760 



INDEX TO THE STEATIGEAPHY OF NORTH AMERICA. 



containing valuable coal beds sharply defined at the base by a conglomerate or conglomeratic 

 sandstone, without doubt marking an unconformity of considerable magnitude. Thickness 

 estimated as about 800 feet on Lay Creek north of Lay. 



Osborn ^^^^ describes the later Eocene deposits of the Uinta Basin as com- 

 prising the Uintatherium zone (800 feet), provisionally equivalent to the upper 

 part of the Bridger; the Eobasileus zone (350 feet), equivalent to the upper zone 

 of the Washakie Basin; and the Diplacodon zone (600 feet), which is the true Uinta 

 of King and properly the Uinta formation. This last he regards as "approaching 

 if not equivalent to the lowermost White River Oligocene — that is, lower Titano- 

 therium zone or Chadron formation." 



Eocene strata extend southward from the Uinta, Valley and White River in 

 Utah and Colorado and, rising, form the Roan or Book Plateau. Their southern 

 margin has been traced by Taff and Richardson in connection with surveys of the 

 underlying (Cretaceous) coal measures. Lee^^"* states the section as follows: 



The Grand Mesa coal field is located on the southern rim of the Uinta coal basin, which lies 

 partly in northwestern Colorado and partly in Utah. It derives its name from Grand Mesa, a 

 high table-land between Grand and Gunnison rivers. The coal beds underlie the mesa and 

 outcrop along the south and west sides. They extend westward into the Book Cliffs field, 

 north of Grand Junction," and eastward into the Anthracite-Crested Butte region,* long known 

 for its anthracite coal, and thence northward along the Grand Hogback, recently described 

 by Gale." 



Generalized section of rocks in the Grand Mesa coalfield, Colorado. 



[Only the Tertiary part of the section is here given.] 



Detailed sections of certain districts covered by the coal surveys made by 

 the United States Geological Survey, in 1907-8, will be found under L 12 and L 13, 

 pages 774-778. 



a Richardson, G. B., The Book Cliffs coal field: Bull. U. S. Geol. Survey No. 316, 1907, pp. 302-320. 

 6 Eldridge, G. H., Anthracite-Crested Butte folio (No. 9), Geol. Atlas U. S., U. S. Geol. Survey, 1894. 

 "Gale, H. S., Coal fields of the Danforth Hills and Grand Hogback in northwestern Colorado: Bull. U. S. Geol. 

 Survey No. 316, 1907, pp. 264-301. 



