merits. According 1 to Holmes (page 37) the conglomerate 
lies about 100 feet above the top of the upper fall and there- 
fore at an altitude of slightly over 7,800 feet. It is interesting 
to observe that this altitude is about 60 feet higher than the 
surface of the existing Yellowstone Lake. 
In order to complete the story of sedimentation in this 
region, it is necessary to descend the canyon by Uncle Tom’s 
trail to the foot of the lower falls. About 50 feet above the 
level of the river the trail has been cut partly in the rhyolitic 
rock and partly in coarse compacted sands which in places show 
conspicuous cross-bedding. These sediments rest on a very 
narrow shelf composed of rhyolitic rocks and were observed 
by Holmes. r ’ 
At about the same or slightly higher level on the opposite 
wall of the canyon there is a conspicuous bluish patch which is 
drenched by the spray from the falls. The appearance of the 
material as seen from this distance reminds one strongly of the 
blue laminated silts in the Red Rock section and it might 
without much hesitation be identified as argillaceous sediments. 
Fortunately, however, we found on consulting the earlier 
reports on the Yellowstone Park that this locality had been 
visited by A. C. Peale 6 who describes it as a bluish gray mud. 
He also observed in close association with it a fine-grained 
argillaceous sandstone, but he attributed these rocks to the 
sliding down of the “soft volcanic ashes” forming the walls 
of the canyon. This cannot be the origin of the cross-bedded 
sandstone on Uncle Tom’s trail on the east side, and we believe 
our interpretation of the mud as a portion of a sedimentary 
filling of the canyon to be the correct one. Taken in conjunc- 
tion with the section near Red Rock, the arenaceous and 
argillaceous deposits near the bottom of the canyon probably 
represent the earlier stages in the filling of the first lake which 
was formed in this part of the canyon, the final stage of which 
is the conglomerate group at the base of the Red Rock section. 
The significance of this deposit did not escape Holmes. He 
remarks : 7 
5 Ann. Rept., U. S. Geol. and Geogr. Surv., 1878, Pt. II, p. 36, 1883. 
8 Ann. Rept., U. S. Geol. and Geogr. Surv., 1872 (1873), p. 233. 
7 Op. cit,, Sup. 
“In regard to that on the east side of the canyon just below the 
lower falls that they have very much the appearance of the lake 
beds which occur on the upper surface of the plateau. If it 
could be ascertained that they really are lake beds, they would 
take a very important place in our discussion of the erosion of 
the canyon.” 
r 
