654 REPORT OF NATIONAL MUSEUM, 1904. 



are well-marked Cretaceous fossils of purely marine types and no 

 others. 



Second. That above the Fox Hill group there are about 200 feet of 

 barren beds, which might be regarded as beds of passage to the lignitic 

 group and which more properly belong with the Fox Hill group below. 

 In this group of transition beds all trace of the abundant invertebrate 

 life of the great Cretaceous series below has disappeared. 



Third, in almost all cases he found, at the base of the true Lignitic 

 group, a bed of sandstone in which the first deciduous leaves peculiar 

 to the group occur. No purely marine mollusks pass above this hori- 

 zon. Estuary or brackish- water shells are found in many localities in 

 great abundance, but these soon disappear and are succeeded farther 

 north by fossils of purely fresh-water origin. He added: 



Whatever view we may take in regard to the age of the Lignitic group, we may 

 certainly claim that it forms one of the time boundaries in the geological history of 

 our western continent. It may matter little whether we call it tipper Cretaceous or 

 Lower Eocene, so far as the physical result is concerned. We know that it plays an 

 important and, to a certain extent, an independent part in the physical history of 

 the growth of the continent. Even the vertebrate paleontologists, who pronounce 

 with great positiveness the Cretaceous age of the Lignite group, do not claim that a 

 single species of vertebrate animals passes above the horizon I have defined from the 

 well-marked Cretaceous group below. 



Peale, in his report for that .year, threw a ray of light upon the sub- 

 ject by suggesting that the reason of the difference of opinion as to the 

 age of the disputed beds might be the existence of two sets of lignite- 

 bearing beds close together, one belonging to the horizon of the Fox 

 Hill beds of the Cretaceous, or possibly a little above it, and the other 

 belonging to the horizon of the Fort Union group (Lower Eocene). 

 He incidentally called up the question relative to the value of different 

 types of fossils as criteria in determining the precise geological horizon. 



He summed up his own conclusions as follows: 



First. The lignite-bearing beds east of the mountains in Colorado are the equiva- 

 lent of the Fort Union group of the upper Missouri, and are Eocene-Tertiary; also, 

 that the lower part of the group, at least at the locality two hundred miles east of the 

 mountains, is the equivalent of a part of the Lignitic strata of Wyoming. 



Second. The Judith River beds have their equivalent along the eastern edge of 

 the mountains below the Lignite or Fort Union group, and also in Wyoming, and 

 are Cretaceous, although of a higher horizon than the coal-bearing strata of Coalville 

 and Bear River, Utah. They form either the upper part of the Fox Hill group or 

 a group called "Number Six." 



In his annual report to Ha} r den for the same year (1874, printed in 

 L875), Lesquereux went over all the ground once more, showing to his 

 own satisfaction, presumably, the conclusive character of the evidence 

 offered by fossil plant remains, and announced again his conviction to 

 the effect that the authority of animal remains should be unquestioned 



