68 GEOLOGY. 
Leaving the valley of the Little Colorado at Camp 76, we ascended the mesa, and crossed it 
in a southwesterly direction, to the headwaters of the Rio Verde. There we struck the trail 
of Captain Whipple’s party, and I was able to connect my observations with those of Mr. Mar- 
cou, the geologist who accompanied that officer. The geology of all this region we found to 
be exceedingly simple, and to include nothing but what had already become quite familiar, 
from our experience of the preceding month. 
A sheet of trap occupies almost without exception the entire surface from. the Picacho to the 
San Francisco mountain. In one or two instances I noticed masses of the underlying sedi- 
mentary rocks forming hills not covered by the trap; as that separating Partridge from Cedar 
creek. These hills had such an altitude that they were not submerged by the lava-flood which 
covered the lower grounds, and concealed all minor evidences of the erosion previously suffered 
by the general surface of the country. The sheet of trap has been cut through by most of the 
streams; exposing the stratification of the underlying rocks, as well as the outline of their 
upper siadiise: From these exposures it was easy to perceive that the lava had covered a 
district precisely similar to that nearer the Colorado on the high mesa, where we crossed it; a 
high plateau of sedimentary rocks, which had been cut into valleys of moderate depth, and 
on which the landscape exhibited a series of gentle curves, like those of the ‘‘rolling country” 
of the Mississippi valley. 
Before the floods of lava covered this region its geological structure was as like that of the 
high mesa further north, as were its physical features. The hills of sedimentary rock—such 
as that referred to east of Partridge creek—are composed, above, of the crinoidal limestone, 
(middle carboniferous,) the same which caps the mesa wall at Camp 70, and forms the surface 
rock of the high mesa east of that point. Here, as there, it is highly fossiliferous, and con- 
tains Productus semireticulatus, P. Ivesi, Athyris subtilita, crinoidal columns, &c., &c., in 
great numbers. Beneath this is the drab cross stratified sandstone, already so fully described, 
which holds the same relative position in all the numerous localities on our route where it is 
exposed. 
Fig. 16.—sEcrion OF STRATA FROM THE PICACHO TO BILL WILLIAMS’S MOUNTAIN. 
o 38 
te - 
. S Bill hee iF 
Breach 8 R 
On, : S ep see nc ee is 
eA ID FIN ICRA reat. 
. i } 14 WY AS 
DD sR = iP vty ad 
AE li acecec ue. Zl Ul anna ccones Beret a 
a ie y ne 
ie 
a Tran 5 Cherty limestone. e Cross stratified sandstones. d Red calcareous sandstones. 
Partridge and Cedar creeks now flow in valleys which had been partially excavated before 
the period of the trap overflow. The walls of the cafion of Partridge creek are generally 
composed at base of the cross stratified sandstone—the limestone being in most localities 
wanting—upon which is a mass of trap, of a thickness varying with the elevation of the sur- 
face over which it flowed. In some places the sandstone is entirely wanting; the trap coming 
down and forming the bed of the stream. 
The description of the geology of this vicinity, given by Mr. Marcou, (Pacific Railroad 
Report, vol. Il, Geological Report, pp. 154, 155 and 156; Geology of North America, pp. 23, 
24, &c.,) is very brief and not altogether comprehensible. While traversing the region about 
the San Francisco mountain the party of Captain Whipple were constantly involved in snow 
