38 GEOLOGY. 
The constituents of these Quaternary deposits are very like those which form the valley- 
drift below, but in our progress up the river the pebbles of sedimentary rock constantly 
increased in number and size. Here, near the southern entrance to the Black cafion, where 
the Colorado issues from the channel it has cut in the first of the volcanic barriers it meets in 
its course, and but few miles below the mouth of the Great Cafion, so long and so deeply 
eroded in sedimentary rocks, the pebbles of the gravel beds are as often limestone and chert, 
derived from the limestone or sandstone, as volcanic materials composing the walls of the 
adjacent cafion. 
In the heaps of boulders, which in many places form the bars or banks of the river, I often 
noticed large rounded masses of Carboniferous limestone, which had been transported by the 
stream at least one hundred miles from their place of origin. 
One of the best exposures of the Quaternary strata of this vicinity is at ‘‘Elephant Hill,” 
between our Camps 56 and 57. This is a conical mound, about one hundred and fifty feet in 
height, which stands in a bend of the river, and is, at its base, continuous with bluffs 
composed of the same materials, forming the eastern boundary of the trough, in the bottom of 
which the present stream flows. 
Fig. 10.—secTion OF TROUGH OF COLORADO AT ELEPHANT HILL. 
a Trap hills rising eastward to the Black mountains. 
b Bluffs of stratified Quaternary, gravel, sand, &c. 
e Elephant Hill. : 
f Tertiary conglomerates, inclined and covered with horizontal beds of gravel. 
A point of peculiar interest, connected with the structure of Elephant Hill, was the finding 
of & very large and perfect tooth of Elephas primigenius in the bed of coarse gravel and 
boulders which forms its base. 
A section of Elephant Hill from summit to base is as follows: 
1. Gravel and sand, twenty feet. 4. Sandy clay, thirty-five feet. 
2. Sand and clay, eight feet. 5. Gravel and boulders with elephant’s tooth, 
3. Gravel, forty feet. fifty feet. 
This lower bed is consolidated into a coarse conglomerate by calcareous infiltration. The 
stratification is entirely parallel with that of the adjacent bluffs, so that there is no room for 
the supposition that the material of the gravel beds has been re-arranged, as so often occurs in 
valley-drift, since its deposition. It is evident, therefore, that the elephant’s tooth is older 
than the hundred feet of gravel and sand which overlie it. 
As the bed of the Colorado is here constantly deepening, while at its mouth it is rising, and 
has been for ages moving towards the west, we can only explain the formation of beds of 
stratified sediment, boulders, &c., of so great thickness, and the occurrence of such a fossil 
near the base of the series, by supposing that the surface of the water of the Colorado was, 
at one time, two hundred feet or more above its present level, while in this locality the bottom 
bolace nearly as low asit nowis. With the great depth of water here recorded, we should hardly 
