VARIETY AND ORDER OF ERUPTION OF VOLCANIC ROCKS. 
153 
not visible on the immediate valley bottom, but can be traced on tlie hillsides, and near tbeir 
summits. On the Peloncillo margin of the valley the dip is 8° south 20° east. On the 
pyramidal hills the dip is 12° north 20° west. East of these, and near the Ojo de la Inez, and 
at Penasquitas, the strata exposed from above downward were— 
Feet. 
Summit capping of felspathic amygdaloid. 30 
Sandstone grit, metamorphic. 25 
Blue silicious chaledonic rock, with seams of talcose rock... 30 
Yellow sandstone shale. 45 
Brown conglomerate flinty pebbles and agatized layers. 66 
Total thickness.° 196 
I searched these sandstones in vain for fossils. They are, in position, superior to the lime¬ 
stone, and inferior to the gypseous beds of the San Pedro. The soil of this, the eastern extremity 
of the valley, is highly gypseous, being, in texture, a reddish sand, with whitish pebbles ; these 
latter derived from the sandstone grit above alluded to as lying under the amygdaloid capping. 
The Peloncillo and Pyramid hills, as they have a striking resemblance in form, so in 
structure are they alike. They are not protrusions of primary rock which have carried up 
their superimposed strata, such as Chiricaliui and the Sierra Calitro, but they are injections of 
plutonic rock, which, rising in a fluid condition, has forced itself through the fissures formed 
hy the subterranean force, and spread over the summit level of the plain, covering over the 
stratified rock, infiltrating itself between the strata, and metamorphosing them to a great 
extent, giving rise to every shade of silicious rock, from ordinary sandstone to chalcedony, 
opal, and chabasite.—(Plate XII, figs. 2 and 3, illustrate this intrusion.) 
Trachyte and porphyry are the two species of rock erupted • most abundantly ; the former 
forming the crest of many of the hills, entering the canon, and spreading itself over the 
surface like a stratum, while the porphryries are found in dykes, and do not appear to have 
been so fluid as the trachytes, A trachyti^ conglomerate is found capping some of the lower 
hills, the pebbles of which are porphyry, while the paste is trachytic ; thus the porphyry 
injection would have been the first which occurred, the trachyte subsequently forcing its way 
through the strata by different fissures, and by the rupture of the crust involving the 
porphyry fragments in its mass. There are as many as five varieties of porphyry found, all 
of them having a fine clay felspar paste from light brown, passing through shades of red to 
violet, including small well defined crystals of orthose. On the east side of the Peloncillo 
range, near camp, August 4, in the canon, a dyke of dense augitic basalt protrudes through 
the trachyte which lines it on either side ; the vein is 25 to 30 feet wide, increasing in width 
downwards. East of it, in the bed of the arroyo at camp, the reddish felspar porphyry, con¬ 
taining quartz crystals of irregular form, is found 60 feet wide. This is the same rock found 
on the north side of the Grila in the canons, and also among the igneous rocks at the foot of 
the Sierra Calitro.—(See Plate XII, figure 3.) 
It may be perceived from the foregoing that there have been three distinct volcanic outpour¬ 
ings in these ranges—the Peloncillo and the Pyramids, considering them geologically as one. 
1st, that of the porphyries ; 2d, that of the trachytes ; and 3d, that of the basalt, the antiquity 
of which were in the order indicated, and the earliest of them subsequent to the deposition of 
the reddish sandstone, and whitish grit, which overlie the lower carboniferous limestone. 
The result of these outflows has been, not only to elevate the district in which the flow aotually 
occurs, but also tbe whole region in aline north and south, lifting it up to a much higher level 
20 U 
