68 GEOLOGICAL REPORT——THIRTY-FIFTH PARALLEL. 
which, as is well expressed by Mr. Marcou, stands as a column in witness of the immense 
denudations to which that part of the country has been submitted. We have an erosion at 
that place amounting to 2,000 feet in depth, and the mountain is almost the only relic of that 
elevation in that region. The long valleys between the the Poteau, Sans Bois, and other ranges 
along the Canadian, are probably for the most part produced by denudation. The denudation 
at Delaware mountain, by which nearly all the carboniferous sandstone has been removed, is 
also worthy of remark. The elevated outcrops of the Albuquerque mountains probably have 
not suffered very greatly by denudation from currents; the slow disintegrating action of snows 
and the atmosphere have been the principal agents in producing their degradation. But it is 
by no means certain that extensive denudations have not taken place further west, and that the 
strata of the Aztec mountains, and under the now wide-spread lavas of San Francisco volcano, 
have not been swept by powerful currents. The bold escarpment presented by the strata 
towards the west, and their sudden and entire disappearance beyond, indicate the most exten- 
sive and powerful denudation. In estimating the probable amount of denudation of the strata 
at the extreme west, we must not lose sight of the fact, that the whole series appears to diminish 
in thickness as we proceed westward. This is first indicated in a marked manner by the outcrop 
at Albuquerque mountains, where the thickness, even estimating it at 2,000 feet, is much less 
than the thickness of the upper portions of the group along the Canadian river. 
Limestone and strata along the Colorado Chiquito.—It will be seen by reference to the general 
description of the series of red strata containing the gypsum, that there is much probability 
that a portion at least of these strata, especially those along the Colorado Chiquito, are in reality 
portions of the earboniferous group. The red color is due entirely to a change in the constitu- 
ents of the beds, or to infiltration, and is not any evidence of a different age, although a strong 
contrast may be presented with other strata of unquestionable carboniferous age. The con- 
formity of the strata also appears more perfect along the Colorado Chiquito than at any other 
point. The limestone of Cañon Diablo, which was met after crossing the Colorado Chiquito, 
was regarded by Mr. Marcou as the equivalent of the magnesian limestone of the Trias, and he 
thought he saw fossil Belemnites, a Nautilas, and a Pteroceras in it. He states that it occurs in 
beds from six inches to one foot in thickness, and dips to the north under angles of from ten to 
fifteen degrees, conformably with the other strata, * the New Red." Strata of limestone were 
also found alternating with the red sandstone. Mr. Campbell afterwards saw this limestone 
greatly developed in the sides of the Cafion Diablo, and states that its appearance is similar to 
that near Camp 96, which Mr. Marcou at first called magnesian limestone, evidently regarding 
itas equivalent to that along the Colorado Chiquito, but which he afterwards found to contain 
abundance of the fossils characteristic of the mountain limestone. It will thus be seen, that it 
is not possible to draw the dividing line between the carboniferous group and that which has 
been described under the title of gypsum formation. I have referred the limestone of Cafion 
Diablo to the Carboniferous, and in the section have represented the red clay and gypsum series 
as resting directly upon it. If it shall be proved by further exploration that this limestone is 
in reality a portion of the lower carboniferous series, a large part of the strata now class 
under the name of Gypsum formation must be referred to the Carboniferous. As the age of 
these strata is so doubtful, little attention has been given to the phenomena which they present 
in the preceding descriptions. Similar uncertainty rests upon the age of the sandstone which 
Mr. Campbell saw in detached mounds on the limestone around San Francisco volcano. 
The probable carboniferous age of the metamorphic limestone found beyond the Aztec 
mountains has already been adverted to. It is most probable that the outcrops in the Cajon 
Pass, and those in the Cañada de las Uvas, and in the Tejon, are of this period. 
