WICHITA FORMATION OF NORTHERN TEXAS 127 



If the upper part of the Carboniferous section of Texas is to be 

 discriminated as Permian, the line of division, as indicated also 

 by the paleobotanic evidence, would probably best be taken at the 

 base of the Wichita. 



An inspection of the faunas collected from the strata immediately 

 concerned in this report shows a rather noteworthy change of facies 

 between the Wichita and the Cisco — a change, however, which is 

 more or less progressive and has its beginning in earlier beds. 

 This shows itself rather in a limitation than in a change of fauna 

 and in the prominence of certain groups more rare below. Thus 

 the brachiopods, pelecypods, gasteropods, etc., are much less in 

 evidence in the Wichita than in the Cisco, but, as already pointed 

 out by C. A. White, 1 they are essentially the same as those of the 

 normal Pennsylvanian fauna. In the Wichita, however, we have 

 a remarkable development of the Cephalopoda, which in the earlier 

 sediments are rare. 



Just what significance faunal changes of this sort possess it is 

 difficult to say. It seems to be a change comparable to that which 

 is more strikingly illustrated when a thin calcareous sheet with a 

 marine fauna occurs in the middle of a coal deposit. Here, of 

 course, there is an absolute change from the animal life of the cal- 

 careous stratum to the plant life of the coal and roof shale, but in 

 this case the significance is not ambiguous and it is clearly not 

 stratigraphic. 2 So I think the faunal change marked by a substitu- 

 tion of one predominating animal type for another may often be 

 more safely interpreted as environmental than as stratigraphic in 

 its import. At the same time the stratigraphic significance may be 

 present also, which would appear to be the case with the Wichita 

 auna, as indicated by the fossil plants. Nevertheless, this change, 

 as marking the evolution from one geologic period to another, would 

 be more impressive if the molluscan and molluscoidean groups 

 were continued into the Wichita and with a difference of facies 

 such as is usually found when the faunas of other systems are con- 

 trasted. 



1 U.S. Geol. Surv., Bull. 77 (1891), 30-39. 



2 1 mean of course that there is usually no time break and no appreciable change 

 of fauna in the general region accompanying the phenomenon. 



