160 GENERAL APPEARANCE OF THE MESILLA VALLEY. 



miles wide. In its upheaval it has elevated the sedimentary heds 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 Doiia Ana. 



The rolling country between this trappean overflow and the river is due to the same trapjiean 

 disturbances ; and the present Mesilla valley and the bed of the Kio 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, iuasmuch 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. 



llesilla valley. — The valley bottom, through which the Rio Grande 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 Galisteo 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 diflerent 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 Doiia Ana down to El Paso. 



