THE CENOZOIC DEPOSITS OF TEXAS. 553 



usually coarse, angular to rounded in shape, forming sandstones 

 of variable degrees of hardness, highly quartzitic in places, and 

 cemented by an opaline matrix at others. Large quantities of 

 opalized wood occur, and chalcedony is abundant, especially in 

 the southwest, where it forms the centers of geodes, the septa of 

 septaria, and even fills crevices in the sandstone. Beds of vol- 

 canic dust and siliceous sinter also occur interbedded with the 

 clays and lignites. In the Nueces valley cone-in-cone structure 

 is widely developed, and considerable aragonite occurs in the 

 basal portion of the bed. Many of the clays are white, and of 

 sufficient purity to be valuable for the manufacture of the finer 

 grades of earthenware. The beds of lignite are, for the most 

 part, small and unimportant. 



There is no sharp line of demarkation between these beds 

 and the Yegua clays below, but the change in the character of 

 the sediments has caused a corresponding change in the topog- 

 raphy. The gently rolling area of the Yegua clays is bordered 

 on the south by a disconnected range of hills, whose northward- 

 facing scarps and bluffs (often 150 feet in height) can be traced 

 from Rockland, on the Neches, westward, by Riverside, Mul- 

 doon, and Tilden, to the Rio Grande. Southward from this 

 scarp the descent is more gradual. The influence of these beds 

 of sandstone on the course of the rivers which cross them is 

 very marked, producing a sharp east or northeast deflection, such 

 as that of the Trinity on the northern boundary of Walker 

 county. 



While the Fayette beds of the eastern part of the State are 

 almost without invertebrate fossils, so far as determined, the 

 fauna increases toward the Rio Grande, and on that river includes 

 large beds of immense oysters. The forms specially character- 

 istic of it are : 



Ostrea alabamiaisis var. contracta Con., Siliqua simondsi Har., 

 Ceronia shigleyi Har., Cormdina armigera var. lieilpriniaua Har., 

 Cerithium pliciferiuji H e i 1 p . 



It is connected with the underlying beds by such forms as 

 Nucitla magnified Con., Venericardia planicosta Lam., Corbida ala- 



