a2 GEOLOGY. 
The determination of the age of this rock is a matter of some little consequence from the 
discussion to which it has given rise. By Mr. Marcou—like that already referred to west of 
the San Francisco mountain—it is regarded as the equivalent of the English Magnesian lime- 
stone; and on that identification he bases his claim to have first detected Permian rocks on the 
North American continent. The evidence which led him to this conclusion will be seen by 
reference to his journal and resumé.—/(Pacific Railroad Report, vol. III, pp. 153 and 170,) 
and (North American Geology, pp. 14 and 15.) It is scarcely necessary to say that the fossils, 
said by Mr. Marcou to have been recognized by him in this rock, if what he supposed them, 
would prove conclusively that it was not of Permian age; the genus Nautilus ranging through 
all the strata from the older Palaeozoic to the present epoch; the others (Belemnites and Ptero- 
ceras) being confined to more recent stata than the Permian. The Paleontological evidence, 
therefore, is rather against than for this conclusion. 
= AS SS SSS. 
Oe A ie - =a 
Fig. 17.—VoLCANIC CONES EAST OF THE SAN FRANCISCO MOUNTAIN. 
The proof furnished by the lithological characters and superposition of this rock will, I think, 
hardly be regarded by the unprejudiced inquirer as more confirmatory of this theory. Itis true 
that it closely resembles the cream-colored Permian limestones of Kansas; hand specimens, 
which I have from the banks of Cottonwood creek, Kansas, being scarcely distinguishable from 
those collected at. the crossing of the Little Colorado; but it is also true that the limestone 
which occupies so much of the space between the Little Colorado and San Francisco mountain 
exhibits nearly the same lithological characters throughout; and in some localities, where con- 
taining quantities of Productus semireticulatus and other unmistakable Carboniferous fossils, is, 
_ perhaps, a little more like the Permian rock of Kansas than is that nearer the Little Colorado. 
The same may be said of the cream-colored portions of the crinoidal limestone in the cafion of 
Cascade river. 
