158 
GEOLOGY OF THE PICACHO OF THE M1MBRES. 
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 hy 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 porpbyritic felspar. This dyke can he traced northwest into the mountain, up its sides, 
and almost to the summit; it is not vertical, but appears to take tbe slope of the sandstone 
strata, which it covers up, and does not apparently cut across. SouthVard this augitic dyke 
can he 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, hut takes advantage of a 
depression in the hill immediately south of its highest point; there a canon leads by a slight 
ascent, but tortuous course, across the mountain to Cook’s spring. 
From the time the Mimbres river was left, fertility may also be said to have disappeared; the 
fine grass found on the mesa and left hank 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; hut 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. 
« Underneath the sandstone, and immediately in contact with the igneous rock, lies the blue limestone, reposing at an 
angle of 28°. 
