CRETACEOUS SYSTEM. 81 



CHAPTER XXXVII. 



CRETACEOUS SYSTEM. 



§ 175. The name Cretaceous is from the Latin Greta, chalk, and was applied 

 to the rocks in Europe long before its use as a geological term. The existence of 

 the Cretaceous on this continent was first ascertained, in 1827, by Morton and Van- 

 uxem. The Cretaceous is found either exposed upon the surface, or covered by 

 the Tertiary, forming a border of variable width on the Atlantic Coast, from New 

 York to Florida. In like manner it occurs everywhere south of the 33d parallel, 

 with the exception of limited areas in the mountain regions. It covers nearly all 

 Mississippi, extends into Tennessee and Arkansas, and reaches Southern Illinois. 

 West of the 97th Meridian, from the 33d parallel to the Arctic Ocean, the whole 

 country is covered with this formation, with the exception of the areas in the mountain 

 regions, exposing older rocks and inconsiderable extensions of land, where it has been 

 swept away, and an area of some magnitude north and west of Hudson's Bay. This in- 

 cludes, of course, the whole extent covered by the Tertiary and more recent deposits. 

 It is found east of the 97th Meridian, extending into Iowa, Minnesota, and some parts 

 of British America. Or, approximately stated, the Cretaceous forms the surface- 

 rock, or is overlaid with the Tertiary and recent strata over nearly half the North 

 American continent, and from the extensive denudation it has suffered, we may 

 fairly presume, at the commencement of the deposit, the land surface was not half 

 its present dimension. In the east and south the formation is exclusively marine, 

 but in the west the marine is succeded by a brackish-water deposit. 



§ 176. Meek and Hayden divided the marine Cretaceous of Kansas, Nebraska, 

 and the great West, in 1861, in ascending order, into the Dakota Group, Fort Ben- 

 ton Group, Niobrara Group, Fort Pierre Group, and Fox Hills Group. The 

 Dakota Group was named from Dakota County, where it consists of sandstones, 

 with alternations of various colored clays, and beds, and seams of impure lignite, 

 silicified wood, and great numbers of leaves of the higher types of dicotyledonous 

 trees, with casts of Pharella dakotensis, Axincea siouxensis, and Gyrena arenaria. The 

 thickness in that locality is 400 feet, in North-western Colorado 600 feet, and in 

 the San Juan region 1,000 feet. It is the supposed equivalent of the Eutaw 

 Group of Alabama and Mississippi, which has a thickness of about 400 feet and 

 contains beds of lignite. 



§ 177. The Fort Benton Group was named from Fort Benton on the Upper 

 Missouri, where it consists of dark-gray, laminated clays, sometimes alternating 

 with seams of limestone. It abounds in Inoceramus, Ammonites, Scaphites, Nautilus, 

 and other fossils, and has a thickness of 800 feet. 



§ 178. The Niobrara Group was named from Niobrara, in Nebraska, where it 

 consists of marls and limestones, and abounds in Inoceramus, Ostrea, and remains of 

 fish, and has a thickness of 200 feet. It has an extensive geographical distribu- 

 tion, but rarely exceeds 500 feet in thickness. 



§ 179. The Fort Pierre Group was named from Fort Pierre, in Dakota, where 

 it consists of clays containing carbonaceous matter, seams of gypsum, and masses of 

 sulphuret of iron, and abounds in the shells of Cephalopods, Lamellibranchs, 



