THE AGE OF THE LIGNITIC FORMATION BY ITS FAUNA. 25 



When both areas, the north and south, are considered in regard to their 

 fossil faunas, they show, however, a marked difference, not in the characters 

 of the species of Mollusks, but in the nearly total absence of invertebrate 

 fossils south of the Laramie Kange. I do not know of any locality where 

 fossil shells have been seen in'the Southern Lignitic Basin except at Mar- 

 shall's, where a bed of clay iron ore, above No. h of the section, has a profu- 

 sion of fragments of Oyster shells {Ostreo subtrigonaUs?). The same species, 

 remarks Dr. Hayden, is abundantly found near Brown and O'Bryan's coal 

 mine, about twenty miles southeast of Cheyenne, also in the Colorado Basin. 

 Anyhow, no remains of invertebrates identifiable as of Cretaceous age have 

 been found in the true Lignitic Measures of the Colorado Basin from Chey- 

 enne to the Raton Mountains. In this whole area, therefore, of the Lignitic, 

 represented by what is called the Fort Union group in the north and the 

 Colorado Basin in the south, we have, from the distribution of the strata and 

 from the fossil faunas, evidence only of the Tertiary age of the formation. 



There is some more difference in the Lignitic of the so-called Bitter 

 Creek series, and, as remarked by Prof. E. D. Cope in his Report, 1873, p. 

 438, the authorities on this formation have presented views more or less at 

 variance with those entertained by him. The whole range from Black Butte 

 to Point of Rocks i.s the slope of an anticlinal whose axis is at Salt Wells; 

 and from the first locality to Point of Rocks or to Salt Wells, in a northeast 

 direction, the series of rocks is passed, which, in their superposition by the 

 southwest dip, has a thickness of three to four thousand feet, according to 

 the measurements of Messrs. Meek and Bannister. A huge Saurian discov- 

 ered by Prof. Meek in the overlying and burnt shale of the main coal of 

 Black Butte has been identified by Prof. Cope as a Dinosaurian {Aga- 

 thaumas sylvestris) of Cretaceous type. Lower in the series, below Black 

 Buttes, at Plallville, Prof. Meek has found shells whose character is not 

 quite definite, but which he considers as Cretaceous, though the same locality 

 was admitted by him in his report of 1870 as Tertiary. But my lamented 

 friend, who has done so much for the paleontology of North America, has 

 so clearly discussed the question of the character of the fauna of the Bitter 

 series formations that I consider it a duty to quote some of the more pertinent 

 passages of the introduction to his list and description of fossils in the Annual 

 Report of Dr. F. V. Hayden, 1872. He says (p. 457):— 



"Returning to the question of the age of Bitter Creek series, it may be 



