Growler Mountains. Thence southward and connecting with similar rocks at the Growler mine the crystalline rocks are exposed. The lava beds of the Growler Mountains and of the butte rest on the eroded surface of the crystalline rocks and form parts of a once continuous lava field. As they dip in opposite directions, however, it is evident that the two parts have been dislocated, presumably along a fault that trends northwest along the front of the mountains, through the hilly tongue that connects the butte and the main mountain mass. That the uplift and dislocation are due to faulting and not folding seems certain for two reasons—an unbelievably abrupt fold would have been necessary to accomplish the movement; and as faulting can be demonstrated for part of the range, on the east side of Growler Pass, it is probable that all the movement was accomplished by this means. The hills on the north side of Growler Pass begin on the east with a low-capped mesa; then follows a high mountain made up largely of limestone and quartzite, presumably of Paleozoic age; and then a high conical hill composed of lava beds that dip sharply to the west and rest on rocks of the crystalline complex. This conical hill is separated by a low pass from the eastward-dipping lava beds of the main part of the Growler Mountains. It is evident that a fault passes through this gap and that a second fault lies between the limestone hill and the lava mesa. South of Growler Pass the Growler Mountains consist of plateaus and ridges, which extend to Cipriano Well. The plateaus are carved from the bedded volcanic rocks of Tertiary age, and their flat tops are inclined at various angles according to the inclination of the constituent lavas. (See Pl. XI, B.) The ridges are also made up of lava, but the original plateau from which they were carved has entirely disappeared, erosion having left only jagged crest lines and palisaded summits. In general the dip of the beds is toward the east on the east side of the mountains and toward the west on the west side, but there are numerous exceptions. This part of the range is a group of fault blocks, each more or less isolated by the erosion of canyons and open valleys along the bounding faults. WELLTON HILLS The Wellton Hills lie 6 miles south of the town from which they are named. They divide the Lechuguilla Desert from the valley of Gila River, though the drainage from the desert finds its way through them to the Gila. The hills occupy an area about 6 miles in diameter over which they are scattered singly or in groups. Many of them are buttes, but the largest hilly area, which lies west of the road between Tinajas Altas and Wellton, is more than 3 miles long. The hills rise from 50 to 500 feet above the flat-bottomed valleys that pass between them. Many of the hills east of the road are carved from lava of Tertiary age, which is not, however, shown on the map (Pl. IX). Other hills are composed of rocks of the crystalline complex, and granite, granite gneiss, and schist were found. The Wellton Hills appear to be the remnants of a complexly faulted elevated area which erosion has reduced to isolated hills. Between the hills are smooth plains cut on rock which correspond in age to the other mountain pediments of the region. This pediment, however, is largely covered with alluvium brought down by streams from the south. BAKER PEAKS The Baker Peaks are an irregular group of hills about 2 miles in diameter on the edge of the Gila Valley 7 miles southeast of Wellton. They comprise three prominent conical peaks, which, because of their height and isolation, are visible for many miles in all directions. The Baker Peaks are composed of granitoid rocks of the crystalline complex and are characterized by a strong westward-dipping sheeting. These rocks contributed the material to form the great series of arkose sandstone and conglomerate which lies at the southern foot of the hills, around Baker Tanks (pp. 127-128). It seems probable that the conglomerate once extended over a large part of the Baker Peaks mass and has been removed by erosion during Pleistocene time, with the formation of a mountain pediment that cuts across the Tertiary conglomerate and into the underlying crystalline complex. Here, as in other mountains, the streams are now cutting sharp gorges in the mountain pediment. AZTEC HILLS The Aztec Hills lie about 3 miles west of Aztec station on the Southern Pacific Railroad. They consist of four somewhat isolated masses strung out in a line about 5 miles long. The northernmost hill is the largest and highest, and its sharp summit makes it a landmark over all the San Cristobal Valley. The hills are composed of granite and granitoid rocks of the crystalline complex. As they lie on the strike of the Aguila Mountains, further study may show that their uplift is connected with the uplift of that range. During a survey in this region in 1923 E. C. La Rue 45 observed a large number of hills that form an almost continuous chain between the Aztec Hills and the Aguila Mountains. 45 Personal communication. O'NEILL HILLS The O'Neill Hills are a detached group of little mountains 4 miles west of Papago Well. North of O'Neill Pass are two detached groups of hills, in all about 11⁄2 miles long. They rise abruptly 150 to 300 feet from the surrounding plain. South of the pass a group of hills extend about 4 miles southeast. Their summits are from 300 to 700 feet above the adjacent plain. To the southwest are a series of detached hills and a prominent mesa. The O'Neill Hills appear to be buried in the alluvium carried out toward Las Playas from the vicinity of Papago Well and the west end of the Agua Dulce Mountains. There is practically no transitional slope from the plains to the hillsides. These hillsides are themselves notably steep, because they are carved from coarsegrained rock, which in O'Neill Pass is a granodiorite. The other hills appear to be formed of the same or similar material. There is no direct evidence that these hills were uplifted in the general period of faulting that affected most of the region. However, the prominent mesa 5 miles southwest of O'Neill Pass is composed of almost horizontal lava beds. It is probable that further study would show that this mesa is the remnant of a once large lava field that covered all or most of the O'Neill Hills. Erosion has removed so much of the covering rocks that the original form and structure of the hills are greatly obscured. HILLS NEAR PAPAGO WELL A number of mountains and hills lie north of the west end of the Agua Dulce Mountains, in the vicinity of Papago Well. The main group of hills occupy an area measuring 9 miles from east to west and 5 miles from north to south. Through this area passes the road from Bates Well to Las Playas (p. 419). The mountains around Papago Well consist of steep-sided hills that rise abruptly from intervening lowlands. These lowlands are usually covered with alluvium, but in many places bedrock is exposed. Their general slope is away from the Agua Dulce Mountains to the north, east, and west. All the hills but one are composed of granite or coarse granite gneiss of the crystalline complex. The larger hills are commonly three to five times longer than wide and taper at the ends. The highest point of each hill is at or near the middle. Each hill is then a miniature mountain of the sierra type, whose very steep slopes are due to resistance to erosion of the constituent rock. Papago Hill, on the west slope of which is situated the Papago mine, rises 900 to 1,000 feet above the base and is the most conspicuous of the The hills directly west of Papago Well differ from the being an irregularly rounded clump. Directly south of the well is a small hill about 400 feet high composed of lava. The upper layer is a black basalt, and the total thickness of lava exposed is about 400 feet. The beds dip about 20° SW. This lava is part of a once more extensive capping of the Tertiary volcanic rocks which formerly mantled the region. Only this fragment, faulted down between higher blocks, remains. The strike of the lava is parallel to the trend of Papago Hill and of two other groups of hills. The trend of four other hills is almost at right angles, and the remaining clump has no definite trend. It seems probable that these trends are the lingering expression of the trends of blocks that formed an intricate fault complex. On this complex, and controlled by its structure, the original drainage began erosion, and here, as in other parts of the region, a mountain pediment was developed. This pediment had its upper limit in the Agua Dulce Mountains, of which this area was a foothill belt. An elaborate system of intersecting valleys was developed largely at the expense of the volcanic rocks, of which only the small fragment previously mentioned remains. The hills are the unreduced remnants of the original fault complex, and their forms and trends are an inheritance from a more or less rectangular system of faulting. The pediment is now largely concealed by débris brought down from higher levels and is not so elaborately dissected as the pediments in certain other areas. Trenching of the pediment is, however, well marked 2 miles east of Papago Well. At distances of 6 and 13 miles north and 10 miles northwest of Papago Well are three isolated groups of hills. These hills were not visited, but their topography is similar to that of other granite hills of the region, and they may be related in origin to the mountains above described. QUITOBAQUITO HILLS The Quitobaquito Hills extend northward from the international boundary for 5 miles and are nowhere more than 2 miles wide. They rise about 500 feet above the surrounding plain. The hills trend slightly west of north, parallel to the Growler Mountains but offset to the west. The north end of the hills is composed mostly of schist and fine-grained intrusive rocks; the south end mostly of granite. No outcrops of Tertiary lavas are known, but the similarity in trend and close association with the Growler Mountains indicate that these hills were uplifted at the same time. VALLEY AREAS YUMA DESERT The Yuma Desert is a plain that extends west from the Gila and Tinajas Altas mountains to the bluffs that separate it from the flood plain of Colorado River. On the north it is bounded by bluffs that border the flood plain of Gila River. The plain is 12 miles wide at the north and increases in width to more than 40 miles along the international boundary. The plain consists of unconsolidated sediments and slopes from the mountains toward the west. Near the mountains the sediments are coarse, unweathered rock débris brought down by the ephemeral streams which drain the western slopes. Near Gila River and especially on the west near Colorado River the material is finer and was in part laid down by the rivers or may even be in part marine. Beginning at Yuma the plain is divided into a lower bench from 50 to 100 feet above the flood plain of Colorado River and a higher bench that is from 50 to 150 feet higher. The lower bench increases in width and definiteness to the south and is about 15 miles wide along the international boundary. At Yuma and at the State rifle range, 5 miles southeast of Yuma, knobs of granite gneiss belonging to the underlying crystalline complex project through the alluvium. The Yuma Desert is mantled with wind-blown sand, probably derived largely from the flood plain of Colorado River. The movement of this sand seems to be slow and insufficient to prevent the growth of a sparse vegetation or to prevent the erosion of the plain by a tributary of Gila River which flows northwestward along the west side of the Gila Mountains from the Fortuna mine to Blaisdell. LECHUGUILLA DESERT The Lechuguilla Desert is a valley extending in a northwesterly direction from the international boundary to Gila River, between the Tinajas Altas and Gila mountains, on the west, and the Tule, Cabeza Prieta, and Copper mountains and Baker Peaks, on the east. It is about 9 miles wide and 35 miles long. The floor of the valley is drained by an axial stream which, throughout the greater part of its course, has a well-defined channel about 100 feet wide but is not dissecting the plain. North of the Wellton Hills the main stream. and others are dissecting the valley deposits. The same condition of dissection exists along the northeastern border of the Gila Mountains. The present level of the valley is determined not by the present base-level but by Gila River when it stood at the level of the terrace of younger alluvium. The valley deposits must therefore be largely of this age, covered by a veneer of more recent materials. The older alluvium is exposed in a bench along the west side of the Tule Mountains (p. 72) and perhaps also in a similar bench north of the Wellton Hills. The older alluvium seems to be somewhat coarser than recent deposits of the same area. Around the borders of the valley there is some dissection of the upper part of the alluvial |