R. T. Hill — North American Cretaceous History. 289 



morphism. It is evident that the extrusions took place at or 

 after the close of the Cretaceous, and probably belong in the 

 same category with similar phenomena reported at Rockwall 

 in north Texas, in the Chickasaw Nation, and in Arkansas. 



Post- Cretaceous events which have concealed Cretaceous history. 



The inequalities between the two great formations of the 

 Cretaceous have been greatly obliterated by subsequent history, 

 as illustrated in the following section along the Arkansas- 

 Choctaw line, across Little and Bed rivers, wherein can be seen 

 the leveling and concealment produced by an early Quaternary 

 subsidence, which has also worn away much of the mountains. 



Fig. 2. Section forty miles in length north and south, along the Arkansas- 

 Choctaw line showing the sequence of the Cretaceous formations, their relation 

 to the Paleozoic Mountain axis F, and their post-Tertiary degradation by the 

 Quaternary subsidence E. Upper Cretaceous, A ; Comanche series, B ; Trinity 

 formation, C ; Intrusive dike, D. 



The whole Cretaceous history, as seen in the region of its 

 most typical sediments, can be summed up as two profound 

 subsidences, separated by a land epoch. These have left in 

 their sediments two great chalk formations, as shown in the 

 following table. The history of these subsidences has hitherto 

 been confused owing to the fact that the littorals of one of 

 them in regions favorable for study have been mistaken for 

 the whole. Although each of the faunas and sediments of the 

 two formations represents an unbroken series, I have mentioned 

 for each horizon distinguishing species of Ammonites, Ostrea 

 and Echinoderms. 



University of Texas, March 7, 1889. 



