much as 2,000 feet. The eroded surface of the crystalline rocks appears to have been fairly smooth, and consequently the lavas are immense plates of rock which, as at present broken by faults, resemble great flakes of ice stranded on a shore by the wind. Toward the east the lava flows are less continuous, the total thickness is more variable, and the individual fault blocks as revealed by the attitude of the lava plateaus stand out less clearly.

The faults are of the normal type and trend in two directions—north and east. The geologic map (Pl. IX) and the cross sections in Figure 2 show the effect of faulting on the distribution of the lavas and in the topography. Sections A-B, B-C, and B-D are arranged on a Y of which A-B is the stem; A-B trends east, B-C east-northeast, and B-D east-southeast. The faults shown in section A-B trend west of north and are marked by valleys. Along section B-D

FIGURE 3.-Geologic cross section in the Tule Mountains, Ariz.

FIGURE 4.-Geologic cross section of the southern part of the Lechuguilla Desert, Ariz.

no lava appears, but a mountain of coarse-grained gneiss stands above the lavas which once covered it, at least in part. Along section B-C a lava plateau that has been uplifted, perhaps, but not tilted still resists erosion. Section E-E, which trends north, at right angles to section A-B, shows the effect of the faults that have an easterly trend. Tilting of the fault blocks is much more marked, and the underlying crystalline rocks are brought to the surface at the base of Jack in the Pulpit and the plateau south of it.

TULE MOUNTAINS

In the pass southeast of Tule Well block faults are well shown by the tilted lavas and lava conglomerate, which rest on a relatively smooth plain composed of the crystalline complex. The cross section in Figure 3, which extends from east to west about 2 miles south of Tule Well, brings out the character of these faults. On the west side of the same range and south of the road a great plate of lava

and lava conglomerate about 750 feet thick dips westward under the Lechuguilla Desert (fig. 4). The contact with the crystalline complex is exposed along the eastern face of the ridge. Hills several hundred feet high project up into the lava and indicate the local irregularity of the surface over which the lava flowed. North of the road what seems to be the same lava caps the mountains at an altitude about 1,000 feet higher. (See Pl. XII, A.) As shown in Figure 4, the westward-dipping block of lava is bordered by a bench of alluvium about 50 feet above the level of the main Lechuguilla Desert. This bench of older alluvium indicates renewed uplift of the Tule Mountains in Pleistocene time, probably accompanied by faulting along the east front of the Lechuguilla Mountains. On physiographic evidence the Tinajas Altas Mountains, near by, are thought to have been reelevated in Pleistocene time (p. 76).

GROWLER MOUNTAINS

The portion of the Growler Mountains north of Growler Pass is a rather simple monoclinal fault-block mountain trending northwestward and about 20 miles long. In the picturesque western escarpment are exposed between 1,200 and 1,500 feet of Tertiary lava, tuff, and conglomerate resting on the crystalline complex, which crops out in places at the foot of the mountain front. The eastern slope is gentle, almost without canyons, and conforms to the dip of the lava sheet.

In Growler Pass there are at least five faults that trend almost due north, but the intervening blocks are rotated in different directions. In the easternmost block the beds are horizontal. In the others the dips are easterly in two and westerly in two. The complex structure of Growler Pass continues in the southern part of the Growler Mountains, which consists of large lava plateaus separated from one another by narrow canyons. The beds in each plateau dip at a different angle or in a different direction from those in neighboring plateaus, so that there must be a complicated system of faults.

GEOLOGIC CLASSIFICATION OF THE MOUNTAINS

GENERAL CHARACTER

The mountains of the Papago country consist either of more or less isolated elevated regions separated by broad valleys underlain by alluvium, or of groups or chains of mountains separated by rather narrow alluvium-filled valleys or canyons. To the traveler crossing the country on the Southern Pacific Railroad the ranges appear to be rather monotonous in their characteristics, showing a recurrent

sameness which implies a common geologic history. They seem to consist wholly of small detached sierras having very similar topographic forms, composed of granite and other coarse-grained crystalline rocks, and to be largely buried in alluvium. This appearance, however, is the result of the fortuitous distribution of certain types of ranges along the route of travel. A further exploration of the Papago country shows that many of the mountains are capped with lava beds much younger than the granite and crystalline rocks already mentioned and have the typical fault-block form common to the ranges of Nevada. Other ranges consist of exceedingly complex plateaus, peaks, and pinnacles, generally carved from thick lava beds. The ranges are thus by no means uniform in their rock composition and topography, and they may be separated into groups according to their composition and structure. There are 68 mountain ranges and groups of hills in the Papago country,s1 and 11 more that lie east of Santa Cruz River are represented on the geologic map (Pl. IX). Of these 79 ranges 15 can not be described, because they have not yet been sufficiently explored. A classified list of all these mountains is given on pages 77-79.

MOUNTAINS COMPOSED LARGELY OF VOLCANIC ROCKS

The mountain ranges composed largely of volcanic rocks are 25 in number and consist in greater part of alternating beds of acidic and intermediate lavas, tuffs, and volcanic products of various sorts. With these lava beds in many places are associated stream-laid conglomerates, the pebbles of which consist of granite, other crystalline rocks, and lavas of various sorts. The lava beds seem to represent a general period of volcanism in Tertiary time, in which a large part of the Papago country was subjected to great flows of lava and enormous ash showers from numerous volcanic vents. The floods of lava were by no means uniform either in chemical composition or in total thickness of lava accumulated, but in a general way the resulting topographic forms are alike.

The mountains composed largely of volcanic rocks have been divided into three groups on the basis of their structure and resultant topographic form. The first group consists of two mountains which appear to owe their height in large part to the accumulation of lava around a volcanic center. They are, in other words, old volcanoes whose activity resulted in the building up of the lava plateaus. In these mountains little or no uplift has taken place and the original topographic form has been only slightly modified by erosion. The eleven mountains of the second group are long, narrow ranges which

Bryan, Kirk, Geology and physiography of the Papago country, Ariz. (abstract): Washington Acad. Sci. Jour., vol. 10, pp. 52-53, 1920. Different figures are given because a smaller area was included in the "Papago country."

trend in a northwesterly direction. The lava beds have a general dip at right angles to the trend of the mountains. These mountains appear to be rather simple fault blocks or horst mountains. Certain of these mountains can be definitely shown to owe their elevation to faulting, and the others are assumed to be due to the same cause. It is probable that more detailed study would show that uplift by faulting produced all the mountains listed in this group. The third group consists of twelve mountains which have no general extension in one direction and are composed of disconnected plateaus and asymmetric ridges. They are thought to owe their elevation to faulting, but because of the great number of faults and because of variations both in displacement and in the direction of the several fault lines, they are so complex that their structure can be made clear only by detailed work. These mountains have a general shapelessness which is in great contrast to the simple forms of the mountains of the second group. Certain of them, such as the Tumacacori and Sand Tank mountains, constitute the largest areas of elevated country in the region. In parts of these larger ranges the lavas have been removed and the core of older crystalline rocks revealed.

MOUNTAINS COMPOSED LARGELY OF GRANITE AND SCHIST, WITH PATCHES OF VOLCANIC ROCKS

Twenty-two mountain ranges are composed largely of rocks belonging to the crystalline complex but have larger or smaller patches of volcanic rocks. The rocks of the crystalline complex are older than the Tertiary volcanic series, from which they are separated by a more or less uneven plane of erosion. In other words, the crystalline complex formed the surface upon which the volcanic rocks were laid down or through which they were intruded. The dominant rocks of the crystalline complex are granite, gneiss, and schist. Though a number of other rocks are found in the crystalline complex and there are areas of Paleozoic and Mesozoic sediments, the topographic expression is fairly uniform and the mountains are of the sierra type. In areal extent granite and gneiss predominate, and these rocks, whatever their age, produce similar topography throughout the region. These mountains are of two general types(1) ranges having a general extension in a northerly or northwesterly direction, rather narrow in comparison to their length, and having side slopes which, though exceedingly steep, are nearly equal on both sides; (2) small, more or less detached steep-sided hills arranged in groups with intervening alluvial or rock-floored plains. In the following classification this distinction in size is disregarded. These mountains all have large or small masses of lava which by their position and inclination indicate that since their deposition they have been uplifted and dislocated. In other words, these mountains,

though composed mostly of crystalline rocks, were once covered by larger or smaller masses of Tertiary lava. Since the lava was poured out the mountains have been uplifted and the lavas tilted at various angles. In the time that has elapsed since these uplifts the mountains have been so eroded as to conceal, to a greater or less extent, their similarity in origin and structure to the fault mountains of the first class.

MOUNTAINS WITH NO KNOWN PATCHES OF VOLCANIC ROCKS

Seventeen mountain ranges appear to be composed wholly of crystalline rocks. These mountains are typical desert ranges, whose characteristics have been frequently described and whose distribution along the common routes of travel has led to the belief that all the ranges of the Papago country are similar in character. In some of these mountains there are no areas of Tertiary lava. Other ranges, however, have been so little explored that it is possible and even probable that small patches of lava will eventually be discovered in them. Two ranges, including one classed in this group, have certain physiographic characteristics which show that they were rather recently uplifted. These are the Sierra Estrella and that part of the Gila Range south of Tinajas Altas Pass, sometimes called Sierra de las Tinajas Altas. Each of these ranges has an exceedingly steep front on the east. This front is sinuous and cut by canyons, but each canyon has a lower portion separated from an upper portion by a falls. Above the falls there is an upland valley at a considerably lesser grade. The eastward-flowing streams then do not have a smooth, continuous grade, but one which is steep at the divide and becomes more gentle. then very steep, and gradually gentle again. (See p. 131 and fig. 27.) As the rock structure is essentially the same along the course of each stream, it seems to be an inevitable conclusion that the upper gradient was developed while these mountains were at a lower level, and that the lower gradient has been developed by the headward cutting of a new canyon by the stream since the reelevation. The time of this reelevation is uncertain, but it is thought to be Pleistocene and more recent than the uplifts that dislocated the Tertiary volcanic rocks.

The mountains of this type, which present little or no evidence of their origin and consist simply of ridges and detached hills of crystalline rock eroded into the sierra type of mountain, equal sided and more or less surrounded by the alluvium arising from their own dissection, are by no means typical examples of old-age desert erosion. The platform upon which the Tertiary volcanic rocks were laid down was not smooth but included relatively high hills and ridges, some of which rose 1,000 to 1,500 feet above the adjacent lowlands. These elevations are commonly composed of the harder