128 CRETACEOUS ROCKS OF TEXAS. 



a peculiar Panopcea. This limestone has been extensively quarried for build- 

 ing stone, and can be seen in the foundation of the postoffice building at 

 Austin. It also receives a handsome polish, and has been used for orna- 

 mental purposes. 



THE WASHITA OR FORT WORTH LIMESTONE. 



Resting upon the upper Caprotina limestone (whether comformably or not 

 has not been determined) commences the Washita limestone — one of the 

 most important beds of the Comanche series. This consists of a compara- 

 tively massive, chalky, fossiliferous limestone. The base and top are compact 

 and the middle more disintegrated. It consists of impure chalky limestones, 

 shell breccia, and calcareous marls in alternating strata, having the same gen- 

 eral aspect upon weathering as the Comanche Peak beds. Lithologically it 

 seems to represent a similar depth of deposition.* 



In its upper beds, however, the Washita or Fort Worth limestone, especially 

 in North Texas, begins to show shallower conditions. At Austin, as can be 

 seen in the railroad cut west of the city, it terminates in a comparatively mas- 

 sive lime stratum with numerous individuals of the only Brachiopod species 

 thus far discovered in the Lower Cretaceous series of Texas, to-wit, Terebratula 

 wacoensis, Roem. The fossils in the Washita limestone show a tendency to 

 occur in zones, which persist over vast areas, and about the same species 

 characterize each zone wherever the writer has observed them. These lime- 

 stones are well exposed at Salad o and Fort Worth, the latter city being situ- 

 ated directly upon them. They contain more clay in the intervening layers 

 in the latter vicinity, however. 



THE EXOGYRA ARIETINA CLAYS. 



In Shoal Creek, at Barton Springs, near Round Rock, and other places 

 in the vicinity of Austin, the Terebratula wacoensis horizon of the Washita 

 limestone is surmounted by about eighty feet of unctuous laminated clays 

 of a greenish blue color previous to long exposure to the elements, and 

 dirty yellow afterwards. The lower half of these clays is filled with the 

 small unique Exogyra arietina or ram's horn oyster, which occurs in no other 

 known horizon in the world. There is no transition between these clays 



* Accompanying this return of conditions is an excessive abundance of life of great generic 

 resemblance to the Comanche Peak fauna, but, with the exception of Gryphcea pitcheri and 

 Neithea texana, of entirely different species. In place ol the small Toxaster texanus of the 

 Comanche Peak, we have the large Macraster elegans, Shumard, Roemer; for the beautiful 

 Exogyra texana there is substituted the similar but larger E. sinuata; while the Ammonites 

 leonensis has superseded A. pedemalis. Here, too, the Gryphcea pitcheri (type, var.) breccia, 

 with E. texana, has its duplicate in a breccia composed of G. washitaensis accompanied by 

 Ostrea carinata. 



