CHAPTER XXII. 



VOLCANIC OR DISTDRBED DISTRICT. 



BtiRRo MODNTAiNs. — Igneous and metamorphic rocks of penasquitas. — Region about ojo de la vaca. — Red sandstone.- — 

 Character of spring. — Trachyte suites. — Enumeration of the stratified rocks near the mimbres. — Stri-ctvre 



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 or the mimbres. — Plain east of the mimbres 



RIVER. PiCACHO.^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 contaiaing 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 but small length, about 15 mile.s, 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 efllux, 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 



