60 GEOLOGY. 
similarity of character is such as to admit of their immediate identification. A most striking 
feature which they exhibit in all the exposures where I observed them is their tendency to 
cross-stratification, a character which seems to indicate they have been deposited by shallow 
and rapid currents of water. 
Middle carboniferous limestone.—The bluff formed by the cut edge of the high mesa where we 
ascended it is capped by something more than 200 feet of bluish gray limestone, not very unlike 
that which covers the lower mesa in its general appearance, but containing larger quantities of 
fossils. These consist of crinoidal columns, very abundant, with many species of Productus, 
Spirigera, Rhynconella, Orthisina, Spirifer, Archwocidaris, &c., all of Carboniferous types, and 
embracing several well known Carboniferous species. 
This limestone forms the surface rock over the greater part of the high mesa, and composes 
the base upon which the San Francisco mountains rest. With its numerous and well-marked 
fossils it forms a clearly defined geological horizon; and from its wide distribution it becomes 
the most convenient and reliable guide to the geologist in the study of all the area lying between 
the Mississippi and the Colorado. 
We shall see as we proceed that in some localities it is separated by an interval of about two 
hundred feet from another thick calcareous mass, which I have denominated the Upper Carbonifer- 
ous limestone. It is probable, however, that these two beds should be considered one, as the 
strata of gypsum and shales which divide them seem to be confined to a comparatively limited 
area; and the strata of limestone above and below, though exhibiting marked differences in their 
lithological characters, and each containing some fossils peculiar to itself, have many species 
in common. The differences which these limestones exhibit are not greater than those which 
distinguish the upper from the lower coal measures, and they doubtless represent precisely 
that geological horizon. The structure of the ‘‘high mesa’’ will, perhaps, be best understood 
by the following section : 
Section of mesa at Camp 70. 
No. Description. Thickness. 
1 | Coarse gray limestone, with nodules of chert, small crinoidal columns, and Productus semireticulatus ....--| 20 
2 | Soft, light, dove-colored, massive limestone, with few fossils, but many geodes containing crystals of 
carb. lime 20 
3 | Harder and darker limestone, with Producti and numerous spines of Archwocidaris_.....---------------- 15 
4 | Gray cherty limestone, with many Producti, (P. Ivesi, P. semireticulatus, &c.,) and lésiie sod meee ———- 27 
5 | Gray cellular limestone, with great numbers of crinoidal columns, Productus Ivesi and Orthisina pyra* 
soe sok 20 
6 | Gray silicious limestone; few fossils 42 
7 | Soft dove-colored limestone, similar to No. 2, with few fossils 7 nd 
8 | Fine-grained laminated sandstone, light brown, ripple-marked ; no fossils 16 
9 | Massive fine-grained sandstone, nearly white < 20 
10 | Coarse yellow sandstone 16 
11 | Hard fine-grained sandstone, in thin layers, weathering purple 4 
12 | Coarse pink sandstone, massive 5 
13 | Coarse white and pink shelly sandstone 45 
14 ee oat ae sandstone, thicker bedded than last; surfaces of layers as smooth as though polished 
75 
15 | Shelly sandstone, weathering reddish brown 55 
16 | Red calcareous shales and soft, fine-grained crimson calc, sandstones 150 
mY § F Cream 1 d tufi is limestone... 10 
18 | Red calc: dst d shales a 368 
19 | Mountain (?) limestone, with Huomphalus, Syringapora, &c aererse 
