COOK'S SPRING BASALT OVERFLOW. 159 



Cook's spring lies at the foot of the Picacho, on its east slope, between a series of porphyritic 

 and trachyte dykes ; a hundred yards northwest of the spring is a seam of porphyritic amyg- 

 daloid rock, running north and soutli. The cavities were filled with small nodules of chalce- 

 dony. The dykes here run north 12° west, which is the direction of the valley east of the 

 mountain. 



The spring lies in a small plain, and is a pool of sulphureous saline water 40 feet wide, 

 which rolls down a short distance before being lost. A reddish quartz rock is abundant, 

 cropping out in the canon approaching the spring, and in the neighborhood of the latter. It is 

 highly probable that this is nothing more than the red sandstone metamorphosed by the 

 igneous action of the porphyries. 



A valley, 15 miles wide, lies east of Cook's spring, in which no exposure of rock appeared ; 

 it slopes gently to the south ; then commenced a broken ascending country, very rocky, made 

 up of reddish jjorphyry, amygdaloid protrusions, and basaltic overflows. This country, a Jor- 

 nada from Cook's spring to the Rio Grande, is generally travelled over without stopping, so as 

 to reach water, but is generally, nevertheless, travelled slowly on account of the rugged charac- 

 ter of the road. A succession of hills in jiarallel rows, connecting by cations, constitute its 

 chief topography ; it is a more elevated district, being 900 feet above the Eio Grande level ; it 

 occupies a breadth of 20 miles from the plain east of the Picacho to Monument Hill, which 

 itself is a trachyte mass, and forms the eastern margin of this disturbed country. Looking 

 from the west, this region presents a long line of trappean hills, conical porphyry, and 

 trachyte pyramids toward the north, raised from 600 to 1,000 feet above the j^lain, and extend- 

 ing from north to south 50 miles ; in the latter direction it sinks into the plain, which becomes 

 more elevated. — (A section is given on plate XIV, fig. 1.) 



East of Monument Hill the plain slopes down to the Rio Bravo, and presents long undula- 

 tions dipping west. Approaching the valley bottom this plain is found to be an elevated mesa, 

 from 150 to 300 feet above the river, which is descended through small caiions formed by 

 the degradation of the strata, which here crop out at the bluffs. The strata dip west, and are 

 of a similar character to those described — white, yellow, and red sandstones. The rapid disin- 

 tegration of these produces the large amount of red sand which is found on the trail descending 

 into the bottom, and the heaps of white quartz sand which there, as well as lower down, form 

 in depots in particular parts of the valley. The town of Mesilla is built upon one of these. 



The upper sandstone beds of this mesa land are highly gypseous — both crystals of selenite 

 and fibrous gypsum being found abundantly. 



The lower portion of the descent was over detritus, which is probably from 60 to 100 feet 

 thick here on both sides of the river, derived from the denudation and decay of the strata 

 exposed, being, therefore, composed of gypseous, arenaceous, felspathic, porphyritic, and 

 trappean pebbles. 



The trail from Monument Hill to Mesilla is over a country iinbroken by any porphyry or 

 lava intrusion, and but one elevation appears to diversify its surface ; this is a conical hill 

 about 800 feet high, opposite the town of Doiia Ana, called the Picacho. This is an upheave 

 of compact quartz and trachyte porphyry, which is connected with the hills to the north and 

 on the opposite side of the river. The gypseous sandstones are well exposed at this Picacho 

 and north of it. Carbonate of copper occurs in small quantity in these hills. 



The basaltic overflow, which upraises the belt of country between Cook's spring and the 

 river so many hundred feet above the level of the Mimbres and Rio Bravo, is about 12 or 15 



