36 GEOLOGY. 
ocean surface, or lines of Titanic elephants crossing a plain; either of which gives a good idea 
of their appearance. 
Of these trappean mountains the most conspicuous is Mount Davis, which stands immediately 
on the left bank of the river. Like the others of the group it is composed of trachyte, tufa, 
porphyry, &c., covered by a thick mass of trap. In some cases this trap covering is horizontal ; 
in others, as Mount Davis, it is highly inclined ; its surface forming one of the slopes. 
All this group of mountains have the appearance of being upheaved fragments of an ancient 
lava-plain, but their forms have been greatly modified by degradation, and I am rather disposed 
to regard them as only the remnants of broad rounded masses of erupted rock, each marking 
the sight of a volcanic vent, worn into the angular forms which they exhibit by the erosion to 
which they have been exposed. 
Tertiary rocks.—The intervals between these trap mountains is occupied by a series of beds 
of indurated gravel, usually covered by detrital matter, forming the long slopes which have 
been referred to. 
These gravels or conglomerates are similar to those exposed above Pyramid cafion, which I 
have considered as belonging to the later Tertiary epoch. 
Fig. 8.—castLe-LIKE CONGLOMERATE BLUFF. 
They are exposed in bluffs often one hundred and fifty feet high on both sides of the river, 
frequently eroded into strange imitative forms. 
The strata of these cliffs are usually horizontal, but in many localities are more or less dis- 
turbed and metamorphosed, as in the westerly bend of the river between Camps 54 and 55. 
Near the entrance to the Black caiion consolidated sands and gravel are seen in detached 
masses and patches, resting on the amygdaloid-trap, trachyte, &c., which compose the foot- 
hills of the Black mountains. Here they have been very extensively eroded—in most places 
entirely removed—leaving the underlying volcanic rocks exposed. 
