rocks and appear to be genuine monadnocks, though the adjacent plains were not ordinary peneplains but confluent pediments. These plains, however, formed an erosion surface typical of a desert region, and the hills and ridges were desert mountains in the old stage of erosion. Upon the reelevation of the region that followed the great volcanism in Tertiary time, many of these desert ranges were resurrected and subjected to renewed erosion by the rejuvenated streams. Such lavas as may have capped parts of the mountains were first removed, and then erosion attacked the original mountains. The reelevation was in places so small or the topographic position of the mountains was such that erosion began approximately in the same place and with approximately the same base-level as in the previous cycle. The result was that an already old mountain was again eroded and retained rather than acquired the slopes characteristic of old age in desert erosion.

SUMMARY OF CLASSIFICATION

The classification of the mountains as above set forth is summarized in the following table:

Classification of mountain ranges in the Papago country

[Mountains east of Santa Cruz River but included in the area shown on the geologic map (Pl. XII) are marked with an asterisk (*) to distinguish them from those belonging to the Papago country proper] Class I. Mountains composed in large part of alternating beds of acidic and intermediate lava, tuff, volcanic conglomerate and agglomerate, and stream-laid conglomerate

Group A. Mountains having a general conical form, smooth slopes, and quaquaversal dips, apparently owing their altitude in large part at least, to accumulation:

Group B. Mountains having a general extension in one direction (northwest) and with prevailing dip in one direction or horizontal; fault-block and horst mountains:

Ajo Mountains..

Crater Mountains.

Tempe Butte.

Group C. Mountains having no general extension in one direction, composed of various more or less disconnected plateaus and asymmetric ridges; complex faulted mountains and dissected plateaus:

Artesa Mountains..

246

IV

• If the outlying hills are excluded, the main mass belongs in group B.

Classification of mountain ranges in the Papago country-Continued Class II. Mountains composed largely of pre-Tertiary rocks but with some masses of rocks of Tertiary age

[Characteristic rocks: (1) Granite, gneiss, quartzite, schist, and phyllite, largely of pre-Cambrian age; (2) limestone and quartzite, of Paleozoic, usually Carboniferous age; (3) white and pink granite and monzonite, very coarse grained and pegmatitic, probably of Mesozoic age; (4) acidic intrusive and extrusive rocks, in places metamorphosed, probably of Mesozoic age; (5) acidic and intermediate intrusives of Tertiary age not easily distinguished from the rocks of subdivision 4.]

Class III. Mountains composed wholly of the pre-Tertiary rocks mentioned under Class II, with no known association of the Tertiary lavas

Classification of mountain ranges in the Papago country-Continued

Erosion and sedimentation are the geologic processes now in action in the Papago country. The rate at which the processes operate, their manner of operation, and their products are controlled by the aridity of the region. The degree of aridity is extreme and is exceeded within the boundaries of the United States only in southeastern California. It is important that the discussion of processes that follows should be considered in respect to the climate of the region, for in other arid regions these processes differ in degree, method, and result.

In the Papago country the same high temperatures and insolation prevail over the whole area. With respect to mean annual precipitation, however, three divisions may be recognized. West of the Growler Mountains the annual precipitation ranges from 32 to 5 inches; between the Growler and Baboquivari mountains it ranges from 5 to 10 inches; east of the Baboquivari Mountains it is more than 10 inches, although not much more except in the Tumacacori Mountains. Though precipitation increases with altitude, the smaller mountains, averaging 2 to 4 miles in width and 1,000 to 1,500 feet in height, appear to have little effect on storms, and their vegetation does not indicate any large increase in effective rainfall over that of the adjacent plains. This is particularly true in the area west of the Growler Mountains. The mountainous district including the Sand Tank and Sauceda mountains probably receives a somewhat greater precipitation than the adjacent plains, as indicated by gramma grass and a slightly more luxuriant vegetation at the higher altitudes. In the same way greater precipitation on the Baboquivari Mountains is shown by scattered live oaks and other small trees not characteristic of the plain. Increase of rainfall due to increase of altitude is best shown in the Tumacacori Mountains,

།

where there are orchard-like forests of live oak and a thick cover of perennial grasses. Even here the ease with which soil is formed on tuffaceous rocks and the consequent slow run-off may have as great an influence in producing the relatively heavy vegetative cover as increased precipitation.

MOUNTAIN SLOPES

DETERMINING CONDITIONS

The mountains of the Papago country rise from the surrounding plain with startling abruptness. Even to the traveler familiar with the scenery of the western United States, the mountain slopes rising without transition from the surrounding plain seem incredibly steep.82 Only in the distant view is it possible to realize that the surrounding plain rises gradually on all sides toward the mountains, which stand up like jagged ornaments on the ridge of a gable roof of low pitch. This appearance is due to contrast in angle of slope between the mountains and the plain. The angles of the mountain slopes range from 15° to almost 90° from the horizontal; those of the plain from 1° to 6°. Between these two sets of slopes there is usually no region of transition; either of intermediate slopes or of low foothills. In many ranges slopes that average 25° to 30° rise directly from the plain to the crest of the mountains. The factors which produced the mountain slopes must then differ radically from those which produced the plain.

The angle of slope is relatively constant for any one type of rock, whether on the border of the mountains or in the side walls of canyons. Even small, isolated hills have slopes of the inclination characteristic of the rock that composes them. This relation between slope and rock evidently is due not to forms inherited from the uplift of the ranges but to the action of the prevailing erosive processes on various rock types. These conditions have been stated by Lawson 8 as follows: "(1) The hard-rock slopes of desert ranges which shed large spalls are steep, while those which shed small fragments have a low angle; (2) ranges composed of hard rock, which are thus naturally steep, maintain their steepness as long as the rock slopes endure."

83

The mountain slopes, owing to the long quiescence of the region, are in few places oversteepened by uplift or by changes in the courses of streams. They are the work of long-continued erosion, and the angle of slope is controlled by the resistance of rock to weathering in a region of warm and arid climate, with only 3 to 10 inches of

82 Hornaday, W. T., op. cit., pp. 38-39 (a humorous statement of this impression). 83 Lawson, A. C., The epigene profiles of the desert: California Univ. Dept. Geology Bull., vol. 9, p. 29, 1915.

rainfall annually. In this region resistance to weathering is largely controlled by the physical properties of the rocks, as chemical action is of small importance because of the prevailing aridity. The spacing of joint cracks largely determines the size of the blocks or spalls that may be detached from the rock face, and in many types of rocks there is a significant relation between the size of block detached and the angle of the mountain slope, as noted hereafter. Resistance of rock to breaking up into small particles by direct weathering is of almost equal importance, and, together with the size, number, and durability of the large blocks, usually determines the angle of slope for any particular type of rock.

In the weathering that produces both large blocks and fine débris mechanical and chemical processes are involved, but alternate expansion and contraction due to changes in temperature are the most effective causes of rock disruption. The outer shell of rock, heated and expanded by the sun, parts from the cooler interior; or, cooled

FIGURE 5.-Diagram to show the range of mountain slopes

and contracted by the wintry desert night, it becomes too small for the warmer mass within. Each mineral grain, expanding and contracting under these same changes in temperature, parts from its neighbor. The disruption and disintegration of rock, complex in the interrelations of the several subprocesses and not wholly understood, is further complicated in its results by the differences in physical composition of the several kinds of rock.

The following analysis of the results of these complicated processes in the formation of mountain slopes and the sculpture of the desert ranges is a mere outline, in which only the outstanding features are treated.

GRADES OF MOUNTAIN SLOPES

Mountain slopes range in steepness from vertical cliffs to grades up which it is easy to ride a horse. Slopes can be divided into three groups-cliffy slopes, boulder-controlled slopes, and rain-washed slopes. The limiting angles and characteristic rocks of these groups are shown in Figure 5. Each group has certain common character