234 WHITMAN CROSS 
his knowledge of some of the experiences of his colleagues in 
the United States Geological Survey work. 
Where lithologic uniformity or regularity in variation of 
character actually stands for continuity of conditions, as far as 
existing facts indicate, the writer is as ready as any one to 
acknowledge that that fact should be recognized on the map, 
because it represents a fact both of geological history and of 
local conditions. The local importance of such a formation is 
surely to be recognized where it does not obscure facts of more 
fundamental significance. In this belief he has, for example, 
discriminated and mapped the Mancos shale of the Telluride and 
La Plata quadrangles in Colorado. This Upper Cretaceous forma- 
tion is nearly two thousand feet in thickness, rests on the Dakota 
sandstone and embraces strata deposited during the Benton, 
Niobrara and Pierre epochs, as indicated by fossils occurring 
near the base and near the top of the shale complex. It is at 
present a matter of inference that some of the shales of this 
complex correspond in time of deposition to the strata assigned 
to the Niobrara formation, as it has been generally discrimina- 
ted elsewhere in Colorado. 
The Ouray limestone of Colorado, first described and named 
in 1900 by A. C. Spencer,’ then the writer’s assistant, was at first 
supposed to be wholly of Devonian age. It is nearly three 
hundred feet in thickness, and while not actually homogeneous 
is in very large part a limestone formation. Near the center a 
marked Devonian invertebrate fauna has been found, and above 
that horizon no persistent line can be drawn on lithologic char- 
acter. It has been recently ascertained, however, that Lower 
Carboniferous fossils occur in certain places in the very upper- 
most layers of this local limestone unit. As study of this forma- 
tion progresses it is plain that some, and possibly a considerable, 
thickness of Lower Carboniferous limestone was eroded from its 
upper portion in the time interval preceding the Upper Car- 
boniferous. In its present partially metamorphosed condition 
the Ouray limestone seems in large degree a unit of continu- 
* Devonian strata in Colorado,” Amer. Jour. Sct. (4), Vol. IX, p. 125. 
