56 



GEOLOGY. 



The surface of the ground adjoining the river hank is a slightly broken slope, extending to 

 the east, and showing a continuous development of the range to the north and south. The 

 general surface presents no indication of a river course, and you are not aware of its presence 

 till you stand suddenly on its abrupt brink ; even here the running water is not always visible, 

 unless advantage he taken of the projecting points, forming angles, along the general course of 

 the river. From this dizzy height the stream below looks like a mere thread, passing in 

 whirling eddies, or foaming over broken rapids ; a stone hurled from above into this chasm 

 passes completely out of sight behind the over-hanging ledges, and one can often count thirty 

 before the last deadened splash announces that it has reached the river bed. From the point 

 formed by its last projecting ledges the view is grand beyond all conception. You can here 

 trace backward the line of the immense chasm, which marks the course of the river, till it 

 emerges from its stupendous outlet. 



Below this the country presents from a bird's eye view an extended basin, set off by the 

 rugged volcanic mountains of the Chisos, we trace the winding of the stream in the basin below, 

 to which distance gives a softening character of fertility not by any means borne out on a nearer 

 inspection. 



OnXLINE YIKW OP THE CHISOS MOnNTAINS, LOOKING TO THE NORTH. 



A. Dark-colored igneous rock of vesicular or close texture, disposed in vertical columns or horizontal masses. 



B. Volcanic breccia in evenly horizontal strata, light-colored and of crumbling earthy texture. 

 a. Emory's Peak, 9,500?" above the waters of the river, having the appearance of a crater. 



Rumor had led us to expect, in connexion with this chasm, an extensive river fall, but such 

 did not prove to be the case. Eapids, indeed, do occur sufficiently severe to render a safe 

 passage by boats a virtual impossibility, but no distinct fall from an upper to a lower ledge of 

 rocks was encountered by the surveying party. Indeed, from a priori reasoning, we should 

 hardly expect to find such a feature in this location, where the strata are of such uniform 

 texture, and where the evident marks of such long continued abrading forces tend to level the 

 river bed. 



All the rapids seen along the course of the river are connected either with a talus, thrown 

 down from the projecting cliffs, or with the irregular deposit brought down from the beds of 

 tributary streams. 



