4 3 RES 
—. 
4 é 
Mesozoic and Cenozoic Geology and Paleontology. 105 
6 
has held its place amid changes of great importance. It was during 
the latter portion of the Tertiary age that much of the volcanic ac- 
tivity took place which was so general over this portion of the country, 
though probably only the closing stages of the lava flows are repre- 
sented by the eruptive deposits of the Pliocene epoch. A section on 
the present lake shore, between Bluff Point and Steam Point, in de- 
scending order, is as follows: 
1. Grass-covered soil passing gradually to loose sand, 2 feet. 
2. Various sand, gravel,.and spring deposits with scattered irony 
concretions, 6 feet. . 
3. White and dark lake sand, very thinly laminated with beach 
structure, and occasional irony layers, 5 feet. 
4, About 15 feet of thinly laminated, blue-black clay, locally con- 
torted and beautifully cut by a small rill, emanating as a spring from - 
one of the irony layers in No. 3. The water is slightly chalybeate. 
Other sections show the same general features with more or less vari- 
ation. They represent the upper portion of the Pliocene series, de- 
posited toward the close of volcanic activity, hence the occasional beds 
of volcanic ejectamenta which were poured out into the lake, are main- 
ly composed of volcanic sand and the finer textured conglomerates, as 
may well be seen near Steamboat springs. As we descend the valley 
of the Yellowstone river, we find the lower members of the group well 
exposed, and the beds of unmodified non-molten material becoming 
more common, with increasing proportions of the molten or lava series, 
until the latter are almost universal, and doubtless represent an earlier 
period, though frequently largely concealed by tlhe subsequent spring 
deposits. Near the close of the Pliocene epoch, the internal fires had 
* so far died out that the igneous ejections were of fitful occurrence, and 
geysers, solfataras, fumaroles, etc., abounded to an almost incredible 
extent, giving rise to enormous deposits of siliceous and calcareous 
material, which has continued to be deposited with decreasing vigor 
until the present day. 
Prof. G. K. Gilbert* found a section of Tertiary on the east face of 
Sam Pitch Plateau, at Wales, Utah, 1,292 feet in thickness, another 
near the head of the main Sevier river, in Utah, 560 feet, and another — 
on the north fork of Virgin river, between Mountain Lakelet and 
Rockville, in Southern Utah, estimated at 3,000 feet. 
Prof. EK. D. Copet described the Puerco marls as in all probability 
* Geo. Sur. W. 100th Meridian, vol. iii. 
+ Ann. Rep. Explr. and Sur., W. 100th Meridian App. L. L. 
