284: R. T. Rill — North American Cretaceous History. 



The first Epoch of Subsidence. 



Along the southern border of these mountains from the 

 Little Missouri in Arkansas westward to the 98th meridian, 

 thence southward to the Brazos along the eastern border of 

 the Central Texas Paleozoic region, can be seen, resting with 

 a slight dip upon the highly disturbed Carboniferous rocks 

 and beneath the more calcareous chalky sediments of the 

 Fredricksburg Division of the Comanche series, a littoral for- 

 mation which marks the beginning of Cretaceous history in 

 the region and whose beds, as far as the writer knows, are the 

 oldest undoubted Cretaceous in the United States, except what 

 are perhaps its eastward continuation, the Tuscaloosa and 

 Potomac formations of Alabama and Maryland. This forma- 

 tion, as seen in its typical exposures along the Murfreesboro- 

 Ultima Thule road in Arkansas, is composed of several hun- 

 dred feet of variegated sands and clays, resembling in color 

 the Potomac formations as seen in the railroad cuts at Balti- 

 more, and, in addition, then fissile layers of shell-bearing 

 limestone and great beds of gypsum and lignites, associated 

 with a vertebrate and molluscan brackish-water fauna, which, 

 notwithstanding our prejudices against trans-oceanic correla- 

 tions, is unmistakably identical in general lithologic and strati- 

 graphic features with the Purbeck and Wealden of England 

 and Germany. This fauna, in addition to a profuse and un- 

 studied flora, consists of Dinosauridse and brackish water 

 Mollusca including millions of individuals of a few species 

 such as Corbiculidas, Yiviparus, Ostrea Franklini of Coquand 

 and the undoubted Pleurocera strombiformis Schloth., so char- 

 acteristic of the Wealden of Europe and not before found in 

 America. These fossils with a single Ammonite are all indica- 

 tive of its Wealden or transitional Jura-Cretacic age. West of 

 the Paleozoic area of Central Texas, the writer has found only 

 the sediments of this formation, but not its fossils. Its eastern 

 termination is covered by the Upper Cretaceous and Quater- 

 nary. South of the Brazos, as at Austin, its position is occu- 

 pied by a great deep marine chalk formation, now metamor- 

 phosed into hardest marble, which has strong Jurassic affinities. 

 The Trinity formation, as it has been named, can be directly 

 seen underlying the more calcareous and deeper marine beds 

 of the Comanche series at many places, and clearly marks the 

 interior shore line of the oldest American Cretaceous, as well 

 as the beginning of a great subsidence which initiated that 

 epoch and gradually covered the whole of the Texas Paleozoic 

 area. How far the waters of the Atlantic extended southward 

 and westward is yet unknown. Its northern limit was the 

 unnamed mountain system above mentioned ; for none of the 

 lower (Fredricksburg) sediments of this division of the Creta- 



