28 RAYMOND C. MOORE AND FREDERICK B. PLUMMER 



chus carboniferum is reported from the Moorefield shale of northern 

 Arkansas, in the Floyd shale of Alabama, and elsewhere in beds 

 which are referred to the Upper Mississippian. Accordingly Girty^ 

 regards the Barnett as late Mississippian in age and therefore 

 distinct from the remainder of the Bend which is undoubtedly 

 Pennsylvanian. Goldman,^ studying samples of cuttings from two 

 wells north of the outcrop, believes that certain lithologic characters 

 differentiate the lower shale, and that it is separated by a strati- 

 graphic break from the overlying beds. Through the courtesy of 

 the United States Geological Survey, Mr. Moore has recently had 

 opportunity to examine particularly the contact between the 

 Barnett and the succeeding Marble Falls divisions. It has been 

 found that although the characteristic Barnett fauna is obtained 

 in a limestone which apparently marks the base of the Marble 

 Falls, there is really a sharp line of faunal demarcation between 

 this and the Marble Falls divisions which is also distinguished by 

 a thin zone of glauconite and phosphatic pebbles. The fauna of 

 the Barnett is unlike that of the succeeding beds and resembles 

 most closely that from formations which are regarded as Upper 

 Mississippian. The Barnett shale is therefore tentatively referred 

 to the Mississippian rather than the basal Pennsylvanian where it 

 was placed in the previous correlation of the writers. 



The Marble Falls limestone is a massive, resistant formation 

 which is well exposed throughout the region of its outcrop. It is 

 somewhat irregularly folded and faulted, and because of the lack of 

 continuous exposures or readily identifiable horizons within the 

 formation it is difhcult to measure an accurate section of the unit 

 as a whole. Its total thickness in the region of its outcrop, how- 

 ever, appears to be about 400 to 500 feet. In the surface exposures 

 no sandstone and practically no shale is observed in the Marble 

 Falls formation; but to the north well records show the occurrence 

 of some shale interbedded with the limestone and also the presence 

 of some limestone in the succeeding Smithwick shale. The Marble 

 Falls limestone is but sparsely fossiliferous in some exposures, but 

 in parts of San Saba County numerous beautifully preserved fossils 



' G. H. Girty, Bull. Amer. Assoc. Petrol. Geol., Vol. Ill (1919), p. 72. 

 "M. I. Goldman, U.S. Geol. Surv., Prof. Paper isg-A (1921). 



