3fesozoic and Ccenozoic Geology and Palceontology. 125 



Dr. F. V. riayden* said the Dakota Group is composed of massive 

 beds of sapdstones, 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 ridges. 

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

 and may be represented by a very narrow belt on the ma}). West of 

 the 100th meridian it has 3'ielded very few organic remains, although 

 it has a ver^- extended geographical range. It is hardly ever wanting 

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

 tains, inUtah. 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 

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

 of 651 feet. •" 



The Fort Benton and Niobrara Groups are found in the valleys 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 

 Joyful creek from Anthracite creek, near station 32, a section of sand- 

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

 a metamorphosed 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 the 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 Roaring Fork. The strata are conformable to the underljdng 



*7th Rep. U. S. Geo. Sur. Terr. 



