R. T. Hill — North American Cretaceous History. 285 



V 



ceous have until recently been found north of it. [Since this 

 paper has been prepared for press, Prof. Crogin has noted the 

 occurrence of rocks which belong, in my opinion, to the un- 

 doubted Comanche Series and probably the Trinity.] This sub- 

 sidence, which has been overlooked in previous geological his- 

 tory, was profound and long continued. The evidence of its 

 depth is recorded in the rocks and fossils of the Comanche se- 

 ries, which throughout consist of a deep infra-littoral deposit of 

 chalk with and without flints, impure chalk and chalk marls 

 often hardened into limestone, uniformly extending over wide 

 areas and gradually succeeding the littoral Trinity beds. The 

 thickness of these sediments increases southward, sometimes 

 reaching 2000 feet. At Austin they are over 1500 feet. The 

 evidence of a greater subsidence southward and absence of 

 sediments northward indicate a continental condition in the 

 latter region during Jurassic time and the possible continuation 

 of deep sea conditions during that period in southern Texas 

 and northern Mexico — a possibility which, as will be shown in 

 another article, may be a fact, as indicated by an nn described 

 system of rocks in those regions. 



The long continuation of this subsidence is well shown by 

 its fauna. First, by the remarkable uniformity in the distribu- 

 tion of its successive horizons. The fauna of the Washita 

 limestone horizon, in the section at the close of this paper, is 

 almost identical at El Paso and at the Arkansas-Choctaw line, 

 some 900 miles apart. The horizon of the remarkable and 

 unique Exogyra arietina clays extends from Indian Territory 

 to Presidio del Norte nearly 500 miles, with no perceptible 

 variations in the outcrops. The long continuation of this sub- 

 sidence is also shown by the gradual change which the species, 

 a large number of which are identical with European Creta- 

 ceous forms, underwent without sedimental break. The spe- 

 cies of Echinodermata, Ostreidse, Gasteropoda, etc., of the 

 Fredricksburg division are replaced in the Upper or Washita 

 limestone by other forms of the same or allied genera, so simi- 

 lar in some predominant feature and at the same time so 

 specifically different as to clearly show a line of progressive , 

 evolution in this epoch." The time of this subsidence, as 

 shown by paleontological evidence was Neocomian and Middle 

 Cretaceous. It is also shown from its absence that this subsi- 

 dence was not so extensive along the margins of the Appala- 

 chian regions. In fact, there is some circumstantial evidence 

 that its northern shore limit, a portion of which is preserved 

 to us in the Arkansas-Indian Territory orographic remnant, 

 must have continued eastward without deflecting northward, 



* The writer has in press a complete revision of the species of this division 

 which will contain further mention of this fact. 



