CARBONIFEROUS LIMESTONES—VOLCANIC CONES 71 
teristic of the Upper Carboniferous limestone; which is, as I have stated, probably only locally 
separated from this by beds of gypsum. I am very confident that none of the strata exposed 
west of the San Francisco mountain and east of Partridge creek are more recent than the 
carboniferous epoch; nor do I believe that the summit of that series is reached in any section 
there exposed. 
From Cedar creek to New Year’s Spring the surface is occupied by volcanic materials, A 
sheet of dark compact or vesicular trap throughout that distance covers the sedimentary rocks, 
except where cut through in the cafions of the draining streams. As we approached the San 
Francisco mountain: this plateau becomes gradually more and more covered by the products of 
more recent eruptions poured over, or piled upon its surface. Of these volcanic masses the 
most conspicuous are Bill Williams’s mountain, Mount Kendrick, and Mount Sitgreaves; each 
of which is a mountain of considerable size, marking the site of a volcanic vent scarcely 
inferior in magnitude to that of San Francisco. Beside these principal cones there are an 
infinite number of smaller size, of which many are distinctly crateriform, and all seem to have 
been the foci of volcanic fires. 
The San Francisco mountain has a general conical form, but terminates above in four distinct 
summits. These seem to surround a central crater; at least, such is the impression we obtained 
from the best views we had of it. On all sides, and especially towards the north, spurs radiate 
from it which mark the lines followed by the lava floods of some of its great eruptions. 
The soil arising from the decomposition of the lava and ashes ejected from these volcanoes 
is evidently of excellent quality, sustaining magnificent forests of pine, interspersed with 
which are prairies covered with luxuriant grass. 
Carboniferous limestone.—About New Year’s Spring and eastward is an area of considerable 
size, which, by some chance, has escaped the torrents of lava poured over all the surrounding 
country. Here the surface of the Carboniferous limestone, forming the summit of the high 
mesa, is fully exposed; and thence to the Little Colorado, with occasional interruptions, it 
forms the substratum of all parts of our route. In a number of localities I noticed its charac- 
teristic fossils, Productus semireticulatus, &c., but in other places it contains no fossils whatever. 
The geodes of crystallized carbonate of lime, so marked a feature at Camp 70, are. usually abun- 
dant. The surface of the limestone has been considerably eroded by streams flowing from the San 
Francisco mountain towards the Little Colorado; and as these streams have-no existence at the 
present day, their dry beds offered us the most agreable line of travel. In the banks of these 
arroyos the cut edges of the strata show a strong dip towards the northeast, which continues 
with great general uniformity to the banks of the Little Colorado. 
At intervals the sedimentary rock is covered with erupted materials, basaltic trap, scoria or 
ashes; and at several points are cones, once minor volcanic vents, and the sources from which 
these igneous rocks were derived. Near our Camp 84 is the most interesting group of these 
craters met with on our route, (Fig. 17.) Several of them are still very perfect in form, and 
long sinuous lines of black lava run down their sides—graphic records of their latest eruptions. 
Ten miles west of the Little Colorado we entered a region where scarce any soil covers the 
tock; and thence, to the crossing of the stream, we were travelling almost constantly upon its 
naked service. Throughout most of this distance we passed over a cream-colored limestone 
undistinguishable in its lithological characters from that which contains well-known Carbonif- 
erous fossils a few miles back, and of which it seems to be the continuation. Fossils are here 
rare, but not far from the point where we left it I observed a well-marked Euomphalus, similar 
in appearance to E. rugosus, which, unfortunately, was imbedded in a block of such size and 
solidity that I could not remove it by any means at my command. . . 
This limestone forms the walls of the Cafion Diablo, where it was carefully examined by Mr. — 
A. H. Campbell, when attached to Captain Whipple’s party. Mr. Campbell found in it 
crinoidal columns, and regarded it as identical with the limestone which occupies most of the 
area between San Francisco mountain and that point. | 
