30 RAYMOND C. MOORE AND FREDERICK B. PLUMMER 



the Pottsville. On account of the unconformity and pronounced 

 change in the character and the considerable thickness of the 

 succeeding Strawn strata which appear certainly to be not younger 

 than Allegheny, the writers are inclined to refer the Bend to a 

 somewhat earlier position in the Pottsville, possibly as indicated 

 by Ulrich,^ the Lower Pottsville. It may be noted that some of 

 the Bend fossils, as among the cephalopods Paralegoceras iowense, 

 which are reported from other portions of the American Pennsyl- 

 vanian, do not appear to be identified correctly with these species, 

 and their significance in correlation is therefore not so important. 



STRAWN GROUP 



The Strawn group includes all the strata between the top of 

 the Smithwick shale and the base of the Palo Pinto limestone in 

 the Brazos River Valley or its stratigraphic equivalent in the 

 Colorado River Valley. The rocks of this group are distinguished 

 chiefly by their clastic character, especially the thickness of coarse 

 sandstones, and by their irregularity in bedding (Fig. 4) . The two 

 main areas of Strawn outcrop, one in the valley of Colorado River 

 and the other in the valley of the Brazos, are broadly similar, but 

 it has not been possible to identify divisions of the one in the other. 

 In the northern area there are exposed in the upper part of the 

 Millsap division a number of beds of limestone which are found 

 nowhere to the south, from which it appears that the waters of the 

 Brazos River Valley were farther from the land of Strawn time 

 than those of the Colorado. The entire section of the Strawn is 

 observable along Colorado River, but in the Brazos Valley a 

 considerable thickness of beds belonging to the lower portion of 

 the Strawn are not exposed on account of the Cretaceous overlap 

 from the east. In both areas there is indication of a very marked 

 overlap of the Strawn from east to west, the successively younger 

 strata of the group extending farther to the west than the older. 



The Strawn of the Colorado River Valley lies in the area studied 

 by Drake.^ In his work the alternating subdivisions of sandstone 

 and shale are differentiated in some twenty units which he termed 



^ E. O. Ulrich, U.S. Geol. Surv., Prof. Paper 24 (1904), p. iii. 

 2 N. F. Drake, op. cit., pp. 375-89. 



