158 GEOLOGY OF THE PICACHO OF THE MIMBEES. 



both species of rock, and in the syenite the pyroxene mineral is in the form of slender crystals 

 of actynolite. This constitutes the main mass of the mountain, whose line of trend is north 

 10° east and south 10° west. These syenites are cut through by felspathic dykes and broad 

 veins or dykes of greenstone. This granite axis carries up with it the sandstone strata ; the 

 uppermost beds of which, the yellow sandstone, is found inclined on the west side at an angle of 

 20°, while up on the eastern slope of the Picacho the dip is 40° and 45". The felspar porphyry 

 dykes, which appear mostly on the west side of the mountain, run nearly north and south, and 

 thus cross the geographical axis. Loose fragments of this mineral are found on the mesa 

 forming the south end of the hill. About 200 yards east of the south end of the hill the trail 

 crosses a dyke of greenstone, running northeast and southwest, about forty feet wide, accompa- 

 nied by porphyritic felspar. This dyke can be traced northwest into the mountain, up its sides, 

 and almost to the summit ; it is not vertical, but appears to take the slope of the sandstone 

 strata, which it covers up, and does not apparently cut across. Southward this augitic dyke 

 can be traced to the Sierra Florida, into which it enters. The eastern flanks of the Picacho are 

 of the red and yellow sandstones and white grit, which appears to be repeated several times by 

 the elevation of the porphyry dykes, which, in conjunction with greenstone, cut up the small 

 hills through which the trail approaches Cook's spring.* — (See plate XIII, fig. 4.) 



There are two or three points of interest connected with the Picacho : 



1st. Its trend is different from any of those further west, which generally varied between 

 north 45° and north 60° west, while the Picacho runs east of north. 



The trail does not travel south of the whole mass of mountain, but takes advantage of a 

 depression in the hill immediately south of its highest point ; there a caiion leads by a slight 

 ascent, but tortuous course, across the mountain to Cook's spring. 



From the time the Mirabres river was left, fertility may also be said to have disappeared ; the 

 fine grass found on the mesa and left bank of the river gradually thinned out, until a sandy, 

 barren trail formed the rest of the route to the Picacho ; for 10 miles nothing was seen but 

 fouquieria, palmetto, and larrea. In the canon, near the spring, good grass, cedar, and walnut 

 trees again appeared, but at the spring itself there was no timber. Osier, three feet high, and 

 bunch grass of a coarse kind, were the only vegetation. 



2d. In the appearance of a distinct granite rock, a rock containing mica in small quantity, 

 and syenitic rock ; generally speaking, felspathic rock of the varieties of leptynite and perlite 

 have constituted the hypogenic axis. 



3d. In the greenstone dykes. Trappean and basaltic dykes and overflows have been de- 

 described as forming a large portion of the district along the Gila. East of the Picacho, these 

 volcanic rocks again make their appearance ; there is nothing new, therefore, in the appearance 

 of this broad dyke, nor hardly anything surprising in observing the geological connection be- 

 tween the Picacho and the Sierra Florida ; but it may be remarked of the dyke, that, in tracing 

 its course up the Picacho, it may be observed running between and separating the sandstones, 

 and metamorphosing them into reddish quartz rock; it has merely insinuated itself between the 

 strata, and has not upheaved them. The upheaval was produced by the granitoid axis, (syenitic 

 rock,) and this upheaval was anterior to the intrusion of the greenstone dyke, for the latter 

 preserves its direction unaltered, whether in the mountain or on the plain ; it is therefore pos- 

 terior in occurrence to the upheaval of the Picacho. 



* Underucatli the saiidstoue, and immediately in contact with the igneous rock, lies the blue limestone, reposing at au 

 angle of 28°. 



