26 RAYMOND C. MOORE AND FREDERICK B. PLUMMER 



Pennsylvanian along a number of selected lines were compared, and 

 data gathered from mapping and evidence from invertebrate fossils 

 were considered. The resulting classification of the sediments is 

 shown in the following table. 



BEND GROUP 



The strata included in the Bend group are exposed in northern 

 San Saba, McCuUoch, western Lampasas, and Burnet counties, 

 areas bordering the Central Mineral Region of Texas. The group 

 is named from McAnnelly's Bend in Colorado River (Fig. 3). It 

 consists of three formations, the Barnett shale at the base, the 

 Marble Falls limestone in the middle portion, and the Smithwick 

 shale at the top. The total thickness in the region of outcrop is 

 about 850 to 900 feet. 



The Barnett shale, named from springs east of San Saba, is a 

 yeUowish-gray to black, bituminous shale ranging in thickness up 

 to 50 feet, the average being about 30 feet. Its outcrop forms a 

 narrow, smooth pathway between the rough, broken terranes of 

 limestone on either side, the Ellenburger (Ordovician) below and 

 the Marble Falls above. This pathway many of the roads in San 

 Saba County follow. Though represented in numerous well rec- 

 ords north of the outcrop, the Barnett is not found ever5rwhere 

 at the base of the Bend group, as at the type locality of the Marble 

 Falls limestone on Colorado River at Marble Falls. Where the 

 Barnett is absent the base of the succeeding limestone commonly 

 appears to be somewhat conglomeratic, containing debris evidently 

 derived from the subjacent Ordovician limestones. The Barnett 

 shale is not very fossiliferous, except locally where certain marly 

 beds contain numerous fossils. The chief distinguishing features 

 of the fauna are the goniatites Glyphioceras cumminsi and G. incisum 

 and the brachiopod Liorhynchus carboniferum. The cephalopods, 

 which were first described by Hyatt, Smith^ referred to the Euro- 

 pean species Goniatites striatus and G. crenistria. Girty^ included 

 G. striatus in the synonymy of G. choctawensis, but it appears that 

 the Bend fossils are most probably distinct from these. Liorhyn- 



' J. P. Smith, U.S. Geol. Surv., Mon. 42 (1903), pp. 68, 80. 

 2 G. H. Girty, U.S. Geol. Surv., Bull. 430 (1911), p. 97. 



