STRUCTURE OF THE COLORADO PLATEAU. 43 
Carboniferous rocks. They lie nearly horizontally upon the granite, forming a series of sand- 
stones, limestones, and shales, about 2,000 feet in thickness. The Carboniferous series con- 
sists of over 2,000 feet of limestones, sandstones, and gypsum, apparently all marine, and often 
highly fossiliferous. The upper members of the latter series form the surface of the mesas 
west of the Little Colorado, upon which the volcanic group of the San Francisco mountains 
rest as a base. 
North of the Colorado, near the Mormon town of Parawan, it is said that the true coal meas- 
ures make their appearance, with workable beds of coal, but south of the river an open sea 
existed during the entire Carboniferous epoch; the “ Mountain limestone’’ appearing, if at all, 
in the cafion of the Colorado, and the Coal measures being represented here—as is the case 
further eastward in New Mexico—by massive beds of limestones heretofore considered, as I 
think erroneously, the equivalents of the Lower Carboniferous or ‘‘ Mountain limestone.”’ 
The strata composing the plateau bordering the Great and Little Colorados by their dip 
form an elongated basin, of which the greatest diameter extends from the Mogollon mountains 
northwesterly into Utah. The Great Colorado crosses that line nearly at right angles; the course 
of the Little Colorado being parallel to and locally coincident with it. Near the western 
margin of the basin some of the older sedimentary strata are seen dipping eastward, resting on 
the flanks of the mountain chains which I have described as bounding the plateau in that di- 
rection. They here present bold escarpments towards the west, oftener the result of erosicn 
than fracture. They have evidently been elevated by the upheaval of the plutonic rocks upon 
which they rest, but as they are usually quite unchanged, the igneous rocks could not then 
have been in a state of fusion, but were themselves the products of anterior eruptions. The 
oldest Palwozoic rocks are nowhere, on our route, included in the elevated escarpments to 
which I have referred; and in the Great Cafion the lower members of the series are seen deposited 
around, and abutting against, pinnacles and ridges of granite, which seem to be spurs from the 
Cerbat or Aztec mountains. Hence it appears that the mountain chains which bound the 
plateau on the west existed, at least in embryo, before the dawn of the Paleozoic period, 
and formed a barrier which, to a great degree, limited the deposition of the Silurian and 
Devonian strata to the basin-like area lying east of them. 
The same phenomena recur on the other side of the plateau, near the Rio Grande, where 
the Carboniferous strata are upheaved in many places, and are seen to rest directly upon the 
granite. The absence of the older rock in both instances is doubtless dependent upon a com- 
mon cause. 
As has been mentioned, I am inclined to suspect that some of the strata composing the great 
plateau recur on the western side of the Black mountains, beneath the Tertiaries of the synclinal 
_ trough of the Colorado basin; and that fused, they form some of the porphyries, trachytes, &c., 
which characterize the mountain chains of that region. 
The Silurian and Devonian sandstones are not recognizable in any of the metamorphosed strata 
of the Peninsular Sierra, (nor are any of the rocks of the table-lands, unless perhaps the Car- 
boniferous limestone,) though they may be represented by the foliated granites and schists. 
It is quite possible, therefore, that the sediments derived from the erosion of the land during 
the older Palzozoic periods, did not extend so far into the ocean which bordered it on the west. 
In crossing the table-lands in a direction from southwest to northeast, or nearly in the line 
of the transverse diameter of the trough formed by the strata, I obtained a section, of which 
the general features are as follows: Leaving the lower Colorado, where its bed is less than 
500 feet above the sea level, we crossed three mountain chains, of which the eastern bases are, 
respectively, many hundred feet higher than the western. When we had passed the third of 
ranges, at an elevation of nearly 5,000 feet, we found ourselves on Lower Carboniferous 
strata, of which the upturned and broken edges form part of the crest of the mountains. They 
thence extend eastward in a plateau, having a distinct dip in that direction. ; 
