Tule Mountains on the south by the broad hill-dotted pass in which lie Tule Well and Tule Tank. The general shapelessness and lack of definite trend of the mountains as shown on the map may be due in part to lack of knowledge. Accounts of travelers indicate that there are several passes through the mountains, and it may be that these passes divide the mountains into parts each of which has a recognizable trend. Cabeza Prieta (black head), the high part of the range, is composed of light-colored granular crystalline rock capped by lava. The lava cappings occur also on other mountains of the range. (See p. 83 and Pl. XII, B.) Northeast of Tule Well a considerable part of the mountain is composed of lavas, which have an estimated thickness of 1,000 feet and are tilted to the east in several parallel ridges cut by a series of north-south faults. On the west flank of the range are two small buttes of lava which dip westward into Lechuguilla Valley. The mountains appear to have been once covered with an extensive and thick series of lavas of Tertiary age and to have suffered from very extensive and complicated post-Tertiary faulting, to which is due much of the present height of the range. Supposed Pleistocene uplift is discussed below in connection with the Tule Mountains. TULE MOUNTAINS The Tule Mountains consist of more or less detached ridges and sierras, which are large and bold in Mexico but become lower on the broad pass that separates this range from the Cabeza Prieta Mountains. The Tule Mountains are composed mostly of granite and gneiss, but on the western border is a monoclinal ridge of lava in which at least 500 feet of beds are exposed. In the pass are lava, lava conglomerate, and arkose conglomerate. (See p. 72.) Both the sedimentary and the volcanic rocks are thought to be of Tertiary age, and by their present attitude, which is well brought out in the geologic sections (figs. 3 and 4), they indicate intense and complicated faulting after the Tertiary volcanic epoch. The bench of older alluvium on the west side is raised about 50 feet above Lechuguilla Desert. (See fig. 4.) This elevated and dissected bench of alluvium seems to indicate uplift in Pleistocene time. The uplift was, however, not sufficient to affect the grades of the streams in the pass, which seem to be working at grade on a mountain pediment of the normal type, developed through the merging of headwater basins of streams draining to Lechuguilla and Tule deserts. PINTA MOUNTAINS The Pinta Mountains are a range 25 miles long extending from the O'Neill Hills northwestward. The range is a narrow comb of sharp peaks, probably not more than 2 miles wide at any point. The highest summits rise 1,500 to 2,000 feet above the adjacent valleys. The slopes are steep, and many narrow canyons lead from the plains to the crest of the range. The Pinta Mountains are remarkable in the sharp division between the two kinds of rocks of which they are composed. The southern part of the range is formed of dark-gray, almost black rocks, reported by prospectors to be black gneiss and schist. Doubtless these rocks are similar to the hornblende gneiss in the vicinity of the Fortuna mine. In the northern part of the range is a pinkishwhite granite, similar in color and topography to the white granite of the Cabeza Prieta Mountains and the vicinity of Tinajas Altas. The black and white rocks are separated by a very conspicuous contact at the center of the range. Many details of this contact are visible at a distance of 7 miles, and the division between the two 'parts of the mountain can be seen for 25 to 30 miles. Great stringers of the granite extend horizontally into the darker rock, where some fork and branch and all gradually thin out. The contact is the result of the intrusion of the granite in a molten state into the gneiss. This intrusion took place far beneath the surface, and the exposure of the contact testifies to the large amount of erosion that has since ensued. So far as known lava of Tertiary age does not occur in these mountains. MOHAWK MOUNTAINS The Mohawk Mountains extend southwestward from the vicinity of Mohawk station on the Southern Pacific Railroad about 18 miles to a gap, and then continue with the same trend for about 7 miles The southern division is on some maps called the Mohawk more. Range. The northern division of the Mohawk Mountains is a narrow range not over 4 miles wide at the widest part, which tapers at both ends, and the most northerly part is a high rocky hill isolated from the main mass by a narrow pass in which the railroad lies. Because the grade of the track is downhill in both directions from this pass, railroad men speak of it as Mohawk Summit. The Mohawk Mountains are composed largely of granite and similar coarse-grained crystalline rocks. To the massive jointing and resistant character of these rocks the steep slopes of the mountains are due. A narrow mountain pediment surrounds the mountains. During its formation a number of hills were detached from the main mass, and these now fringe the mountain border. Such isolated hills are the one just south of Mohawk station and one near the Red Cross mine, on the east side of the mountains. The mountain pediment, like others of the region, is dissected, as is well shown in the pass at Mohawk Summit, where steep-walled trenches cut 20 to 30 feet deep along the streams are in marked contrast to the smooth curves of the old valleys that connect with and form part of the pediment. Along the west side of the Mohawk Mountains erosion and dissection of the mountain pediment is retarded by drifting sand. This sand, swept forward by westerly winds, mantles the east side of the valley and reaches up to the very foot of the mountains. The southern portion of the mountains is divided from the northern portion by a low pass 2 miles wide marked by three rather prominent hills. This portion is said to be about 7 miles long and is separated by a pass from an unnamed range which extends 15 miles farther southeast. The so-called Mohawk Range is broader but not so high as the northern portion of the Mohawk Mountains. It is composed of similar granitic rock and has similar topographic forms. AGUILA MOUNTAINS The Aguila Mountains, known among the Mexicans and some Americans as Sierra de Águila, form a small range which begins about 10 miles southeast of Aztec on the Southern Pacific Railroad and extends about 10 miles farther southeast. On many maps these mountains are confused with or included in the Granite Mountains. farther south. The northern part of the range is a great lozenge-shaped plateau of lava about 3 miles long. It is highest on its southwestern point and slopes gently to the northeast. The western front is a great cliff and talus slope, which gradually decreases in height to the north end of the plateau. In the same fashion the cliff and talus of the plateau extend around the south end and decrease in height. more rapidly to the east. This plateau is commonly called Malpais Hill. The eroded edges of the constituent lava beds appear along the western face. The slope to the northeast corresponds to the dip of the beds. South of the plateau the mountains extend in a hook about 4 miles. long to a gap through which the road passes. Near the plateau the hills are low, and then begins a remarkable series of peaks, whose sides rise sheer for several hundred feet and whose forms are so fantastic that they are often called "the Eagletails." This metaphor is derived by analogy from the Eagletail Mountains, north of Gila 44 River, and from the name of the principal watering place, Eagle Tank. The principal rock of the peaks is porphyritic andesite.* To the jointing of this rock the fantastic erosion forms are doubtless due. South of the gap three low and somewhat inconspicuous hills extend to the south about 3 miles. The Aguila Mountains appear to be the remnant of an earth block faulted and tilted after the Tertiary volcanic eruptions had ceased. Of the original block only Malpais Hill remains. The rest of the range is carved from rocks of the crystalline complex and probably also from intrusive rocks contemporaneous with the Tertiary lava. GRANITE MOUNTAINS The Granite Mountains lie about 25 miles southeast of Stoval on the Southern Pacific Railroad and the same distance northwest of Bates Well. On many maps they are confused with the Aguila Mountains (Sierra de Águila), or the two ranges are combined under a single name. The range has a general trend in a northerly or northeasterly direction, and the main mass is about 5 miles long. According to reliable prospectors, the whole extent of the range is about 10 miles, and from a point near the center of the west side a range of small mountains extends about 5 miles to the west. These mountains are called Devils Hills or Sierra del Diablo. From a distance the Granite Mountains have the sawtooth outline and sierra type of topography characteristic of the erosion of granite and similar coarse-grained crystalline rocks. Prospectors report that the range is composed wholly of "hungry" (hard and barren) granite. AGUA DULCE MOUNTAINS The Agua Dulce Mountains are named from Agua Dulce, on Sonoita River, where ground water comes to the surface. From this point the mountains extend northwestward about 18 miles in a great curve. The trend is almost due north where the range crosses the international boundary but swings to the west, so that south of the Papago mine it is almost due west. The mountains consist of three parts of about equal length, which are divided by narrow passes. Each of the parts is a lozengeshaped mountain mass of the sierra type of topography. The plains rise steeply to the foot of the mountains, and there appears to be an extensive mountain pediment. The topography of the mountains and the rock waste found in Growler Valley, to the north, indicate that these mountains are composed wholly of the coarse. grained granitic rocks of the crystalline complex. 44 Jones, E. L., jr., personal communication. GROWLER MOUNTAINS The Growler Mountains are a range about 40 miles long, which lies west of the Valley of the Ajo. The range is divided into two rather distinct parts by the pass at Bates Well, known as Growler Pass. The northern part of the range is a rather simple asymmetric ridge extending from Growler Pass in a northwesterly direction to Childs Valley, west of No. 3 Well. The eastern slope of the ridge is smooth and gentle, parallel to the inclination of the Tertiary lava beds that cap the mountains. This slope merges rather gently with a narrow alluvial valley cut off from the main valley of the Ajo by a series of hills and small mountains-an unnamed ridge about 4 miles long just north of Bates Well, the Little Ajo Mountains, and Childs Mountain. The western slope of the Growler Mountains is very steep, a veritable façade sweeping northwestward from Growler Pass in a slightly sinuous line. (See Pl. XI, A.) The façade begins at the crest of the ridge with a rim rock of lava beds and descends in cliffs and talus slopes to the Growler Valley, a drop of about 1,500 feet in a distance of about 1 mile. Alternate bands of varicolored lava and tuff ranging from yellow-white to purple-red combine with the crenulations of the mountain front to make this façade a picturesque and imposing sight. The simplicity of the façade is broken by three hills at the base of the mountains. Sheep Tank Peak lies about 16 miles northwest of the Growler mine. It is visible from all parts of the valley between the Growler Mountains and Papago Well as far north as the Granite Mountains and is the landmark for Sheep Tank, which nestles against its northern flank. The peak is conical, very sharp and pointed at the top, and situated at the foot of the façade just west of a deep canyon. This peak appears to have been part of the mountains which has been isolated by the headward cutting of the stream that carved the canyon. About 8 miles northwest of the Growler mine a small ragged hill lies close to the foot of the mountains just north of a trail over the crest. From a distance this hill appears to be made up of granite and to be part of the base on which the lava rests. Halfway between this hill and the Growler mine is the most prominent and significant break in the uniformity of the western façade. Here there is a black butte a mile long and half a mile wide, with a rather gentle slope on the southwest side and a steep northeastern face. (See Pl. XI, A.) In this face may be seen the outcropping edges of the lava beds that form the hill, dipping sharply to the southeast. At the base these beds rest on rocks of the crystalline complex (granite, gneiss, etc.), which form a tongue of rough, hilly country connecting the butte with the base of the |