STRUCTURE. 21 
Fossils are not abundant in the lower part of the Eocene series in 
the area under consideration, and collections were made in only two 
localities, each from thin beds or lenses of limestone. The following 
determinations were made by W. H. Dall: 
Fossils collected near divide of Hay Creek, north of Turner's ranch, about 200 feet above 
top of Mesaverde. 
Goniobasis tenera Hall var. carteri Conrad. 
Cast of Polygyra (aff. leidyi Hall). 
Impression of Unio sp. 
Fossils from Horse Canyon, Utah, 200 and 350 feet above top of Mesaverde. 
Physa, probably bridgerensis Meek. 
Vivipara panguitchensis White. 
Vivipara, probably wyomingensis Meek. 
Goniobasis sp. 
Dall reports that these are fresh-water shells which do not indicate 
with much precision their horizon within the Eocene, though they are 
probably Wasatch. Probably part of the area colored as Eocene on 
the map includes other Eocene formations in addition to the Wasatch, 
but the region has not been studied in detail and their differentiation 
was not attempted. 
The presence or absence of the Fort Union formation, which nor- 
mally occurs beneath the Wasatch, has not been determined in the 
Book Cliffs field. In the Grand Hogback, northwest of Newcastle, 
Colo., T. W. Stanton and H. S. Gale collected fossil plants which F. IT. 
Knowlton refers to the Fort Union, but Gale did not find it practicable 
to map the rocks containing these fossil leaves distinct from the Wa- 
satch. In the Book Cliffs field no Fort Union fossils have been found 
and the age of the thin, variable zone of conglomerate and buff sand- 
stone at the base of the Eocene remains to be determined. 
STRUCTURE. 
The strata of the Book Cliffs coal field, forming as they do part of 
the southern margin of the Uinta synclinal basin, dip gently north- 
ward. The dip is not uniform, however, for this area, besides being- 
included in the zone of folding of the Uinta Mountains and the Uinta 
Basin, is affected by the uplifts which produced the San Rafael Swell 
and the Uncompahgre Plateau. That part of the Book Cliffs coal 
field which is included in the present report is gently warped, the 
eastern part constituting the end of a low, northward-plunging anti- 
cline, and the western part the end of a northward-plunging synclino. 
Conforming with this structure, the irregular S-shaped outline of the 
cliffs coincides with the strike of the rocks. There are, also, local 
faults of small displacement 
