186 Cincinnati Society of Natural History. 



Hills, this group is somewhat difficult to recognize, but in Colorado it 

 maA- be traced for long distances in well defined north and south lines. 



The Niobrara Group, although much thinner, is more easily recog- 

 nized. It frequently" blends so completely with the overlying Fort 

 Pierre Group that it is extremely diflScult to separate them. 



The Fox Hills Group, feast of the Colorado Range, is characterized 

 throughout by great uniformity in texture and physical habit, and con- 

 sists of a coarse sandstone formation, showing on I3' variations in color 

 from reddish brown to reddish yellow. The strata pass b}^ imperceptible 

 gradations, into the Laramie series, offering no well-defined line of 

 separation, both formations from top to bottom consisting of coarse 

 sandstone. The Laramie Grpup may be ti-aced along the Big Thomp- 

 son and Cache la Poudre vallej'S, and then eastward up the valleys of 

 the northern tributaries to the South Platte. The sandstones form the 

 exposed banks along Crow and Lone Tree creeks, and ma}" be traced 

 northward, passing under the Tertiary- of Chalk Bluffs. This group 

 includes the valuable coal deposits at Erie, and the Marshall and 

 Murph}' mines, north of Golden, extending from within one-half mile of 

 the base of the range far out upon the plains into Eastern Colorado. 



The Laramie beds form the ^ippermost members of the great series' 

 of conformable strata that lie upturned against the Archaean mass of 

 the Rock}' mountains; all overlying strata resting unconformabl}- upon 

 the older rocks. 



The Cretaceous rocks are distributed over the surface of the Laramie 

 Plains. On Rock creek, a branch of Medicine Bow river, north of the 

 Little Laramie, and near Rock Creek Station, the Fort Benton Group is 

 exposed from 350 to 400 feet in thickness. In the North Park, the 

 Dakota Group is estimated at 350 feet in thickness, and hei-e the Fort 

 Benton, Niobrara and Fort Pierre Groups have a combined thickness 

 roughly estimated at from 1,500 to 2,000 feet. 



The Medicine Bow river, after leaving the mountains, runs almost 

 excluslvel}' through beds of Cretaceous age. its course being guided b}' 

 the clays and marls, and the overlying Fox Hills sandstone. 



On the northern slopes of Elk .Mountain, the most northern point of 

 the Medicine Bow Range, are found all the beds from the coal measures 

 to the Fox Hills sandstone, uplifted at high angles, lying against the 

 Archaean formation. All the geological divisions are well represented. 

 In the valley of the North Platte river the Fox Hills Group has an esti- 

 mated thickness of between 3,000 and 4,000 feet. 



The strata containing the coal beds, at the town of Carbon, fi56 miles 

 west of Omaha, Mr. Hague supposed to be Upper Cretaceous. 



