160 
GENERAL APPEARANCE OF THE MESILLA VALLEY. 
miles wide. In its upheaval it has elevated the sedimentary beds in opposite directions, so as 
to form a synclinal axis, running north and south, the strata dipping east and west toward the 
middle of the disturbed region. Tracing this overflow northward, it does not appear to cross 
the river, but turns northwest toward the Copper Mines and the Burro mountains, lying 
directly north of the axis, where the trail crosses an elevated plain which stretches far to 
the northeast, north of the Picacho and northwestward from Dona Ana. 
The rolling country between this trappean overflow and the river is due to the same trappean 
disturbances ; and the present Mesilla valley and the bed of the Rio Bravo at that locality prob¬ 
ably lie in the angle of a fault produced by such dislocation. 
This rupture of the sedimentary crust and the dislocation of the strata dividing the latter 
into so many minor areas, bounded by trappean dykes and overflows, renders the district 
unprofitable as a means for obtaining water from deep sources, inasmuch as each minor district 
is thus fed only by the fall of water on its own area ; in other words, artesian well borings are 
not likely to be successful in their result. 
Mesilla valley .—The valley bottom, through which the Rio Grrande rolls, is made up of 
alluvium and the finer detritus of the upper country, and varies in breadth from less than two 
miles in a few places, to nearly six. Through this the river meanders in a serpentine and 
not always constant channel, changing its banks so as to encroach yearly some feet occasionally 
on either side, and carrying suspended a large quantity of mud of a fine reddish tint, derived 
from the reddish sandstone and fine felspathic clays. After freshets, the waters of the Puerco 
and GJ-alisteo carry down large amounts of mere silicious matters, which are sometimes strewed 
by overflow upon the lower and fertile bottoms of the valley to the injury of its productiveness; 
the variability of the force of the current and the body of water carried down leads to a con¬ 
tinual shifting of the bed of the river, rendering the fording of the river unsafe, except at points 
which are rarely the same in different years. 
The mesas on the east side of the Mesilla valley are chiefly covered over by this detritus, 
which overlays the whole surface within a mile of the river bottom ; further back the super¬ 
ficial matters are derived from the decay of the underlying strata, and affords soils having but 
little clay, and either yellowish or reddish as the subjacent sandstone is white or reddish. 
These soils lie at the base of the mountains, and for five or six miles toward the river ; it then 
becomes more argillaceous and calcareous, until the thick bed of detritus alluded to, in forming 
the river bed, is reached. 
These deposits are cut through by arroyo beds, and toward the north of the valley expose a 
layer of greenish sand with silicified wood, belonging to the cretaceous period. 
The soil of the valley everywhere is porous, and saturated with the waters of the river, which 
give it thus a remarkable fertility. Cotton-wood is the only timber on the bottom, but it is 
much used even in the simple architecture of the towns ; abundant crops of grass and roots, 
grapes, quinces, peaches, melons, and (other) garden vines. The fields are generally without 
fences, and watered by the sequias led from the river higher up. The soil is a light sandy 
clay, containing an evident quantity of carbonate of lime. The valley preserves the same 
fertility and general appearance from Dona Ana down to El Paso. 
