190 Cincinnati Society of JSfafurctl History. 



patches of it. It is found along the western side of the Gunnison and 

 forms the floor of the San Miguel plateau. Going north on the San 

 Miguel plateau, we find the massive sandstones of the Dakota Group 

 broken, and forming the tops of mesas between the streams rising in 

 the Uncompahgre plateau and flowing into the San Miguel and Dolores 

 rivers. Still further north it disappears altogether, until we approach 

 Grand river, near the mouth of the Dolores. 



In the Uncompahgre valley, on both sides of the river, until the 

 canon is reached, there are exposures of the Fort Benton and Niobrara 

 Groups. East of the Uncompahgre Agenc}^ the thickness of the beds 

 is about 3,000 feet. 



F. M. Endlich, geologist of the southeastern division, found the 

 Dakota Group in the San Juan region forming a ridge parallel with 

 the Piedra river, and having a thickness of more than 1,000 feet. He 

 also discussed the age of the Lignitic Group of the Trinidad region, 

 which spreads over an area of 750 square miles, and with Prof. Lesquer- 

 eux supposed it to be of Tertiary age. 



Dr. B. F. Mudge* said the Cretaceous in Kansas covers an area of over 

 40,000 square miles, or more than half the surface of the State. The 

 Fort Benton, Fort Pierre and Fox Hills Groups are entirely wanting. 

 The Dakota Group rests upon the Permian, and is succeeded b}" the 

 Niobrara Group. 



The averaoe width of the Dakota is less than 50 miles, beino- some- 

 what less than that in the north part of the State, and more on the 

 Smoky and Arkansas rivers. Tiie dip is to the northwest, and very 

 slight. It is conformable to the formation above it, and has a max- 

 imum thickness of about 500 feet. 



The Niobrara Group occupies a belt of countr}- about 30 miles in 

 width, in the northern part of the State, but gradually widens to more 

 than twice that extent in the Smokj^ Hill valle3\ The upper part is 

 composed of chalk and chalky shales, the lower part which is called 

 the Fort Ha3's Group, consists in its higher strata of heav^' bedded 

 limestone, under which is a friable, bluish black, or slate colored shale? 

 which abounds in concretions or septaria, of all sizes, from one inch to 

 six feet in diameter. The body of the concretions is of hard cla}^- 

 marl, with cracks lined with beautiful crystals of calc spar. The lower 

 part has a thickness of 260 feet, and the upper part of 200 feet, making 

 the total thickness 460 feet. It is succeeded b}^ strata of Pliocene 

 age. 



* 9th Kep. Hayden's U. S. Geo. Sur. I'err. 



