GEOLOGIC VERSUS LITHOLOG/C 237 
elsewhere known. The actual Triassic age of the upper portion 
of the ‘‘Red Beds” has been established by scanty but signifi- 
cant vertebrate, invertebrate, and plant remains, occurring within 
a limited vertical range. The problem, alike for the geologist, 
endeavoring to make an adequate geological map, and for the 
lithologist, required to express the grouping of the many small 
lithologic individuals in the structural divisions of the Carbonifer- 
ous and Triassic systems, is here to find the correct line between 
them. 
In the Rico Mountains it was found that above the complex 
of two thousand feet of beds characterized by the Lower Car- 
boniferous fauna there was a series of strata about three hundred 
feet in thickness, generally somewhat reddish in color, sometimes 
markedly so, containing a fauna described by the paleontologist 
as distinctly different from that known in the two thousand feet 
of strata below and as intermediate in character between the 
Carboniferous and Permian faunas thus far known. The change 
takes place abruptly, z. ¢., within fifteen feet. The Permo-Car- 
boniferous fauna occurs in thin limestones through a section 
containing lithologic members much like strata above or below. 
Above this series comes an apparently unfossiliferous section of 
frequent and irregular changes in lithologic character. Many 
of the strata here are extremely like beds of the complex con- 
taining the Permo-Carboniferous fossils. 
In the Report on the Geology of the Rico Mountains by the 
writer and A. C. Spencer the heterogeneous group of strata con- 
taining the Lower Carboniferous fauna was designated as the 
“Hermosa Formation;” that containing the Permo-Carbon- 
iferous fauna as the ‘‘ Rico Formation ;’’ and the remainder of 
the section was provisionally assigned to the ‘‘ Dolores Forma- 
tion.’* The latter formation had already been defined as 
intended to include the Triassic portion of the heterogeneous 
“Red Beds”’ section of the region and apparently including the 
whole. The object in making the three divisions was to express 
‘WHITMAN Cross and ARTHUR COE SPENCER, “ Geology of the Rico Moun- 
tains, Colorado,” Twenty-first Ann. Rept. U. S. Geol. Surv., Part II, 1900. 
