26 GEOLOGY. 
separated from each other; sand had subsequently been deposited upon and around them, and 
now the cut edge of the stratum appeared only as a line of detached and imperfectly quadran- 
gular masses of clay in a sand bank. 
Fig. 5.—EDGE OF SUN-CRACKED STRATUM OF CLAY IN ALLUVIAL BANK OF COLORADO. 
aa Stratified sand. 
& Detached masses of clay, formerly united. 
The bottom lands of the river here, as below, are in many localities fertile; but are generally 
charged with alkalies, which frequently appear in the form of a snow-like efflorescence on the 
surface. Where there is little alkali, the Indians successfully cultivate corn, beans, pumpkins, 
wheat and melons. Little rain falls here, and the necessary moisture is supplied to the culti- 
vated lands from the annual overflow of the river. If a larger amount of water could be 
secured at intervals throughout the season, by irrigation, a portion of the salts would be 
removed, and much of the alkaline soil be redeemed from its sterility. 
HALF-WAY AND RIVERSIDE MOUNTAINS. 
These mountains do not cross the Colorado, but their eastern bases are washed by it for 
many miles. The Half-way mountains we had little opportunity to examine; but the spurs of 
this range which reach the river are composed of coarse, gray, felspathic granite, with masses 
of dark cellular trap. 
Riverside mountain exhibits a greater variety of constituent material than any other range 
which we had an opportunity to examine. It is composed of granite, gneiss, mica, and talcose 
slate, basaltic and amygdaloid traps, with masses of quartzite and veins of quartz. With the 
micaceous and talcose slates are associated highly crystalline limestones, blue and yellow in 
color, and containing many veins of iron. These latter rocks are all, doubtless, metamorphic 
in character, and I have supposed them to be the representatives of the Palaeozoic limestones, 
micaceous grits, and perhaps dolomites forming the table-lands east of the Colorado. 
This group of rocks composing the Riverside mountain is such as leads me to suppose they 
will be found to contain rich mineral veins. I had an opportunity of examining only a very 
small portion of the range, but I shall be surprised if gold, silver, and lead are not found in it. 
Iron and copper are known to exist there, and the rocks which I have enumerated very closely 
resemble those containing the auriferous veins of California. Argentiferous galena is contained 
in greater or less quantity in all the ranges of this system, from Sonora northward, wherever 
the granitic and metamorphic rocks appear. In the porphyries, traps, trachytes, and tufas 
which, over large areas, replace or conceal the metalliferous rocks to which I have referred, I 
was able to detect no metallic minerals, except black oxide of manganese, and cobalt. 
The variety of materials which compose Riverside mountain is made evident by the striking 
contrast of colors which they exhibit. This mountain is entirely destitute of vegetation, and, 
when seen at the distance of several miles, the patches of purple, brown, blue, ash, cream, 
red, &c., form a picture which would scarcely be exaggerated if cg by the colored 
Gaceenge: the geological lecture-room. 
