RECONNAISSANCE IN COLORADO AND UTAH 675 
collections and those of the lower Grand River lies in the assumption 
of insufficiency of collections at Moab, which by chance failed to 
include a large number of the species of the Powell collection. In 
view of the fact that the collections from Moab and Sindbad Valley 
were made by several different collectors at different times, this 
explanation seems hardly a plausible one. Another interpretation 
is that there may be a stratigraphic break, due to uplift and 
erosion, through which the Aubrey strata found by Powell 
and Newberry have been removed at Moab, in the Sindbad 
Valley, and to the mountain region to the east. This implies 
that the Hermosa beds of Moab are present beneath the section 
examined by Powell and Newberry. Such a break must occur at 
the base of the Paleozoic ‘‘Red Beds,” and no suggestion of such a 
hiatus has come from observations in Colorado; but it is to be remem- 
bered in this connection that in southern Utah and northern Arizona, 
Powell, Gilbert, Dutton, Walcott, and others have noted a persistent 
unconformity by erosion between the Aubrey and the succeeding 
strata now commonly referred to the Permian through Walcott’s dis- 
covery of fossils in the Kanab Valley (37). All of the above-named 
geologists have observed a conglomerate more or less widely distributed 
at the base of the Permian series, composed in large part of 
pebbles derived from the Aubrey rocks, as shown by fossils contained 
in them. It is, of course, possible that the denudation at this horizon 
may have been much more extensive than the observations 
thus far reported would suggest. The situation is really not very 
different from that concerning the unconformity and stratigraphic 
break now known below the Dolores Triassic formation, which a few 
years ago was supposed to consist in comparatively slight unconformity 
by erosion. 
The foregoing suggestion is in some degree confirmed by the fact, 
determined in the Colorado region, that the Hermosa fauna is succeeded 
by one having a distinct, and apparently a younger, facies (16, pp. 
245-56), that of the Rico formation, which has not been found at 
Moab. Mr. Girty informs me that recent data tend to correlate 
Rico with the Aubrey formation (in part) through the Manzano 
group of New Mexico. In its lithologic, stratigraphic, and faunal 
relations the Rico is said to be suggestive of the Manzano group. which 
