AND PHYSICAL HISTORY 535 



(mainly clays and sands, some greensand, 300 feet), the Schn<i 

 Chalk (1,000 feet) in the middle, and the Ripley (mainly sand, 200-500 

 iVrt) above. The Eutaw is believed to be the equivalent of the 

 Matawan formation of the Atlantic coast, and the Ripley is thought 

 to be older than the Rancocas. The Cretaceous beds of the Gulf 

 coast have been disturbed more than the corresponding beds alon.u 

 the Atlantic coast. They have been bent into low anticlines and 

 syiu liiu's in some places (Alabama), and faulted to a slight extent. 



Western Gulf region. The general stratigraphic relations of the 

 system here are the same as farther east, but deposition seems to 

 have been well under way in Texas before the oldest exposed beds 

 of the system farther east were laid down. The system has a 

 maximum thickness of about 4,000 feet. Three principal subdivis- 

 ions are recognized: (i) The Dakota; (2) the Colorado; and (3) the 

 Montana. The Dakota formation, 600 feet and less thick, is largely 

 of sandstone, with some lignite, and is, for the most part, of non- 

 marine origin. The Colorado series contains much limestone (or 

 chalk) of marine origin. Its thickness is about 1,000 feet. The 

 Montana series is more largely clastic, and from it the oil of the 

 ( '< n^icana oil field of Texas is derived. Locally, the system is much 

 faulted. From Texas it is continued northward into Arkansas, and 

 westward into New Mexico. 



The Cretaceous of the western Gulf region differs from the cor- 

 responding system farther east in its greater thickness, and in its 

 greater proportion of calcareous matter, largely in the condition of 

 chalk. Of limestone or chalk, the Cretaceous of the Atlantic coast 

 contains little, that of the eastern Gulf region (Alabama and Mis- 

 sissippi) more, and that of Texas much; nor is the chalk confined 

 to the Gulf region, as will be seen. 



Western interior. One of the standard sections of the Creta- 

 ceous system of the western interior consists of the following sub- 

 divisions, commencing at the bottom: i. Dakota; 2. Colorado 

 (including the Benton and the Niobrara formations); 3. Mon- 

 tana (including Ft. Pierre and Fox Hills); and 4. Laramie. This 

 classification, however, does not fit all parts of the west. 



The Dakota formation, mainly of non-marine origin, is wide- 

 spread in the Great Plains, though most of it is buried. It extends 

 westward beyond the Rocky Mountains at many points. The 

 formation is largely sandstone, though it contains much conglom- 

 erate and clay, and some lignite. It is perhaps to be regarded as 



