22 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



1861, and afterward by his annual reports. Considering his opinion on the 

 age of the Lignitic, he remarks, in the last report:* — "It is well known that 

 I have held with some tenacity the opinion that the coal formations of the 

 West are of Tertiary age, and I still regard the Lignitic group as transitional 

 or Lower Eocene until the evidence to the contrary is much stronger than 

 any which has been presented up to the present time." 



It has been seen already that, in his explorations of 1854, Dr. Hayden 

 carefully surveyed the Lignitic beds along the Missouri River from their first 

 appearance near Fort Clarke to the mouth of the Yellowstone, and then up that 

 river to a point near the mouth of the Big Horn. "In all this distance, about 

 six hundred miles, following the windings of the river, the Cretaceous beds 

 appear but once, and then only along the bed of the river for a few miles, 

 while the entire country, with this exception, is occupied with the Lignitic 

 groups. It rests everywhere upon well-defined Cretaceous beds, No. 5, which 

 we have all along regarded as the highest known in the West, and have re- 

 ceived the name of Fox Hills group, from a locality on the Missouri River 

 called the Fox Hills or Fox Ridge, where this formation was first studied and 

 found full of Molluscan life.f There is a gradual passage upward from the 

 black, plastic, shaly clays of No. 4, or the Fort Pierre group, to the yellow 

 calcareous clays of the Fox Hills group, and at the upper portion the sedi- 

 ments are arranged in thin layers, very arenaceous, indicative of their deposi- 

 tion in turbulent as well as shallow waters. In these arenaceous sediments, 

 the well-marked marine life ceases to exist, and soon after appear the brack- 

 ish-water species.'' From this kind of formation of the Fox Hills group, it is 

 not surprising that it is not of universal extent. It is the true transition 

 group, locally of a thickness of five hundred feet, but it is not constant. For 

 example, its presence is clearly marked from Rock Creek to Medicine Bow 

 along the Union Pacific Railroad; but I have not seen it anywhere under the 

 coal strata along the base of the Rocky Mountains, at least not with its 

 characteristic fossils. At the Raton Mountains, and all around Trinidad, 

 where the succession of the Lignitic to the Cretaceous is exposed at many 

 places, the brackish beds overlying the Cretaceous No. 4 are already Lignitic 

 by their characters; for they do not contain any traces of Cretaceous remains, 

 but a profusion of fragments of dicotyledonous wood, evidently rolled with the 



•Annual Report, 1874, p. 20. 



t See section of the Cretaceous of Nebraska aud Kansas in Cretaceous Flora, p. 14. It is copied 

 from Dr. Hayden's Annual Eeport, 1870, p. 87. 



