EASTERN CHAIN OF CERBAT MOUNTAINS. 53 
That experience, with a careful examination of the surface nearly to the brink of the 
precipice bounding Yampai valley, together with the observations of Mr. Marcou and Mr. A. 
H. Campbell, who with Captain Whipple crossed the edge of the table-lands a few miles 
southeast of where we came on to it, have led me to the conclusion that the sedimentary strata 
covering the eastern flanks of the Cerbat mountains include only portions of the Carboniferous 
series. 
I think it will be seen, as we proceed, that there is even reason to suppose that the lower 
member of that series, the ‘‘ Mountain limestone,’’ is wanting in all the sections examined by 
Mr. Marcou west of the San Francisco mountain. 
The Carboniferous strata in this region, where all are represented, are at least 2,500 feet in 
thickness, and consist of three great beds of limestone, separated by red, white, and gray 
sandstones, beds of gypsum, &c.; the whole forming a series of such magnitude, and of a 
structure so complex, as almost certainly to confuse the most experienced geologist, unless he 
chanced to find such a section as that exposed in the cafion of the Colorado. 
The mountains which I have called the eastern range of the Cerbats form the last of the 
ranges which intervene between the ‘‘mesas’’ and the lower Colorado. Along the course of 
the upper Colorado, in the direction pursued by our party, there are no other mountains till 
that line strikes the first ranges of the Sierra Madre, (‘‘Sierra de Chusca,’’) east of Fort 
Defiance ; although north of the Colorado isolated mountains are discernible, and opposite 
them in the south, more than a hundred miles distant, is the similar isolated but more lofty 
San Francisco group. 
Along the 35th parallel the table-lands are not so continuous or well marked—scarcely more 
than the edge of the basin of the Little Colorado being crossed by it; and the route followed 
by the party of Captain Whipple, from the San Francisco mountain west, is much broken by 
effusions of trap, and by lines of upheaval and erosion. Some of the mountain ranges mentioned 
by Captain Whipple, as that of the ‘‘Black Forest,’’ including the Picacho, subsequently 
visited by us, entirely disappear before reaching the Colorado on the north, and the sedi- 
mentary rocks in its line show only slight traces of its disturbing action. 
The Aztec and Aqariuus mountains, of which interesting though very brief notes have been 
given by Mr. Marcou, I was not able fully to identify on our route, but have been disposed to 
regard them as the equivalents of the range under consideration. The Aztec mountains are 
its precise geological equivalent, and I should not hesitate to consider one the continuation of 
the other had we not seen a distinct connexion of both the ranges, crossed by us east of the 
Black mountains, with the principal granitic mass of the Cerbats; and the Picacho range disap- 
pearing in the north led me to suspect the Aztec mountains had done the same. 
Issuing from Yampai valley through the caflada of Yampai creek, we followed this until we 
came up on to the table-land to which reference has been so frequently made. Throughout 
its course the cagada, or cafion, of Yampai creek is the effect of erosion. Its lower and broader 
part is bordered by hills of coarse gray granite, as at Camp 64. Thence to Camp 65 it becomes 
narrower, running between hills of granite on the west and mesas of trap on the east. The 
granite is plainly much the older, having once been completely covered and concealed by floods 
of lava, apparently flowing from the southeast, and doubtless of common origin with those 
which surround the Picacho, and form mesas in Partridge creek. 
The drainage from the mesa has followed the line of junction between the granite and trap, 
leaving walls of the latter rock several hundred feet in height on one side, and on the other 
granitic masses, from which the trap has been all eroded, or remains in detached capping 
masses, 
From Camp 65 we turned into a branch of the cafiada leading down from the east, which, 
being cut through the trap plateau, has perpendicular walls on either side, and for several 
miles is a typical cafion. 
