34 3 GEOLOGY 
fragments of the masses of metamorphosed chert, so common in the limestones of the upper 
Colorado. 
As far as inacved. these conglomerates contain no fossils properly their own, except the 
silicified trunks of trees. These are abundant, generally broken; their extremities more or 
less rounded—as is usually the case with drift-wood—and are principally coniferous. 
The trough which the Colorado has excavated in the series I have described is partially filled 
with accumulations of loose gravel of still more modern date. It is quite impossible to draw 
the line separating these two formations, although a long time must have elapsed between the 
eras of their deposition—the time occupied by the Colorado in deepening its bed two hundred 
feet. * 
I am disposed to class these conglomerates with the more recent Tertiaries ; but resting, as 
they do, horizontally and undisturbed on the traps and porphyries of the Black mountains, 
rocks which have thrown up and metamorphosed the Tertiary strata, both above and below 
this locality, they may be more recent. The identity of composition which their enclosed 
pebbles exhibit with those of the modern gravel beds, though affording presumptive evidence 
of their common Quaternary age, is not entirely conclusive, as similar te eR features 
Fig. 7.—HILIs OF TERTIARY CONGLOMERATE AND MODERN GRAVEL, CAMP 49. 
were exhibited by this portion of the continent during the Tertiary era to those of the present 
epoch. The lines of drainage were, in general terms, the same, and, consequently, the sources 
from which transported materials were derived. 
In several excursions to the base of the Black mountain range, made from our Camps 49, 59, 
51, &e., I procured a very complete series of the rocks which compose these mountains. 
Among them the porphyries are very conspicuous, including at least a hundred varieties, of 
which some are unequalled in beauty by any I have elsewhere seen. 
