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 cañons 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 cañon 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 here 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, alternating with thick deposits, of an earthy form, of volcanic breccia.-(Specimen Rocks, No. 59 to 62 inclusive.)

The general arrangement of these formations shows them to be variable in thickness, and disposed in regular strata one above the other.

The dark-colored lavas form usually the upper capping, together with one or more intermediate seams. The intervening lighter colored breccias are often of considerable thickness, showing 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, varying 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 BUFASILLA MOUNTAIN, NEAR THE COMMENCEMENT OF THE CAÑON OF THE RIO BRAVO, THIRTY MILES BELOW

PRESIDIO DEL NORTE.

600

200

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 formation, and

connecting the upper and lower strata of igneous rock.

C. Vesicular trap-rock, dark colored, 400 feet? thick.

D. Volcanic breccia in horizontal strata of light brownish color, 200 feet thick

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

singular character of formation has given the name of "BOFECILLOS" to some of the more striking mountain ledges.

Owing to the crumbling nature of this underlying stratum, we find it variously washed and often fantastically shaped by the peculiarities connected with its various exposures; it thus frequently forms burrowing caverns and dark grottos, set off with misshapen pillars. Quite invariably we find its base occupied by a talus, derived from the overhanging rock, forming a rough slope strewn with irregular blocks; thus its connexion with the underlying rock stratum. This connexion is, however, occasionally brought to light, and shows a lower development of lava rock, differing but little from that above, except in a greater closeness of texture, the upper capping being generally vesicular, while that below is compact.

Frequently, however, a direct connexion between the separate lava strata is made by narrow dykes, identical in character with the rock above and below, passing through the intervening breccia. At other places, veins are seen shooting from above and below, and terminating in the intervening series.

SECTIONAL VIEW, SHOWING A SERIES OF VOLCANIC PRODUCTS, TRAVERSED BY INJECTED IGNEOUS VEINS; CAÑON OF RIO BRAVO,

THIRTY-FIVE MILES BELOW PRESIDIO DEL NORTE.

B

A. Dark-colored vesicular trap.

B. Volcanic breccia of a lightish brown color in horizontal strata.

C. Lava or trap-rock of close texture, dark-colored.

D. Breccia as above, having a light greenish color.
E. Igneous veins.

We are now sufficiently prepared to appreciate the external features of the region thus characterized. We can understand how the unequal development of these several layers may give shape and character to the mountain ranges, and what diversified features they will necessarily assume, under the influence of denuding causes, acting so unequally on their separate members. The geological formation seems to conspire with the atmospheric influences to give a ruggedness and character of desolation to this region, of which description can give but a most meagre idea. It is, however, in the line of the river-course that these rugged features present their grandest developments. We see the turbid waters of the Rio Bravo here contracted to a narrow channel, barely a stone's throw across, sweeping on a resistless current beneath bristling crags; now tumbling over foaming rapids, connected with some abrupt turn in the course of the stream, and then gliding smooth and unbroken through mountain clefts with perpendicular walls on either hand, rising to the dizzy height of 1,200 to 1,500 feet perpendicular.

In our progress through the range, the breccia deposit becomes less developed, and finally disappears altogether, or is seen only in narrow seams along the sides of the mountains.

The course of the river at first is not entirely hemmed in by abrupt rocky walls, a rough talus at the base affording a rude pathway, occasionally lined by narrow strips of sand beach.

At other places, however, all approach to the river, except by the route of its dangerous channel, is out of the question.

Along the course of the river, the mountain barriers are occasionally pierced by side chasms for the drainage of tributary mountain valleys. One of these is so remarkable as to deserve some separate notice.

At a point about seven miles from the entrance of the cañon, where the river is completely hemmed in on each side by the largest development of the mountain range, being unapproachable except in boats from above, there is a cut-off on the American side, leading by an open country over a gentle swell of ground, reaching the river about five miles below. This cut-off passes directly at the base of the high mountains intervening between this route and the river, having an average breadth of half a mile. At the summit of this swell is a depressed valley, the drainage of which leads directly toward this mountain barrier in its course to the river.

In following the dry-stream bed thus marked out, we find it entering by a narrow portal, about 15 feet in width below, thence cutting its way by a uniform cleft through the entire breadth of the mountains to reach the Rio Grande.

It thus presents a miniature picture of the larger cañon made by the Rio Grande. Its floor shows a smoothly-washed rock surface, in which basins frequently occur, bedded by washed sand and pebbles, and receiving the limpid issue of a small trickling stream. In its general course toward the river, it makes frequent zigzag angles, thus giving a new feature of scenery at every turn, and presenting altogether a most varied combination of the grand, grotesque, and beautiful. Along its sides is plainly observed a high-water mark, with an average height of 15 feet above the rocky bed, indicative of the sudden floods, derived from copious rains, to which this chasm is subject. This fact serves to give a somewhat nervous interest to its exploration. The height of the perpendicular walls on each side, corresponding to the thickness of the mountain range, is from 300 to 800 feet. The chasm thus formed opens up gradually towards the summit, forming a broken yawning abyss, untouched by sunlight, and having its depth exaggerated by the comparative dimness that shrouds it below.

Thus sheltered from the sun's scorching rays, and cooled by evaporation from its brimming basins of clear water, with its entrance fanned by a constant stream of cool air, this cañon forms a grateful retreat. Further toward the river the descent is made by several abrupt falls, forming extensive basins below. These are filled with clear water, and offer natural bathing places of a most attractive character. Its exit on the river presents the same general features of chasm, the final debouchment being marked by a débris of rocks and pebbles, which project into the main stream and form a difficult and dangerous rapid.

About three miles from this latter point, and twelve from the head of the cañon, the main development of the mountain range forming the Sierra Rica is passed; the final exit is through a narrow rocky portal, and presents the appearance of an immense gateway. The width of the river at this point is barely 80 feet; the adjoining mountain ridge on either side is so broken and rugged as to be impassable for animals.

On passing this narrow outlet we come upon a more open but still broken country, consisting of basins of limited extent, set off with the usual form of gravelly table-land. The course of the river is frequently obstructed by low rocky ranges, forming cañons; again pouring out of these cañons into the more open basins, it becomes expanded, and forms limited sand beaches, patches

of bottom-land, and occasionally small islands. This character continues for ten or twelve miles, when we enter on a more extended basin, through which passes the Comanche trail, leading from Upper Texas into Mexico, by the adjoining Mexican settlement of San Carlos.

SECTIONAL SKETCH AT COMANCHE CROSSING, ON THE RIO BRAVO, BELOW PRESIDIO DEL NORTE.

A. Cretaceaus limestone marked with large ossils of Inoceramus, inclining N. N E., at an angle of 80°.
B. Mesa, or gravelly table-land formation, resting unconformably upon A.

At this point the rock exposure exhibits outcrops of limestone belonging to the cretaceous period, being quite abundantly marked by fossil impressions of Inoceramus, often of large size. The rock exposure exhibits a very variable dip, mostly inclined towards the west, occasionally at a very sharp angle. It rises at various points in the adjoining table-land, forming ochreous colored rocky bluffs, where at several points the gravelly table-land is seen to rest unconformably on the sharply-tilted strata.

Further down the river, in an eastern direction, this cretaceous formation assumes a nearly horizontal position and a closer texture. It is here seen overlaid by a variable sheet of dark colored lava rock, corresponding in character to that noticed above in connexion with the Bofecilla mountains. This sheet of igneous rock is seen to conform closely to all the inequalities

SECTIONAL VIEW ON A RAVINE LEADING TOWARD THE RIO BRAVO DEL NORTE, NEAR SAN CARLOS, SHOWING IGNEOUS ROCK DIRECTLY ASSOCIATED WITH CRETACEOUS LIMESTONE.

A. Cretaceous limestone, having an earthy texture, containing fossils of Inoceramus, 150 feet..

B. Dark-colored igneous rock, 80 feet in thickness.

C. Cretaceous limestone of closer texture than that above, 15 feet thick.

D. Débris.

of the underlying limestone, exhibiting in the walls of the cañon below a distinct line of separation, traceable for a long distance. The westerly dip of the cretaceous formation underneath gradually thins out this upper igneous capping, which finally disappears, and solid limestone walls continue along the line of the river.

At one point on the line of the trail leading round the broken ranges of the mountain ledges, directly bordering the river, to reach its bed some eight miles below the Comanche Ford, the sides of a deep washed ravine bring to view the successive and relative thickness of the various exposures alluded to above.

We here see the upper members of the cretaceous rocks forming the tabled summits of the adjoining mountains, and marked by frequent cretaceous fossils, resting on a bed of igneous trap-form rock 50 to 80 feet thick, this again overlaying the closer layers of the limestone strata below.

Our further route, adjoining the river on the Mexican side, passes over high ground, based on limestone rock, and attaining a height of 800 feet or more above the river, the strata here dipping slightly to the west. We again reach the river-bed at the mouth of San Carlos creek, which, draining a considerable valley extending to the south some fifteen miles, affords a constant stream of clear water.

Just below this point commences the gigantic cañon of San Carlos, through which for ten miles the Rio Grande, pursuing a nearly due east course, makes its way. This cañon presents unbroken walls of cretaceous limestone.

The course of the river here cutting the strata in a line directly opposed to the dip, there is a

constantly increasing elevation of the cañon walls. These walls commence with a height of between 200 and 300 feet; but the fall of the water combined with the rise of the strata, develops, in the course of ten miles, a clear perpendicular height of at least 1,500 feet above the river level. A faint conception only can be formed from these facts of the truly awful character of this chasm. Its course can be marked along the mountain slope in a regular zigzag line, terminating by an opening cleft, which rises high and clear above the surrounding mountain ranges.