142 
DEVONIAN GROUP-SAN PEDRO VALLEY. 
which forms an angular, sharp crested hill, of small elevation. Further south and west of this 
raised bosses of granite protrude. The sandstone on this side of the river is a coarse con¬ 
glomerate of granite and feldspar porphyry, with reddish porphyritic pebbles interspersed. On 
the north side the sandstone is made up of quartz and feldspar, in fine particles, including 
syenite and greenstone and basaltic pebbles. If this bed extends under the river, and is one 
with its congener, the thickness of the strata would then be over 2,000 feet. 
No trace of fossils could be found in the sandstone strata. This rock corresponds lithologically 
to the old red sandstone or Devonian system. It is here found underlying the carboniferous 
limestone situated further east, on the Sierra Calitro, and is the furthest point to the west to 
which it has been traced along this line. The conglomerates found on the desert have not the 
reddish tint, nor the coherence of this rock, and are similar to that flanking the east side of the 
Cordilleras. 
In the valley between the Sierra Catarina and the Sierra Calitro the San Pedro river flows 
and empties into the Gila. From that point the Gila turns northeast, while the course of the 
San (Jose) Pedro is south, 20° east, and runs at the base of a range parallel and almost con¬ 
tinuous with that in which Saddle mountain lies. These are the Sierra Calitro. The river 
rolls on the west side of the chain and receives several small creeks flowing out from it. The 
valley of the river becomes wider for several miles up, forming a splendid bottom of fine grass 
and wild vine, the bottom being in many places a perfect thicket with jungle and bush. 
From the mouth of the San Pedro, for seventy miles up its stream, the bottom is cut 
out from nearly horizontal strata of gypseous rocks. The mesa and low land which rises 
gradually up to the base of the Calitro hills are altogether of this formation. Of a loose texture, 
they are easily eroded by the freshets from these mountains, and not only are deep channels 
worn for the passage of Arivaypa creek and other small streams, but even the whole mesa is 
cut through; so that what at first sight appears a smooth ascent along a sloping mesa, is found 
to be a series of hills and valleys, with dry arroyo beds, whose sections show the denudation 
which has occurred. The Calitro hills lie about eight to ten miles east of the river. The west 
bank of the San Pedro is bounded by foot hills of the Santa Catarina range; these are of 
lamellar felspathic rock of the same character as the Spire hills. They approach the river 
closely, and generally the river keeps on the west side of the valley. 
There is a slight dip to the east in these gypseous beds, which are of great depth ; in some of 
the valleys as much as 240 feet of yellow sand rock is exposed, (half a mile east of river, July 9.) 
It contains broad veins and seams of gypsum, massive and amorphous anhydrite in seams two 
to four inches thick, and repeated five times in a thickness of 30 feet, with muriacite, and also 
seams four to six inches wide of selenite. At the base of the Calitro range it is overlaid by a 
rough conglomerate of jasper, porphyry, and-granitic pebble, from 80 to 100 feet thick. This 
bed also dips eastward toward an elevated crest of felspathic granitoid rock, which forms the 
outlier of the Calitro range ; reposing conformably on the east of this crest, and apparently 
upraised by it, are thick beds of hard yellow sandstone, (grit,) of which the lower strata, in 
proximity to the granitoid rock, are metamorphic, and contain quartz crystal in abundance. 
These strata might be 500 feet high. Still east of this, in a deep valley in these hills, a bed of 
red granitoid rock (red felspar porphyry) occupies the bottom of the valley, and underlies the 
thick beds of red sandstone which form the elevated hills of the range, that are cut through 
by seams of porphyry, and capped by basalt and amygdaloidal trachytic rock. The gypseous 
beds are cut off from any contact with these elevated hills by the felspathic crest, which consti- 
