and marked by a conspicuous molluscan 

 fauna, to be enumerated later. Succeeding this lower limestone group 

 is a great bed of unctuous green clay some 80 feet in thickness. This 

 clay is in turn succeeded by a peculiar limestone, to which I have given 

 the name Shoal creek limestone. 



Shoal Creek Lijnestone. — This has its characteristic exposure, north of 

 the Colorado, in the beautiful scarps of Shoal creek, in the city of Austin 

 and in the bluffs. on the south side of the river, where Bouldins creek 

 enters the valley, at the crossing of the International railroad and the 

 Oatmanville road. It forms precipitous cliffs, with top[)ling abutments? 

 owing to its jointed structure and the undermining of the clays beneath, 

 and is the principal scenic feature of Pease park in Austin. 



Its outcrop is whitish yellow, with blotches of pale pink, as if it had 

 been subjected to fire, and is much darker in exterior color than the 

 other limestone strata of the Colorado section. In ]:)laces it is very 

 hard, Init of varying texture; sometimes soft and efflorescent in spots, 

 the I'ocks deca3ang into a soft, pulverulent material with slightly saline 

 taste. 



The red and pink blotches in this limestone are peculiar, and have 

 given to it the local name of " burnt limestone." The writer once sus- 

 pected that these spots were derived from the oxidation of intrusive or 

 interl)edded igneous material, like that found in certain portions of the 

 adjacent Austin chalk ;''^ but recent microscopic study has revealed the 

 fact that the limestone is made up largely of foraminifera, filled and 

 coated with a mineral which in all probability is glauconite, but in such 

 small quantities that its oxidation only results in faint discoloration. 



♦Pilot Knob -A Marine Cretaceous Volcano: American Geologist, November, 1890, and Pre- 

 liminary Check-List of Invertebrate Cretaceous Fossils, pp. xxiii, xxiv. 



