STRATIGRAPHY. 19 
The invertebrate fauna of the Mesaverde includes two distinct elements; one con- 
sisting of marine species is found chiefly in the lower portion, sometimes in beds alter- 
nating with those containing the other, which consists of fresh- water and brackish- 
water forms. The marine element is a direct continuation of the upper Mancos fauna 
and is not safely distinguishable from it without full stratigraphic data. The non- 
marine fauna is closely related to that of the Laramie, with which it has some species 
in common, especially in the genera Ostrea and Corbicula. During the past season 
[1907] the fact has been established that Tulotoma thompsoni, hitherto regarded as 
a characteristic Laramie species, occurs in both the Laramie and the Mesaverde. 
The invertebrate fossils that have been collected from the coal-bearing rocks of the 
Book Cliffs all occur in the Mesaverde of northwestern Colorado, and Doctor Knowlton 
finds that this is essentially true of the plants also. It is admitted that most of the 
fossils in question from the Book Cliffs would not seem out of place in the Laramie, yet 
their close agreement with those known to occur in the Mesaverde of a neighboring 
area, and the general stratigraphic and areal relations of the rocks in which they are 
found, make their reference to the Mesaverde most reasonable. The unconformable 
relations that doubtless exist between those rocks and the overlying Wasatch will 
explain the absence of the later Cretaceous rocks from the area. 
In the Book Cliffs field the unconformity between the Mesaverde 
and the overlying Eocene is marked not only by the absence of the 
Lewis sl^ale and the Laramie, but also by the westward thinning of 
the Mesaverde formation (p. 17), by the basal Eocene conglomerate, 
and by the distinct general difference in stratigraphy between the 
underlying buff sandstones and shales, which are brackish-water and 
fresh-water deposits, and the overlying variegated formation which 
accumulated under more diverse conditions, probably in part sub- 
aerial and in part lacustrine. 
TERTIARY SYSTEM EOCENE SERIES. 
Strata of Eocene age cap the Book Cliffs and for several thousand 
square miles constitute the surface of the Uinta Basin to the north. 
In the east end of the basin the Hayden Survey a mapped the Wasatch, 
Green River, Briclger, and Uinta formations of the Eocene, and in the 
west end G. II. Eldridge 6 also recognized the same formations. The 
present reconnaissance survey was not extended north of the crest of 
the Book Cliffs, and the large area designated Eocene on the map is 
taken from the authorities above mentioned. 
The lower Eocene beds in the area here considered are composed of 
local conglomerate, varicolored shale, buff sandstone, and subordinate 
thin lenses of limestone. The stratigraphy is characteristically 
varied and many adjacent sections are very unlike; in one place the 
varicolored shale predominates and in another it is inconspicuous. 
The conglomerate also is variable in occurrence. In some sections 
none was seen, while elsewhere there is considerable. One of the best 
exposures observed is north of Thompsons, where from 10 to 20 feel 
a Eayden's Atlas of Colorado. 
b Eldridge, Q. H., Asphalt and bituminous rock deposits of the United States: TWenty-second Ami. 
Rept. U.S. Geol. Survey, pi. 1, 1901. 
