82 CRETACEOUS SYSTEM. 



remains of fish and saurians, and has a thickness of 700 feet. In Northern Colorado 

 it is 800 feet thick, and in Alabama and Mississippi it is known as the Rotten 

 limestone, and reaches a thickness of 1,200 feet. 



180. The Fox Hills Group was named from Fox Hills, in Dakota, where it 

 consists of gray, ferruginous, and yellowish sandstones, and arenaceous clays, abound- 

 ing with shells of Cephalopods, Lamellibranchs, Gasteropods, remains of fish and 

 saurians, and has a thickness of 500 feet. East of the Colorado Range its thickness 

 is 1,500 feet, in the valley of Bitter Creek 3,000 feet, and in that of the North 

 Platte 4,000 feet. It is the same as the Ripley Group of North Carolina, Alabama, 

 and Mississippi, which has a thickness of about 400 feet. 



181. The thickness of the marine Cretaceous in New Jersey is about 700 

 feet. It is valued in that State for its fertile marl, and beds of kaolin in its lower 

 part. In Louisiana its thickness is more than 1,000 feet, in the Uintah Mountain 

 region 7,000 feet, and in New Mexico and British America more than a mile at 

 many places. The canon of San Carlos, on the Rio Grande, exposes a clear perpen- 

 dicular height above the river level of 1.500 feet of Cretaceous strata. The Cre- 

 taceous is the Coal-bearing formation at Vancouver's Island and other points on the 

 Pacific Coast. 



182. There is in the West, superimposed upon the marine Cretaceous strata, 

 rocks which were deposited in brackish water, and form transition-beds from the 

 strictly marine condition of the Cretaceous to the epoch of numerous fresh-water 

 lakes, which were scattered all over the country west of the Mississippi, and north 

 in British America to the Arctic regions. These rocks were named in 1861, by 

 Meek and Hayden, the Fort Union Group. They consist of beds of clay and sand, 

 with numerous seams and local deposits of lignite and beds of coal. The passage 

 from the marine to the brackish-water deposits, and from the latter to the fresh- 

 water deposits, is without abrupt change in the sediment, and with complete con- 

 formability. There is no evidence of any important physical or climatic change, 

 beyond the gradual filling up of the basins of the sea and the recession of the salt 

 and brackish water, appearance of fresh-water lakes, and their gradual disappear- 

 ance. The Fort Union Group has been called the Judith River Group, the Bitter 

 Creek Group, the Bear River Group, the Laramie Group, and by divers other names. 

 It has a thickness, in Bitter Creek Valley, Wyoming, of 6,000 feet, and iu Bear 

 River Valley, in Utah, of 7,000 feet. Its geographical distribution extends for a 

 thousand miles in length, and a maximum width of 500 miles or more, with a varying 

 thickness from 100 feet or less, to 7,000 feet or more. It abounds in plants belong- 

 ing to Eocene genera, which connect the Cretaceous and Tertiary flora by insensible 

 degrees, while the Dinosaurian remains demonstrate its Cretaceous age. 



183. Before the discovery of this Group, absolute nonconformability was sup- 

 posed to exist between Cretaceous and Tertiary rocks, and this is the case where 

 marine Tertiary follows the marine Cretaceous, wherever known in the world. But 

 here, where the marine Cretaceous is as recent as elsewhere, and the continuance of 

 the period is represented by brackish-water deposits, and then fresh-water deposits 

 in lakes cut off from the ocean, the rocks are conformable, and the vegetable and 

 animal kingdoms show the slow progress of advancing ages. About one-third of 

 the genera of plants belonging to that period have become extinct, but the living 

 plants, Corylus americana, C. rostrata, Davattia tenuifolia, and Onocka sensibilis, have 



