ERODED VALLEY OF LITTLE COLORADO, 67 
of the valley of the Little Colorado we were constantly upon beds of drift, or the Upper Car- 
boniferous limestone, until we reached the edge of the trappean flood already alluded to as 
covering so large an area about the mountains toward which we were moving. 
The limestone is in many places well exposed, being deeply cut by the extreme branches of 
the cafions which traverse the plain. No change was noticed in its lithological character or 
fossils from Cascade river to the point where it is covered by the trap; but the sandstone and 
conglomerate, red-sbale and clouded limestone disappeared as we progressed southward. 
The drift to which I have referred is entirely of local origin, being principally composed of 
the cherty portions of the carboniferous limestone, and derived from the erosion of that rock. 
Mingled with these are quantities of pebbles of obsidian, phonolite, basalt, &c. Pebbles of 
this character are quite numerous in the accumulations of drift high up on the western slope 
of the valley, where we first entered it; and, as they could have been derived from no other 
-source than the vicinity of the San Francisco mountains, they afford satisfactory evidence that 
the drainage from that surface flowed in this direction before either the cafions or valleys were 
excavated to their present depth, and confirm the view that has been taken of the manner in 
which they were formed. 
~ 15.—TRAP MESAS NORTH OF SAN FRANCISCO MOUNTAIN. 
The ‘hie of the trap tivdiliow was reached near our Camp 75. This material was evidently 
poured out in immense quantities, and, spreading over the surface of the stratified rocks, con- 
formed entirely to that surface, forming a new table-land at a higher level. In this plateau of 
trap the draining streams have cut innumerable channels, some of which have passed through, 
not only its entire thickness, but that of the limestone below, and have exposed the upper 
surface of the drab sandstone, No. 11, of the Cascade river section. On the borders of the 
valley of the Little Colorado the trap and the underlying strata have been eroded to such a 
degree that they form mesa walls of four or five hundred feet in height, of which the upper 
fifty feet are trap. . 
