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 south. 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 porphyry, amygdaloid protrusions, and basaltic overflows. This country, a jor- 
nada from Cook’s spring to the Eio 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 parallel rows, connecting by canons, 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 trappe'an hills, conical porphyry, and 
trachyte pyramids toward the north, raised from 600 to 1,000 feet above the plain, 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 XIY, fig. 1.) 
East of Monument Hill the plain slopes down to the Eio 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 canons 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 unbroken 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 Dona 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 Eio Bravo, is about 12 or 15 
