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Show 80 Btono forms extensive barren slopes of 15° to 20°, constituting the northwest flank of the gradually- rising Gallinas Mountains; but farther south where the mountain reaches its greatest elevation, it is steeper and more broken. The structure of the region west of the Sierra Madre from this point as far as my investigation extended ( ufby miles) is a beautiful repetition of that observed and described on the east slope of the Rocky Mountains so far as the Mesozoic strata are concerned. The mountain- axis itself exhibits great variations in its surface- formation and elevation ; but the position of the beds on its flanks is remarkably uniform. These form a series of hog- backs, formed by Cretaceous Nos. 1 and 3, and occasionally by harder beds of Nos. 2 and 4, which are separated by parallel valleys whieh are often grassed and timbered and rarely occupied by sage- brush. The most important of these is that lying between Nos. 1 and 3. The upper portion of the Chama flows through a similar valley on the eastern side of the Gallinas axis, and is turned aside by that line of elevation, and then cuts through the beds of No. 1 and the overlying formations, and finally through the axis of elevation farther eastward, reaching the Trias before entering the Santa F< 5 marls. On the western side of the axis of the Gallinas, the valley of Cretaceous No. 2 exhibits two points of elevation. The most northern is near the Rio Chama; the southern and highest, at the head of the Rio Puerco. From the latter the drainage is carried through the Gallinas Creek northward, which flows along the valley until it is turned aside by the rise toward the divide already mentioned, when it flows to the east through a cafion of the Gallinas Mountain and joins the Chama below. The appearance of the No. 2 valley is as follows: On the left ( east) the barren slopes of brown sandstone rise, marked with regular cleavage- lines, from which scattered pifiones gain subsistence. On the east, perpendicular bluffs extend in a regular line, parallel with the mountain- axis. They Teach 700 feet and more in height; but the strata are undulating in long waves, reaching the valley- level at intervals of several miles, where the depression opens a view of the country to the west. The face of the bluffs is the outcrop of the bluish shaly beds of No. 2, which are full of Ostrea and Inooeramus. The summit of the bluffs is the light- yellow sandstone of No. 3. This sandstone varies much in thickness, increasing toward the south, where it constitutes the entire bluff. The valley widens to the south for a distance, and a line of low hills of the shales of No. 2 rises from its surface. Another line of hills, less constant and less elevated than that of No. 3, is formed by the yellow beds of No. 4, and first appears near the mouth of the Gallinas Canon, and continues to approach No. 3, until, to the south, the two combined form a single hog- back. The axis of the Gallinas range appears to be undulating; at least, a series of undulations of the strata on its flanks are due to axes of elevation at right angles to the principal one. The side of the Gallinas Mountain at the north appears to be composed mainly of Cretaceous No. 1; but at the cafion of the Gallinas the colored beds of the Jurassic appear in its summits. South of this point these beds, capped with the white gypsum, extend entirely across the anticlinal; the sandstones of Cretaceous No. 1 appearing on the eastern as well as the western flank. Further south these are abruptly removed, leaving a plateau of the hard " Triassic" sandstone at a somewhat lower level, this bed resting in turn on the deep- red marls of the same age. Farther south the Triassic sandstone forms the summit of the highest line of the range; the Jurassic and Cretaceous No. 1 reposing on its sides. Still farther south the Nacimiento Mountain rises to a greater height, and is composed of the red feldspar- porphyry of the Rocky Mountain axis. It forms the culmination of the Sierra Madre, and extends southward as far as my examination was carried. The first and most northern section ( Plate III) was carried across the flank of the mountain twelve miles south of the entrance of the oafiou of the Gallinas Creek. The oldest beds of this section form a plateau surrounded by greater elevations, from which it is separated on the south and east sides at least by deep ravines. The walls of these are composed of a deep- red marl of the Trias, capped by the usual heavy bed of gray sandstone. The north side of this plateau is bounded by an abrupt precipice of Jurassic strata, the red below, yellow in the middle, and the bed of snowy gypsum on top ; the relations of the Triassic and Jurassic here being precisely as described above at the entrauce oi the cafion Canjelon. The sandstones of Cretaceous No. 1 are observed on both east and west flanks of this open anticlinal; on the eastern side without the intervention of the gypsum bed. The yellow bed is also deeply scored, and in some places isolated, showing that a stronger eroding action had been at work on this side than on the west prior to the deposit of the Cretaceous No. 1. Immediately to the west of the plateau, a more elevated wave is also covered with the Jurassic beds: the entire summit of the mountain for many miles being composed of the gypsttin. This soft material is worn into innumerable gullies. It is separated from the plateau by a gorge, which is the seat of a fault. The Triassic plateau has evidently been thrust upward so as to continue the level of the yellow beds of the Jurassic at this point, the fault thus amounting to not more than three hundred feet. But the Jurassic beds dip southward, forming the descending slope of a longitudinal wave of their axis of elevation. As the Triassic is level at the point of descent |
