70 GEOLOGY. 
a striking discordance between that section and that of the cafion of the Colorado, which, in a 
like manner, includes the junction of the granite with the sedimentary rocks. 
Mr. Marcou represents the cliffs on Pueblo creek as twelve hundred feet in height, of which 
six hundred are granite; and from the spacing of his figure we may infer that the thickness of 
the red sandstones is not greater than two hundred feet, the remaining four hundred being 
made up of ‘‘gray sandstone and carboniferous limestone.’’ Even supposing this limestone to 
be as he has represented it, the Mountain limestone, there is no room between it and the granite 
for the two thousand feet of strata which should occupy that interval. We can hardly imagine 
that a group of rocks of such thickness and character as those which underlie the Mountain 
limestone in the cafion of the Colorado should have been so much reduced in thickness within 
a distance of fifty miles. From all these considerations I am compelled to ascribe to the 
‘*Devonian sandstones’ of Mr. Marcou, not only in the locality referred to, but wherever 
appearing immediately beneath what he considers the Mountain limestone, a date not earlier than 
the Carboniferous era. 
The questions involved in the determination of the age of the strata exposed on Pueblo creek 
have not simply an abstract interest, but are of the highest scientific importance as affecting 
the mode of formation and original extent of the great central plateau. If, as supposed by 
Mr. Marcou, we find the granitic masses now forming the natural boundaries of the table-lands 
crowned with the oldest rocks of the series, we prove that these mountain chains had no 
existence during the older palaeozoic periods, and that the strata forming the table-lands once 
extended indefinitely westward. If, on the contrary, we find the Silurian and Devonian strata 
abutting against, but never covering, the granite ranges, none but Carboniferous rocks crowning 
their summits, we must conclude that the mountains west of the table-lands, like those border- 
ing the Rio Génie, existed, at least in embryo, anterior to the palaeozoic epoch, and that 
they limited the area of deposition of the Silurian and Devonian rocks. The evidence which 
we at present have, bearing on this question, seems to me decidedly in favor of the latter 
supposition; and while our views of the general geological structure of this portion of the con- 
tinent are all liable to be changed or modified by the observations which shall be hereafter 
made in the wide areas yet unexplored, we may safely say that this is one of the generalizations 
sanctioned by all the knowledge we yet have on the subject. 
Magnesian limestone.—Mr. Marcou, in several places in his journal and resumé, speaks of 
finding, both east and west of the San Francisco mountains, a magnesian limestone, the equiva- 
lent of the European Permian rock of the same name. Among other localities of its occurrence, 
cited by him, is a hill three miles east of Captain Whipple’s Camp 96.—/(Pacijic Railroad 
Report, vol. III, Geology, p. 156.) 
In his reswmé, pages 23 and 24, he again refers to this magnesian limestone, and to this among 
other localities where it is exposed. His words are: ‘‘ This formation, which is placed between 
the carboniferous and the trias, corresponds, without doubt, to the magnesian limestone (Per- 
mian) of England, and is a new member, which I add to the series of the secondary rocks in 
‘* North America.’’ 
Of the limestone exposed near the crossing of the Little Colorado—the point where Mr. 
Marcou supposed he first met with Permian rocks—I shall have occasion to speak again. In 
reference to that observed by him between Lava and Partridge creeks, I am compelled to say 
that, on an examination of the same localities, made under the most favorable circumstances, I 
failed to detect any evidence of the presence of Permian strata. The hill which he describes 
_ as formed at summit of magnesian, at base of mountain limestone, was ascended by our party. 
To me it seemed composed throughout of the crinoidal or Coal measure limestone; exhibiting 
precisely the same lithological characters and fossils as at our Camps 70, 73, &c. Some of its 
_ layers are dolomitic, and some are nearly destitute of fossils. The nodules of chert, mentioned 
by Mr. -taatia are abundantly distributed through most parts of it, and are still more charac- 
