Mesozoic and Cainozoic Geology and Palmonfology. 181 



Dr. F. V. Haydcu* said the Dakota Gioiip is composed of massive 

 beds of sandstones, intersected with layers of cla^^, and forms some 

 of the most conspicuous ridges or "liogbacks" along the eastern base 

 of the Front or Colorado range. Its * importance, however, varies in 

 different localities as much as its texture; sometimes it is scarcely 

 seen, and then again it forms one or more of the most important rid'ges. 

 Its aggregate thickness is never great, varying from 200 to 400 feet, 

 and may be represented by a very narrow belt on the map. West of 

 the lOOth meridian it has j^ielded very few organic remains, although 

 it has a very extended geographical range. It is hardly ever wanting 

 along the margins of the mountain ranges east of the Wasatch Moun- 

 tains, in Utah. From its structure in the far West he regarded it as a 

 sort of transitional group between the well-defined Cretaceous and the 

 Jurassic below. 



Dr. A. C. Peale measured a section of the Dakota Group beneath 

 station 73, north side of Gunnison river, that presented a thickness of 

 5.36 feet, and another section at station 60, that presented a thickness 

 of 651 feet. 



The Fort Benton and Niobrara Groups are found in the vallevs of 

 Grand and Gunnison rivers, and on the North Fork of the Gunnison. 

 A partial section between station 38 and station 80 gave a thickness of 

 753^ feet, and another section on Gunnison river, opposite Roubideau's 

 creek, measured 687 feet. The estimated thickness, however, including 

 the Fort Pierre Group, is from 1,500 to 2,000 feet. 



On Coal creek there is a bluff, in the face of which are exposed 

 1,500 feet of light-gray and yellowish sandstones and shales, referred 

 to the Fox Hills Group. And on the North Fork of the Gunnison the 

 exposures are of greater thickness. On the ridge dividing Oh be 

 Jo3'ful creek from Anthracite creek, near station 32, a section of sand- 

 stones occurs 883 feet in thickness. Most of these sandstones have 

 a metamoi'phosed appearance, and the ridge, in which they are ex- 

 posed, is intersected with dikes. Below the strata of this section 

 there are propably 1,000 feet of shales and sandstones to a series of 

 coal-bearing strata on Oh be Joyful creek. The latter, according to 

 Mr. Holmes' estimates, is about 2,000 feet above tli'e Dakota Group. 



Above these beds there is a series referred to the Lignitic Group 

 from 7,000 to 8,000 feet in thickness, covering a large area extending 

 from the Grand river to the Gunnison, beneath the basaltic plateaus 

 west of Eoaring Fork. The strata are conformable to the underlying 



•■■■Tth Rep. U. S. Geo. Sur. Terr. 



