234 GEOLOGY. 
plain. The abrupt terraces, therefore, exist only along the Colorado, and as far west as the 
narrow part of the alluvial clay formation, a few miles west of the Alamo. 
It appears from the observations of Mr. Charles H. Poole, of the United States land survey, 
that the surface of the Desert, northerly from the Big Lagoon, is a gradual slope, and that a 
very considerable depression exists in that direction. This slope appears to extend from the 
low ground to the level of the upper plain, without any break corresponding to the terraces 
that have been described. Mr. Poole also states that a distinct beach-line extends along this 
slope, and marks the shore of a former sheet of water. 
The dry arroyo, or canal-like water-course, which was found between the lagoons, is in all 
probability the channel of New River. This stream derives its supply from the Colorado only 
at times of high water, and it is probably fed through numerous and ramified channels of slight 
depth. The precise locality of the entrance to the stream is not known; this renders it more 
probable that there is not a well defined channel at the Colorado, but that the water spreads 
over the banks and afterwards collects into one channel. 
GEOLOGICAL FORMATIONS. 
The geological structure of the Desert is very simple. "The bluish clay is of alluvial or 
lacustrine origin, and the gravelly plains or low hills around the bases of the mountains are 
Tertiary or Post-Tertiary. The only other formations known are the granitic and metamorphic 
rocks of the mountains on each side, with erupted dykes here and there. 
The metamorphic rocks of the mountains on the western side of the Desert have already 
| been sufficiently described in the Itinerary. Nearly all the points of rock that we passed were 
highly laminated, and contained lenticular beds of limestone. The peculiar angular character 
of most of the outerops is well exhibited by View XIII. In this instance the rocks rise above 
the level surface of the alluvium or clay of the Desert. The point of rocks forming the end of 
one of the long and high ranges extending on the northeastern side of the Desert is also meta- 
morphic, if the structure of the rock is regarded as good evidence of it. It is laminated and 
gneissoidal. The relative position of the ridges, their parallelism, and overlapping one beyond 
another, is regarded as indicative of their stratified origin and subsequent plication. The out- 
crop at Pilot Knob is very probably a continuation of one of these ridges, or the summit of 
another, in great part buried by the Tertiary of the Desert. At Fort Yuma the rock is eruptive, 
or so far metamorphosed as to lose all traces of a sedimentary origin. It is a dense porphyritic 
granite. The ranges extending from the Gila southeasterly through Sonora are properly a 
prolongation of the chain bounding the Desert on the northeast, and, without doubt, present à 
a similar geological structure. This chain is very much broken, and partakes of the character 
of the short, overlapping ridges of the Great Basin. 
Of the sedimentary and recent formations of the Desert, the alluvium has by far the gr eatest 
extent of surface, and is connected with the bottom-land of the Colorado. It, in fact, may be 
said to extend from the borders of the stream and the head of the Gulf for about 170 miles 
northwest to the base of the Bernardino Pass. Where first seen—at the Deep Well on the south- 
eastern side of San Gorgofio mountain—it was a fine blue clay, mingled with sand, and it had 
nearly the same appearance and composition in all of the ravines that were crossed in the transit 
from the Pass to Carrizo Creek. In some places a part of the clay was observed to have a reddis 
color, and this was particularly the case at tho Alamo Well. The clay at the Deep Well was 
more sandy than at other localities, it generally being very fine and indurated, so that it could 
