MESOZOIC TIME — CKETACEOUS. 817 



Western Gulf border. — In Texas, and to the north and northeast in 

 Indian Territory and Kansas, west and northwest in New Mexico, and west 

 and southwest in Mexico, the Lower Cretaceous beds are mainly marine. 

 They are the Comanche series of R. T. Hill. They have fresh-water beds at 

 bottom, but consist above largely of thick limestones, which are partly chalk. 

 They abound in fossils, and indicate, for the most part, the presence of pure 

 subtropical oceanic waters. The thickness is 1000 to 2000 feet in central 

 Texas, and 5000 feet on the Rio Grande. 



The subdivisions adopted by Hill (on which the division of the Lower 

 Cretaceous into epochs is based) are as follows : — 



3u Washita Epoch. — Washita group. 

 4. Shoal Creek limestone. 

 3. Denison beds, sands, clays, limestones ; Exogyra arietina clays. 



2. Fort Worth, or Washita, limestone. 



1. Preston beds, Duck Creek chalk, Kiamitia clays. 



2. Eredericksburg Epoch. — Fredericksburg group. 



3. Caprina limestone, Austin marble. 



2. Comanche Peak chcdJc. 



1. Walnut clays, with Exogyra Texana and Gryphsea Pitcheri of 



Roemer (G. mucronata of Gabb). 



1. Trinity Epoch. — Trinity group. 



3. Pahtxy sands. 



2. Glen Pose beds, sandy below, calcareous above, containing marine 



fossils and some vegetable and Reptilian remains. 

 1. Trinity sands, with fossil leaves and lignite. 



As above indicated, the Cretaceous limestones of Texas are partly chalk, 

 like the Cretaceous of southern England; and the chalk contains flint. Chalk, 

 as already explained, is made from sea-bottom accumulations consisting 

 largely of the minute shells of Rhizopods, corresponding to the Globigerina 

 ooze of modern seas ; and flint, from the siliceous spicules of sponges and 

 siliceous shells of Diatoms or Radiolarians that may exist in the same calca- 

 reous deposits. Chalk is supposed to show therefore that the seas in Avhich 

 it was formed had a depth of at least some hundreds of feet. The various 

 fossils in the beds are also evidence of deep water. The beds continue to be 

 thick over the Indian Territory, but thin out in Kansas. The Ouachita 

 Mountain range was emerged land, and the Cretaceous sea, as Hill observes, 

 had a shore line at its base. 



In Mexico, the Lower Cretaceous, on the map of Castillo (1891), extends 

 nearly to the city of Mexico ; and it is continued beyond to the southward 

 and westward, in isolated patches. According to Hill (1893), all, except a 

 small portion to the northeast, is a continuation of the Comanche group of 

 Texas, but with less distinct subdivisions ; and he concludes further that 

 Dana's manual — 52 



