PENNSYLVANIAN STRATIGRAPHY OF TEXAS 29 



have been collected. Altogether more than 150 species have been 

 identified in studies reported by one of the writers/ which show 

 that in spite of a very considerable proportion of species which 

 have not previously been described the fauna is undoubtedly 

 representative of the Lower Pennsylvanian. Forms which are 

 commonly found only in Mississippian strata, as the coral Paleacis, 

 occur in the Marble Falls, but the dominant element of the fauna 

 belongs without question to typical Pennsylvanian species. 



The Smithwick shale is typically a fine-grained, black or olive- 

 green, rather fissile, bituminous formation which rests conformably 

 upon the Marble Falls limestone. It has a total thickness of about 

 400 feet. Excellent exposures are found along Colorado River 

 near Smithwick, Marble Falls, and Bend, but the outcrop is in most 

 places marked by areas of very gentle relief where the shale has 

 been covered by debris from weathering. The drainage, as shown 

 in the course of San Saba River, appears to have become adjusted 

 to the surface geology, the larger streams following the outcrop of 

 the easily eroded Smithwick shale. Fossils are few in the black, 

 bituminous portion of the Smithwick, but in the gray portion a 

 varied fauna has been obtained in which the mollusks are, as might 

 be anticipated, the dominant element. The fauna is related to that 

 of the Marble Falls formation, but contains a number of forms not 

 found in the beds below. 



The Bend group may be correlated with the Morrow group of 

 northeastern Oklahoma and northern Arkansas and with the 

 horizon of the Wapanucka limestone in southern Oklahoma. Not 

 only is there a notable similarity in the faunas of these beds, but 

 a number of species which have been reported only from them 

 appear to be in common. However, Pentremites and Archimedes, 

 which have been found in both the Morrow and the Wapanucka, 

 are not known in the Bend. As has been indicated, the Pennsyl- 

 vanian aspect of the fauna is evident. If the Morrow belongs to 

 the Upper Pottsville, as indicated by plant fossils studied by 

 White,^ it would appear that the Bend belongs to a late portion of 



^ R. C. Moore, loc. cit. 



= David WTiite, U.S. Geol. Surv., Nineteenth Ann. Rept. (1898), Part III, p. 469; 

 Twentieth Ann. Rept. (1900), Part II, p. 817. 



