GEOLOGICAL SUEVEY OF THE TERRITORIES. 53 



fartlier northward into Iowa and Minnesota, and probably far up 

 into British America. It is believed also to occur all along the Rocky 

 Mountains, although as yet no positive proof from fossils has been ob- 

 tained. 



There is a series of beds between well-known Cretaceous and Jurassic 

 rocks in those regions, which has been regarded as belonging to the 

 Dakota group ; also, near the sources of the Missouri, is a series of 

 beds differing from any other yet described, containing many species of 

 shells and a bed of lignite, which seems to belong to this group. These 

 latter beds need more careful study before the position can be positively 

 fixed in this section. 



Along the Atlantic coast, especially in New Jersey, the lower Cretace- 

 ous beds seem to be lithologically similar, in con . aining numerous dicoty- 

 ledonous leaves, so that it is now regarded as the equivalent of the 

 Dakota group of the West. It is, therefore, evident that this formation 

 is very widely distributed, perhaps even east and west from one ocean 

 to the other. 



FORT BENTON GROUP. 



This group bears the above name from the fact that it is largely de- 

 veloped in the vicinity of Fort Benton, near the sources of the Missouri 

 Eiver. In ascending the Missouri, it is first seen in thin outliers below 

 the mouth of Big Sioux River, and on the Big Sioux six miles above its 

 mouth. It is characterized as a dark leaden-gray plastic clay, but when 

 saturated with water it is of a black color. 



A few fossils have been found at various localities, as Inoceramus 

 problematicus, Ostrea congesta, Ammonites, Serpula, &c. Near the mouth 

 of Iowa Creek there is the best exposure of this group, as well as grou^js 

 above and below. 



3. Gray and light-yellow calcareous marl or chalky limestone, with 

 great numbers of Inoceramus problematicus, Ostrea congesta, and remains 

 of fishes. Niobrara division 40 to 50. 



2. Dark plastic clay, with abundant remains of fishes, I. problematicus, 

 0. congesta, Ammonites peracutus, Serpula tenuicarinata, and a species of 

 oyster, like 0. congesta. Fort Benton group 30 to 40 feet. 



1. Variegated sands and clays of Dakota group, 15 to 20 feet above 

 water's edge ; impressions of leaves of willow, laurel, and many crystals 

 of sulphuret of iron. 



The beds of the Fort Benton group are widely distributed through- 

 out the West, but in no portion has it revealed any useful minerals or 

 economical rocks of any kind, to my knowledge. The black plastic clays 

 may be rendered useful at some period, but it is quite doubtful. They 

 are everywhere filled with sulphuret of iron. 



At the locality where the above section was taken I obtained some of 

 the finest specimens of crystallized sulphuret of iron I have ever seen. 

 There were also many species of selenite. So far as I know, this forma- 

 tion does not exert any favorable influence on the country. 



The beds of impure coal near the mouth of Iowa Creek ^re very inter- 

 esting in a geological point of view. At no other locality do I know of 

 the existence of any seams of Carbonaceous matter. This coal is too 

 impure and contains too much sulphuret of iron ever to be made avail- 

 able. 



