638 WHITMAN CROSS 
beds formerly called Jurassic into ‘Lower Dakota” and Jurassic, 
consistent with the map. It may be well to make a summary state- 
ment in this place concerning the origin and application of the term 
“Tower Dakota,” since the use of the Hayden map is rather confus- 
ing without explanation. Moreover, the McElmo beds were named 
from a locality where the Hayden map shows no Jurassic beds. 
While surveying the Rico quadrangle the Dakota and subjacent 
strata were traced southwesterly down the Dolores Valley to the 
great bend of the river, where it turns due north. But a few miles 
west of that point several branches of McEImo Creek, a large tributary 
of San Juan River, cut below the Dakota sandstone into the under- 
lying Jurassic beds. Flowing at first in narrow canyons rimmed by 
the Dakota, these various forks finally widen into valleys presenting 
broad exposures of the strata in question. In 1897, H. S. Gane, who 
had been my assistant in the Telluride quadrangle work, traversed 
the main McElmo Valley from its head to the San Juan River and 
upon his report the name McEImo was chosen for the Jurassic forma- 
tion beneath the Dakota. 
The Hayden map, Sheet XV of the Ailas oj Colorado, represents the 
“Lower Dakota” as the principal formation below the plateau sand- 
stone, in McElmo and Montezuma Valleys and in the adjacent portion 
of the broad San Juan Valley. Apparently the “Lower Dakota” 
of that map represents in fact the general distribution of the McElmo 
formation. No Jurassic formation is shown upon the Hayden map 
of that region. 
This cartographic representation is based on the work of W. H. 
Holmes, and its explanation is to be found in his report for 1875 (17, 
p. 260). Describing in general terms the section below the Dakota 
proper Holmes says ‘‘The variegated series which succeeds it [down- 
ward] I at first felt inclined to call Jurassic, since it resembles so closely 
the variegated beds that on the eastern slope of the mountains have 
usually been credited to that age.” After mentioning details of 
differences in lithologic character, Holmes refers to a much more 
important basis for his conclusion. He states (17, p. 261) that: 
“‘In Middle Western Colorado Dr. Peale has found Cretaceous fos- 
sils in a stratum of sandstone some three or four hundred feet beneath 
the bed of conglomerate [referring to the basal Dakota conglomerate], 
