CHAPTER XXII. 
VOLCANIC OR DISTURBED DISTRICT. 
Burro mountains..—Igneous and metamorphic rocks of penasquitas.—Region about ojo de la vaca.—Red sandstone.'— 
Character of spring.—Trachyte buttes.—Enumeration of the stratified rocks near the mimbres.—Structure 
of the mimbres valley.—Agua caliente.—Vicinity of the spring ■—Properties and temperature of the water.— 
Carbonic acid gas.—Volcanic disturbance of the region.—Giants of the mimbres.—Plain east of the mimbres 
RIVER.-PlCACHO.-ELEVATION AND STRUCTURE.-TRACHYTE AND GREENSTONE DYKES.-OBSERVATIONS ON THE PICACHO.- 
Structure of the vicinity of cook’s spring.—Character of the water.—Jornada.—Basalt district.—Extent.— 
Trachyte outpouring.—Monument hill.—Mesas of rio grande valley.—Sandstone detritus.—Picacho of 
mesilla.—Topography of vicinity.—mesilla valley, extent.—River bottom, fertility. 
PELONCILLO HILLS. 
These have been so called from containing a few hills whose conical shape bore a strong 
resemblance to the Sugar Loaf, and whose form is so distinctive as to make them easily recog¬ 
nizable from a distance. This range is of hut small length, about 15 miles, dividing the great 
plain east of the Chiricahui mountains into two, which interlock round the north and south 
extremities. Geologically, these hills are unimportant. They are upheavals of plutonic rock, 
extensive overflowing of trachyte amygdaloid, and basalt, covering up the stratified rock, the 
only one of which observable there is a reddish conglomerate, now appearing for the first time ; 
dykes of felspar, augite, and porphyry, (reddish,) run from north to south. Some of the 
felspar dykes are seventy feet in width, and run north and south. The axis of all the conical 
hills in this range are made of this rock, the dyke being readily traced by the eye to the summit. 
Milk-white and opaque chalcedony is very common, the amygdaloid rock often containing large 
nodules. 
The amygdaloid and basalt capping of these hills dip toward the centre, as if there were two 
rents of the crust, and two upheavals, with a synclinal axis between. The whole breadth of 
the range is small, and the latter must be looked on as the termination of a basaltic efflux, which 
further north, at the junction of the San Carlos with the Gila, has produced a much greater 
amount of local dislocation. 
A line of less disturbance, of a precisely similar character, lies a few miles east of these hills, 
in the middle of the Valle de las Playas. There are a few pyramidal shaped hills made up of 
felspar dykes, with amygdaloid and basaltic rock, both compact and laval. 
The valley “ de las playas ” is an extensive plain, without any well defined slope to the north 
or south. The soil is much more arenaceous than that of the Sauz valley, and is much less 
deep. It is composed of a reddish felspar sand, mixed with white quartz pebbles. No water 
was found in the valley bottom. Opuntia and echino-cacti, palmetto were very abundant, with 
yucca, larrea, obione, and dwarf mesquite. 
The blue limestone of Chiricahui is not seen here ; whitish metamorphic sandstone, with 
beds of yellow, slaty grit, and flinty conglomerate, form the valley basin. These strata are 
