12 THE BOOK CLIFFS COAL FIELD. 
fication differs from that of Peale, of the Hayden Survey, who in his 
report on the Book Cliffs field separated the rocks here assigned to 
the Mesaverde formation into the "Fox Hills" and the "Laramie." 
It was recognized by the Hayden Survey that there is no distinct 
lithologic break in the Book Cliffs between the "Laramie" and the 
"Fox Hills," and the nomenclature employed was an attempt to 
conform to subdivisions used in other fields. It has been found desir- 
able, however, to restrict the use of the name "Fox Hills" to the origi- 
nal area in South Dakota, 5 and, as shown below, these rocks are not 
Laramie, but belong in the Montana group. The classification here 
adopted is based on the general stratigraphic and areal relations of 
the rocks and on fossil evidence, as explained on pages 17-19. The 
Uinta Basin section differs from that of southwestern Colorado 
and the Yampa coal field in northwestern Colorado 6 ' by the 
absence of the Lewis shale and the Laramie formation between 
the Mesaverde and the Eocene. This hiatus in the Book Cliffs field 
appears to be accounted for by the unconformity at the base of the 
Eocene, which implies that these formations, if they were ever pres- 
ent in the area under discussion, were removed by erosion previous 
to the deposition of the overlying Tertiary rocks. 
In the Book Cliffs field the general character and sequence of the 
rocks is shown by the section in Plate III. The strata are separable 
into four distinct lithologic divisions, the three Cretaceous forma- 
tions named and the Eocene rocks, all easily recognizable throughout 
the field by their physical character and sequence. 
CRETACEOUS SYSTEM. 
]>.\ KOTA SANDSTONE. 
The Dakota sandstone in the area here considered possesses the char- 
acteristic features common to the formation in this general region. It 
is composed of buff quartzitic sandstone, generally conglomeratic, and 
local beds of carbonaceous shale and low-grade coal are provisionally 
included in the formation, although no fossils have been found in them 
in the Book Cliffs field. The Dakota varies in thickness from about 
200 feet to less than 25 feet. The outcrop forms a narrow belt of low 
hills parallel to and about 12 miles south of the Book Cliffs. 
The formation is extremely variable in composition and arrangement 
of the beds, as shown by the following sections measured in different 
parts of the field. At the mouth of Gunnison River, south of Grand 
« Peale, A. C, Geologic report on the Grand River district: Tenth Ann. Rept. U. S. Geol. and Geog. 
Survey Terr., 1878. 
b Stanton, T. W., Geology and paleontology of the Judith River beds: Bull. U. S. Geol. Survey No. 257. 
1905, p. 66. 
'• Fenneman, N. M., and Gale, Hoyt S., The Yampa coal field: Bull. U. S. Geol. Survey No. 297, 1906. 
