VARIETY AND ORDER OF ERUPTION OF VOLCANIC ROCKS. 153 



not visible on the immediate valley bottom, but can be traced on the hillsides, and near their 

 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 Peiiasquitas, the strata exposed from above downward were — 



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 tbese 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 bighly gypseous, being, in texture, a reddish sand, with whitish pebbles ; tbese 

 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 Chiricahui and the Sierra Calitro, but they are injections of 

 plutonic rock, which, rising in a fluid condition, has forced itself through the fissures formed 

 by 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 caiion, 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 trachytic 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 difl'erent fissures, and by the rupture of the crust involving the 

 porphyry fragments in its mass. Tliere 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 cauon, 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 Gila 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 actually 

 occurs, but also the whole region in a line north and south, lifting it up to a much higher level 

 20 U 



