R. T. Hill — North American Cretaceous History. 287 



The second Epoch of Subsidence. 



Following this mid-Cretaceous land epoch there was another 

 profound subsidence. This epoch may be said to include 

 geographically, stratigraphically, and paleontologically what 

 was lately known as all of American Cretaceous history and 

 which with slight modifications in correlation, is the section 

 of Meek and Hayden and includes all the Upper Cretaceous 

 of the Northwest, New Jersey and Alabama, except the basal, 

 Tuscaloosa and Potomac beds, in the two latter regions. The 

 paleontologic and sedimental sequence is continuous. In all 

 these regions the grand development of the Lower Cretaceous 

 strata of the Comanche series is missing, and the upper divi- 

 sion rests either unconformably upon the basal littorals, equiv- 

 alents of the Trinity beds, as in New Jersey and Alabama, or 

 upon the pre-Cretaceous rocks of the mid-Cretaceous conti- 

 nent, as in the northwest. In Texas, however, this Upper 

 Cretaceous system, which attains an even greater development 



Fig. 1. Section across the Austin-New Braunfels unconformity, Travis 

 county, Texas, showing disturbances at close of Upper (A.) and lower Creta- 

 ceous (B), and unconformity between these systems. Basaltic extrusion (Pilot 

 Knob) at D. 



than in the typical " Nebraska" region, rests every where uncon- 

 formably upon the Comanche series. The unbroken succes- 

 sion of the formations of this Upper Cretaceous, recorded both 

 in Texas and the northwest by its sediments, is as follows : — 

 (1) sands, (2) clays, shales changing upward into calcareous 

 shales, (3) chalk, (4) chalk marls, (5) sandy marls, (6) sands 

 with littoral fossils indicating a period of slow prolonged sub- 

 sidence and gradual emergence. 



This was the most profound submergence in all Mesozoic 

 time, the Atlantic ocean having extended continuously, as 

 shown by the remarkable identity of sediments similarly sit- 

 uated in relation to the shore line and by its fossils, from 

 British America southward around the Appalachian continent. 

 Its history is similar to that of the lower division — a long con- 

 tinued and gradual submergence, the sedimentation of which 

 is marked by an immense chalk* deposit followed by a gradual 

 transition upward without break into arenaceous littorals. 



* The question of chalk in the North American Cretaceous is fully discussed in 

 my report on the Geology of Southwestern Arkansas. It is sufficient to say 

 that in sections of this basal Upper Cretaceous chalk, kindly prepared by Mr. J. S. 

 Diller of the U. S. Geological Survey, were found almost a repetition of the 

 foraminifera of the European Upper Cretaceous chalks. 



