FROM PRESIDIO DEL NORTE TO THE PECOS. 51 



Rica," forming the topographical limits of the basin in that direction. Through this range 

 the river passes, and forms here the first of a series of gigantic canons below. 



There is no occasion to dwell longer on the general features of scenery connected with this 

 Presidio del Norte basin. To apply the term Rio Grandeish would convey at once a clear idea 

 to any one at all acquainted with the general aspect of scenery invariably connected with this 

 desert stream. 



This first caiion commences about twenty-five miles below the town of Presidio del Norte. 



The general course of the river for this distance bears south 70° west, {mag.,) passing at 

 several points rocky knolls of igneous character which abut on the river. On approaching the 

 mountain range directly in front, it will be seen that the river, winding through the lower line 

 of adjoining hills, suddenly contracts its channel, and thence tumbling over a series of foaming 

 rapids, enters the mountain range. 



The rock exposure liere is of a most remarkable character, and different from any heretofore 

 met with. When the adjoining mountains, reaching a height of 1,000 to 1,500 feet, present a 

 clear sectional face, we see a somewhat regular series, composed of lavas, vesicular or compact 

 in texture, alterbating with thick deposits, of an earthy form, of volcanic breccia. — (Specimen 

 Eocks, No. 59 to 62 inclusive.) 



The general arrangement of these formations shows them to be variable in thickness, and dis- 

 posed in regular strata one above the other. 



The dark-colored lavas form usually the upper capping, together with one or more interme- 

 diate seams. The intervening lighter colored breccias are often of considerable thickness, show- 

 ing in some places a development of 300 feet or more, while at other points it is reduced to a 

 thin seam. The usual appearance of these breccias is that of an earthy-stratified deposit, vary- 

 ing in color from a whitish brown to a dull green ; its texture is more or less crumbling, being 

 composed of a whitish paste, which contains, occasionally, minute pebbles of quartz rock. 



In entering into the composition of mountain masses, these several formations assume very 

 distinct and peculiar characters. Thus, where the earthy breccias are considerably developed, 

 we see them exposed, along the sides of mountains, in perpendicular walls, capped by the 

 darker colored lava rocks, which are frequently seen overhanging and forming a regular line of 

 terraced platforms, thence rising upward in broken ledges to form a flattened summit. This 



SECTIONAL VIEW OF BCTASILLA MOUNTAIN, NEAR THE COMMENCEMENT OF THE CASON OF THE EIO BBATO, THIBTT MILKS BELOW 



PKESIDIO DEL NORTE. 



^";?- ^ *>, <* * ^* ^" *^ u 



A. Lower exposure of trap-rock of closer texture than the upper stratum. 



B. Vein of trap, six feet in thickness, traversing the intervening breccia forraation, and 

 connecting the upper and lower strata of igneous rock. 



C. Vesicular trap-rock, dark colored, 4U0 feet ? thick. 



D. Volcanic breccia in horizontal strata of light brownish color, SOO feet thick| 

 E> Talus strewn with blocks derived from the upper igneous stratum. 



