114 
GEOLOGY. 
bounding tbe vision in that direction. Northward and eastward, the most striking feature of 
the landscape is found in the extraordinary outline of the mountains. Gigantic columns and 
domes of mountain size rise high into air, above the lower hut still rough and angular outlines 
of the ridges. An idea of their forms may he obtained from the little outline sketch, hut no 
conception of the desolate grandeur ot the scene can he obtained without looking out upon it. 
The highest of these peaks is called A-melle-e-quette, by the Yuma Indians, and Chimney Peak 
by residents of the fort. The “Dome Rock” has the Indian name of Ar-with-a-que , (pro¬ 
nounced as written.) They are about twenty miles distant from the fort. These pinnacled 
ranges are represented to be of granitic rocks by intelligent parties, who have visited portions of 
them, from Camp Yuma. Chimney Peak is also reported to he formed of sandstone strata 
standing on edge. 
The principal and most distinct of the dome-like rocks is called “ Capitol Dome it is not 
included in the sketch, hut one is seen of a similar shape. The vertical, wall-like sides of these 
rocks simulate the weathered remnants of horizontal fields of hasalt, or the vertical bluffs of 
trappean formations. These elevations were too far removed and inaccessible to he visited 
during my visit at the fort. They are very singular, and are worthy of a special examination. 
The granite of the knob contains large crystals of feldspar, and is thus a porphyry or por- 
phyritic granite. It is probable, however, that the rock varies in different parts of the hill. 
Several irregular veins of dark-colored hornblende and epidote were exposed fn a natural section 
along the river, and, from their ramified and peculiar character, I concluded that they were of 
volcanic origin, and probably of comparatively recent intrusion. 
It is a curious fact that the Colorado cuts directly through this elevation of granite, instead 
of circling around it, where it could readily excavate a channel in the clay. Indeed, there is 
an old, dry channel on the north of the butte, which becomes filled at the time of high water, 
thus, for a time, making an island of the butte. 
SECTION OF THE COLORADO RIVER AT FORT YUMA. 
a, Alluvium, g, g, Porpliyritic granite, r, Colorado river, t, Tertiary, or post tertiary sandstone. 
The section from north to south, across the butte, will serve to show the relative positions of 
the granite, the Tertiary, or Post Tertiary, and the alluvium of the river. The walls of granite 
rise nearly vertical on both sides of the river, and being of tbe same height and outline, 
indicate that the butte has been cleft asunder, or fissured by some great convulsion of nature. 
Even the rock appears as if freshly broken, for it is not discolored and blackened by age as the 
outer portions are. The fissure, if such it may be considered, affords a good opportunity for 
