20 GEOLOGY. 
barriers which opposed their progress towards the sea, have cut them down from summit to’ 
base, forming that remarkable series of deep and narrow cafions through which its turbid waters 
* now flow, with rapid and almost unobstructed current, from source to mouth. The effect of 
the removal of the barriers which once checked the flow of the Colorado has been to confine the 
stream to the channel occupied by its current; to limit its vivifying influence to a narrow mar- 
gin along the banks, leaving the open areas through which it flows—and which were once lakes 
and afterwards fertile valleys—arid and sterile wastes. Instead of the broad sheets of water 
which once, by evaporation, filled the air with showers, covering with verdure all the southern 
portion of the ‘‘ Great Basin,’’ it now presents but a narrow thread hastening to the Gulf, and 
yielding almost no moisture to the region through which it flows. , 
The mountain chains that traverse the Colorado basin exhibit throughout a marked similarity 
of geological structure. The more important ranges are, in great part, composed of granitic 
and metamorphic rocks, similar to those which form the mass of the Peninsular mountains. 
With these are associated nearly every known phase of erupted material—porphyries, trap, 
trachyte, obsidian, tufa, scoria, &c., in endless variety. Some of these volcanic products are 
evidently of modern date, and show that the eruptive forces which have left on all that region 
such surprising evidences of their energy were but recently in vigorous action. _ 
The solfatara west of the mouth of the river, and the mud volcanoes of the desert, prove 
that the fires below are not yet wholly extinguished. 
The general features of the Colorado basin seem to have existed previous to the deposition 
of the later Tertiary strata, for nearly all its area is underlaid by rocks of that age, which, 
though generally covered and concealed by Quaternary deposits of gravel and sand, are upheaved 
and metamorphosed around the bases of most of its mountains. 
LOCAL GEOLOGY. 
MOUTH OF THE COLORADO TO FORT YUMA. 
flats. At Fort Yuma and above, the sediment consists of fine micaceous sand and red clay, 
1, Tunning northwestward from Sonora, cross the Gila many miles 
olorado at some distance above the fort. 
_ ‘THE PURPLE HILLS. 
re miles above - ‘ort Yuma the Colorado issues from the first of the series of passes 
Thave spoken of as giving character to its course. This pass is cut through the most 
coo 
ee 
