GENERAL FEATURES. 5 



the eye in every direction, the intervening surface spreads out into wide basin plains of an allu- 

 vial character. These basins receive and absorb the scanty streams of the adjoining mountains. 

 Rarely indeed, except in the highest mountain recesses, is running water visible, the occasion- 

 ally copious rains furnishing only a temporary current along the course of the numerous stream- 

 beds. The water thus accumulated in rocky basins or marshy lagoons, affords the only supply 

 for travellers, over these arid wastes. During the dry season these plains spread out their 

 dreary tracts, unrelieved by a single feature of fertility, occupied by innutricious grasses or a 

 scattered growth of dry shrubbery, among which the repulsive form of the " Spanish bayonet" 

 {Yuca) is a conspicuous feature. Owing to their exposed and elevated position, these plains are 

 subject to great extremes of temperature. They are mostly shut off from the Rio Grande 

 by a variable mountain range, composed of the carboniferous limestone, variously associated 

 with igneous rocks. The passage to the valley is accomplished by following down the natural 

 cleft made by some rain stream. These passes exhibit fine sectional views of the tilted lime- 

 stone strata, exposed in various conditions of disturbance, in some places inclined at an angle 

 of 80° to the west, and at other points exhibiting evidences of igneous action in metamorphic 

 changes. 



Kactc Spnngs 



We have thus reached, on the line of the ordinary wagon-road, the upper valley of the Rio 

 Grande, the external features of which, as more directly connected with the line of boundary, 

 will claim a more detailed notice. At first, however, a more rapid sketch must suffice, while 

 continuing to notice the general features of scenery and geological structure presented on the 

 route westward to the lower valley of the Rio Gila. 



As we pass from the rocky canon, by which we enter on the Rio Grande valley, we first come 

 upon a gravelly plain, generally presenting a smooth and more or less uniform surface, sloping 



