USGS Logo Geological Survey Bulletin 845
Guidebook of the Western United States: Part F. Southern Pacific Lines

SHEET No. 23
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Elevation 1,621 feet.
New Orleans 1,545 miles.

Elevation 1,425 feet.
Population 1,100.
New Orleans 1,564 miles.

The line from Picacho to Wellton by way of Phoenix (turn to sheet 23) was built in 1925 and 1926 at a cost of $15,600,000, in order to pass through the great irrigation district of the Gila (he'la) and Salt Rivers. It is 42 miles longer than the old line but has the advantages of better grades, fewer curves, and long tangents, which almost compensate for the detour. The Gila River is crossed twice, one bridge being 5,000 feet long and the other 3,800 feet.

This route leaves the old main line at a switch tower a mile west of Picacho siding and goes north across the wide alluvial plain to the Gila River, 20 miles distant. It passes through the sidings of Peak, Topaz, and Randolph and the town of Coolidge in this interval and also crosses the great ditch that carries water from the Gila River to Casa Grande and other irrigation settlements to the westward. This water conserved by the Coolidge Dam, on the Gila River in the mountains 50 miles above Florence, is let out into the river as required and deflected into the main canal near Florence. (See p. 210.) About 40,000 acres of the land to be irrigated is in the Gila River Indian Reservation, and the remainder of the water is available for settlers outside, who have taken up much of the land and are raising cotton, lettuce, and other crops with satisfactory results.

Two miles beyond Coolidge the ruins of Casa Grande are in sight, not far west of the railroad. For many years they had no protection against the weather, but finally after some restorations a roof was erected to protect the ruins from rain and in some measure from wind blown sand, a powerful erosive agent in regions of dry climate. (See pl. 24, A.)

PLATE 24.—A (top), CASA GRANDE, ARIZ. The ruins of a communal house of the aborigines, now under a protecting shed.


Casa Grande,42 as the name indicates, is the "large house" mentioned by the early explorers; it was the work of aborigines of 700 to 1,000 years ago. It was discovered by the Jesuit Padre Kino in 1694; he reached it again by way of the San Pedro in November, 1697, and said mass within its walls. It stood 1-1/2 miles south of the Gila River, with which it was at that time connected by a wide ditch. It was visited by Padres Garcés and Font in 1775 and minutely described by Font. It has always been one of the best preserved of the prehistoric ruins and has been restored to a considerable extent by the United States National Park Service, which took possession of it in 1892. There were three buildings within a space of 150 yards, two of which were practically ruined. The walls of the main building, which was three and in the central part four stories high, were massive and 4 or 5 feet thick at the base. The inner sides of the wails were vertical, but the outer sides sloped inward in a slightly curved line. The house contained 11 rooms and had a watchtower estimated to have been 39 feet high. The material used was the local mud and gravel packed into rectangular blocks until hardened. There is some ornamentation in red on the inner polished walls, but no inscriptions. There are doors east and west, but no windows except circular openings in the upper part of the chambers. The framework of the building evidently was burned, presumably by hostile Apaches. Near by are ruins of other buildings and of an elliptical amphitheater more than 100 feet long, probably all used for religious or communal ceremonies.

42It has been suggested that this may be the ruined house called "Chichiltic'cale" (red house), where Coronado changed his course from north to northeast on his way to the Seven Cities of Cibola (Zuñi) in 1540, but many authorities believe that his route was farther east. (See p. 161.)

Excavations in 1930 a mile east of the Casa Grande ruins revealed several large houses, several crematory pits, and much pottery, carved bone, and stone and shell artifacts. Fragments of mirrors whose reflecting surface was a close mosaic of iron pyrite crystals were also found, showing that the people took considerable interest in their personal appearance.

In the river flat just north of the ruins are remains of old irrigation ditches which conveyed river water to fields. The people of this early settlement were evidently experienced in agriculture, and the irrigation ditches, some of them large, show considerable engineering skill. (See p. 201.) It seems clear from the broken pottery and ruins that the Gila Valley and the valley of the Salt River supported a good-sized agricultural population in the early days. The Pima Indians called these people "Hohókam." They lived in small huts not unlike the Pima "jacales," made of rude masonry. It is supposed that they came from the south. It is an Indian tradition that a hostile faction from the east drove these agriculturists from their settlements in the Gila Valley, but some who remained in the general were the ancestors of the Pima, Papago, Yuma, Chemehuevi, Mohave, and Maricopa tribes of the present day.

In the times of the Conquistadores and the missionaries most of the sedentary Indians of Papaguería lived on adjacent ranches and in villages palisaded for protection against roving Apaches or other enemies. It is stated that there were about 6,000 of these Indians and 100 rancherías in the lower Gila region in 1742. There was constant warfare among the tribes or among allied tribes. For the white man there seems to have been a hearty welcome until ill treatment roused hatred that prevailed for a long time. The Pima Indians, however, have always been friendly to the white settler and helped to fight the Apaches, who were hereditary enemies of the sedentary tribes, stealing their crops and wives. Now Papago, Maricopa, and Pima Indians live in harmony on the reservations south of Phoenix. The Pimas and Maricopas have the first rights on the irrigation waters of the Gila River, which they use extensively for the more common field crops.

The lower Gila region was never the scene of such extensive and bloody Indian warfare as other parts of Arizona, because of the more peaceful character of its aboriginal inhabitants and partly because of the scarcity and poverty of the white settlers in the early days.

Two miles north of the Casa Grande ruins the broad bed of the Gila River is crossed on a long bridge. In the main channel there is usually considerable water, which is allowed to flow from the Coolidge Reservoir to sustain irrigation, together with some ground water and seepage of local origin. The Gila River is one of the major streams of the Southwest, for it drains an area of about 7,200 square miles and is about 500 miles long. It rises in western New Mexico and crosses all of Arizona to join the Colorado River just above Yuma, receiving many large affluents, including the San Simon, San Pedro, and Santa Cruz Rivers, which are crossed by the railroad in eastern and central Arizona. Up to 1853 (the time of the Gadsden Purchase) the Gila River was the boundary between the United States and Mexico. The Gadsden Purchase brought into the United States the portion of Arizona south of the Gila River, an area of 40,000 square miles (see map, p. 151), at a cost of $10,000,000. The international boundary was surveyed in 1855, and the United States took possession in 1856 by sending troops to Tucson. The river was called Río del Nombre de Jesús by Oñate in 1604. The heroic Father Kino in 1694 applied the name Rio Grande de Gila to the river, but generally called it Rio Grande. The Indians on its headwaters were called Xila or Gila, and this name was applied by the Spaniards to a savory but bony fish called matalote by the Indians. It is stated also that there is a Yuman word Hila, meaning salty stream. Later, Kino's was given to the entire stream.

After crossing the Gila River to Poston siding, near which a branch line leads to Florence, 6 miles east, and thence to the mining town of Christmas, in eastern Pinal County, the railroad deflects northwestward and follows near the north bank of the river through Blackwater and Olberg sidings. To the east and north of the railroad are many buttes of granite, the highest of which, Walker Butte, is capped by lava. At Olberg is a quarry in lava and scoria, which are used extensively for road making.

North of Olberg is Malpais Mountain (mal-pah-ees'), which consists of lavas and tuff43 capping granite which appears also in ridges and detached buttes to the east; it also constitutes Santan and Goldmine Mountains, farther north. Yellow Peak and Rock Peak, a few miles north of Olberg, are capped by conglomerate of Tertiary age.

43These volcanic rocks cover an area of about 9 square miles and consist of several flows, in all several hundred feet thick, dipping gently south-southwest. At one locality a sheet of olivine basalt is exposed lying on a 200-foot sheet of latite, in part tuffaceous, which in turn lies on the old granite. Under the microscope the latite is seen to consist mostly of volcanic glass crowded with microliths; it contains some orthoclase, albite, biotite, and olivine.

South of Olberg are the prominent granite ridges of the Sacaton Mountains, with various outlying buttes. These are all typical desert mountain ridges, with steep rocky surfaces rising abruptly from the long, gentle slopes of wash and valley fill, which is very thick in the adjoining valleys. At most places large parts of the flanks of these mountains are buried by detritus and only the tops protrude, and doubtless there are many others that are entirely buried. If this valley fill were removed the Salt River-Gila plain would present a very rugged topography, with ridges and buttes 1,000 to 2,000 feet high. The filling has progressed for centuries, is still actively going on, and will continue until the present ridges and buttes are worn very low and the smaller ones buried entirely. A view of a typical desert valley in this region is given in Plate 25, A.


B (bottom), INDIAN PICTOGRAPHS NEAR SACATON, ARIZ. Crude figures of animals, snakes, birds, etc. depicting records or messages. Probably very old.

Just south of Olberg is a dam that diverts water from the Gila River into canals to supply the lower portion of the Gila River Indian Reservation. This reservation occupies a wide area in the Gila Valley and according to the report of the United States Commissioner of Indian Affairs for 1932 contains about 4,000 Pima Indians, 500 Maricopa Indians, and a few others. Many of these Indians irrigate farms, using the water provided for them by the Government and raising alfalfa and other crops which under irrigation flourish in the rich soil of the valley lowlands. In Padre Garcés' time (1775) the largest Pima settlement was located in this immediate neighborhood, with a population of about 5,000. He called it Sutaquison, but Padre Kino 80 years earlier had named it Encarnación.

Leaving the bank of the Gila River near Olberg the railroad skirts the rocky slopes of Malpais Mountain and passes through Dock and Santan sidings. There are many sahuaros, or giant cacti, and cholla (mostly Opuntia bigelovii) on these slopes. Indian houses are in sight at many places (see pl. 24, B), and the Pima Indian village of Santan, with a large school, is a mile east of Dock siding. Three miles south of Dock is the larger settlement of Sacaton, with the Indian agency that administers the Gila River Reservation.44 The reservation consists of 371,422 acres of which a small part is under irrigation. Now that water of the Gila River is conserved by the Coolidge Dam a much larger area can be cultivated than formerly. Near Sacaton is a field station of the United States Department of Agriculture investigating the crop conditions of the region.

44The Pima agency also administers the Gila Bend and Chiu-Chiuschu Reservations, occupied by small groups of Papagos. Many of the Indians are engaged in farming or serve as laborers on ranches in the adjoining regions. About one-quarter of them speak English, and many speak or understand Spanish.

East of Santan is a group of rugged ridges and hills culminating in a peak 3,093 feet high known as Santan Mountain, which is a conspicuous feature from the wide desert plain to the north. This mountain and the surrounding ridges consist of pre-Cambrian granite and schist cut by younger granite.

Near the Maricopa-Pinal county line the railroad bends due north and goes through Chandler to Mesa.

Near Serape siding the Salt River Valley is entered, consisting of almost continuous irrigated fields in a high state of cultivation, utilizing water from the Salt River conserved by the Roosevelt Reservoir. (See p. 214.) The contrast between desert conditions and vigorous plant growth is strikingly shown on the margins of the irrigated areas, especially at the foot of slopes of the rocky ranges rising out of the plain. The use of Salt River water for irrigation dates hack to an early time, for the aborigines had many ditches, some of them of considerable size and length. These and the later ditches of the white man were washed out or damaged every few years by floods, which are especially prevalent in the arid region. In 1877 many settlers began coming into the valley, and since that time its development has been rapid as irrigation has been improved and extended.

Elevation 1,215 feet.
Population 1,378.
New Orleans 1,597 miles.

Elevation 1,245 feet.
Population 3,711.
New Orleans 1,604 miles.

Chandler, in the southeastern part of the great Salt River irrigation district, is an attractive rural settlement founded in 1912 by Dr. A. J. Chandler. It is also a noted pleasure and health resort with an artistic winter hotel. From Chandler and northward there are fine views of Four Peaks, the high summit of the Mazatzal Range, and of the bold west front of Superstition Mountain.

Mesa was started in 1878 by a colony of 77 Mormons who followed the original Mormon colony from Utah, established the preceding year at Jonesville (now Lehi) by Brigham Young. The new colony at once commenced the construction of a ditch costing $43,000, to irrigate about 5,000 acres. At present there is a very large area under irrigation and many crops are produced, including dates (pl. 26, A) and citrus fruits. From a small village in 1883 Mesa has grown to an area of 1 square mile, parts of which are closely built. The near-by population is about 11,000. The Mormons have a large temple, several churches, and an auditorium.



Two miles west of Mesa is a 160-acre farm of the State Agricultural Experiment Station, where practical tests of many kinds are made on a tract of heavy silt-loam soil, which is typical of much of the Salt River Valley. Here cotton, alfalfa, lettuce, melons, and other plants are grown under various conditions of irrigation, fertilization, crop rotation, and cross breeding. Experiments are also made with cattle and sheep pasturing.

In this vicinity are fine views of the west front of Superstition Mountain, 20 miles east of Mesa. (See pl. 31, B.) It consists of flows of lava (rhyolite) and beds of white volcanic tuff, in all more than 3,000 feet thick, yet greatly eroded from its original size and extent. On its slopes are many sahuaros and other desert plants, and in early summer the showy scarlet flowers of Beloperone californica, which also grows on the Picacho Mountains, and is very attractive to humming birds.

Elevation 1,159 feet.
Population 2,495.
New Orleans 1,611 miles.

From Mesa the railroad turns sharply west, and near Tempe (tem'pay) it deflects north on joining the branch line from Maricopa. At Tempe is the State experimental date farm, the United States Entomological Laboratory, a large normal school, and a condensed-milk factory which utilizes much of the product of dairying, now a great industry in the Salt River Valley. Tempe, established in 1870, is the second oldest town in the valley. It was first called Haydens Ferry and later renamed for the classic Vale of Tempe.

At Tempe a great bridge carries the railroad over the Salt River. This large stream rises in the mountains of eastern Arizona and flows into the Gila River about 15 miles southwest of Phoenix. Formerly it experienced many freshets, with disastrous results to irrigation ditches and near-by fields, but these no longer occur since its waters have been impounded by the Roosevelt and other dams. Now its flow is regulated to meet the needs of the farms and orchards it irrigates, and its utilization has resulted in an agricultural development which has made the Salt River Valley a celebrated garden spot. Kino called the river Río Azul, and Garcés Río de la Asunción.

Just east of the bridge over the Salt River is Tempe Butte (see fig. 50), a prominent landmark due to a heavy mass of lava (andesite) lying on shale and sandstone, which with the lava is tilted to the southeast at an angle of 45° or more. The base is a massive sandstone quarried to some extent for building. The strata are more and more mixed with clay toward the top, where most of the material under the lava is red shale. More red sandstone in massive beds is exposed north of the river opposite Tempe; it grades down into a coarse granitic arkose or breccia lying on an irregular surface of old granite. It dips 65° NW., nearly at right angles to the dip of the exposure in Tempe Butte. This sandstone was found in a well 1-1/2 miles northeast of Tempe, but a well 2-1/2 miles northwest of the town was entirely in granite. Similar arkose and conglomerate lie on granite in Camelback Mountain, near Phoenix. Probably the age of the formation is late Tertiary. (Lee.) Other buttes, including Bell Butte,45 rise out of the valley a short distance southwest of Tempe.

45The rock of Bell Butte under the microscope proves to be a hornblende-pyroxene andesite showing phenocrysts of hornblende and feldspar of the soda-lime group. The groundmass is glassy, in part microlithic. (Lee.)

FIGURE 50.—Section through Tempe Butte and Tempe well, Ariz. After Lee

Just north of the river north of Tempe sedimentary rocks of Tertiary age form a small group of picturesque hills included in the Sahuaro National Monument. Here the material is an arkosic conglomerate in massive beds lying in part on granite gneiss and in part on a porphyritic felsite. The conglomerate contains much granite and some schist and felsite with many fragments from 6 inches to 6 feet in diameter. In places there is but little matrix, but in general the coarse material is embedded in sand composed of grains of quartz and feldspar. It has been suggested that these rocks are of Triassic age, but here, as in Tempe Butte, they include a thin basalt flow and are capped by basalt, a succession closely resembling that which is found in the Tertiary of the surrounding region. The tilting of the Tertiary beds here and elsewhere in the Phoenix region shows that there have been earth movements in this region in post-Tertiary time, and the similar tilting and faulting of the volcanic succession in adjoining regions indicate that deformation was widespread in southwestern Arizona.

In the Sahuaro National Monument are many fine sahuaros and other plants of the desert flora which will be preserved under Government supervision. The rocks are eroded in many fantastic forms, one of which is the natural window called "Hole in the Rock." The gravel-covered plains surrounding the hills are typical of the wide desert valleys of the Southwest.

Elevation 1,083 feet.
Population 48,118.
New Orleans 1,620 miles.

Phoenix, the metropolis of western Arizona and capital of the State, occupies an area of about 10 square miles on the plain extending north from the bank of the Salt River. Although in the midst of a desert, the city has developed great landscape beauty and many cultural and educational resources. It was established by Jack Swilling in 1867 as a colony for irrigation, a fact commemorated by the Swilling memorial fountain in the courthouse grounds. Phoenix was incorporated in 1881. It was reached by a branch line from Maricopa, on the Southern Pacific Railroad, in 1887 and by a branch from the Santa Fe lines (Santa Fe, Prescott & Phoenix Railroad) from Ash Fork in 1886. Prescott was the State capital from 1864 to 1867 and 1878 to 1911, and Tucson from 1868 to 1877. The Phoenix region was first visited by Padre Kino in 1694.

The growth of Phoenix has been rapid, especially since 1910, when it population was only 11,134; the growth was 70 per cent from 1920 to 1930, and this increase was closely paralleled by the growth of the populous surrounding ranch territory. The name Phoenix (given by Darrel Duffa) refers to the fact that the settlement has "risen from the ashes of the vanished civilization of the aborigines of long go." In the valley there are many miles of ditches of great antiquity, capable of watering many acres. There are also ruins of numerous settlements and many remnants of utensils and implements. Large collections of archeologic material are on exhibition in the Arizona Museum in Phoenix and also in the Heard Museum. At the latter are collections from the ruins of "La Ciudad" or the "Indian mounds" near the city. At Phoenix there is a large Indian school sustained by the United States Government.

Irrigation has gradually been extended over level lands of the Salt River Valley until now a large area is occupied by farms and ranches in a state of high cultivation, connected by fine roads in greater part lined with cottonwoods and other trees. The valley population is about 150,000. Many crops are raised, including a large production of grapefruit and alfalfa, and for a wide area the region is a veritable garden, in great contrast to adjoining unirrigated lands that remain in their original desert condition, as shown in Plate 27, A. (See also pls. 26, B, and 27, B.) In 1929, according to the United States Bureau of Reclamation, the agricultural products were valued at $38,000,000 from an irrigated area of 404,315 acres. Production and irrigated area have about doubled since 1920.

PLATE 27.—A (top), THE DESERT FROM WHICH THE SALT RIVER VALLEY IRRIGATION DISTRICT HAS BEEN RECLAIMED. It is covered by cacti and other desert plants and marginal by bare rocky mountain slopes. Note sahuaros in fruit, also cholla at right.

B (bottom), IRRIGATION IN SALT RIVER VALLEY. The water is derived from the Salt River and from wells.

The development of irrigation was slow and irregular under private management, and there were many complaints of inadequacy of water supply and much conflict in respect to claims for water and canal rights. Finally the United States Bureau of Reclamation46 reorganized the project and built the Roosevelt Dam to hold the water of the Salt River and its tributary Tonto Creek in a huge reservoir in the mountains 80 miles east of Phoenix. (See p. 213.)

46This bureau of the Government was an outgrowth of plans of Maj. J. W. Powell for the reclamation of the arid lands of the West, and it was brought to existence by the irrigation act of 1902, fostered by President Theodore Roosevelt, with the late F. H. Newell as the first director. The Roosevelt Dam was the first large project completed. Up to June 30, 1929, the Government had invested about $186,000,000 in reclamation projects in the United States (not counting interest), and the total repayments have been $36,350,000. The repayments in the fiscal year of 1929 amounted to $6,308,000 (U. S. Bur. Reclamation).

In the Salt River Valley, as in most other irrigated lands in the Southwest, alfalfa is the most extensive crop, yielding from 5 to 8 tons to the acre; other forage plants are also raised, most of them giving two crops a year. The value of the cotton crop in 1929 is estimated at $12,435,000 by the State College of Agriculture, including much of the long-staple variety introduced from Egypt, for which the region is well suited. Cotton was a minor product prior to 1912, when its area was only 400 acres. The cost of producing cotton in the Salt River Valley in 1928-29, according to careful investigations by the State College of Agriculture, ranged from 8.72 to 20.46 cents (average 13.4 cents) a pound for ordinary cotton and from 17.2 to 38.8 cents (average 23.8 cents) for long-staple cotton. This included picking, which cost 1.5 and 2.5 cents respectively, and ginning, 45 cents per 100 pounds of seed cotton. The ginning is more than paid for by the value of the seed.

Cattle feeding and dairy farming have the advantage of having open pastures the year round, but a staggered system of pasturing is used to provide for regrowth of the grass. About 25,000 dairy cattle were reported in 1929. Many sheep are wintered in the Salt River Valley to be fattened on alfalfa.

The sugar mills are busy for much of the year, the cane crop coming in as the beet crop ends. Citrus fruits are extensively produced, to the number of 453,330 boxes in 1929 (Census report). The very young grapefruit trees can not be left out in winter, so they are taken up in December and placed under cover until spring. This processed is "balling," because a ball of earth is taken up with the roots. It was in the suburb of Ingleside, at the foot of Camelback Mountain, that the first orange orchard and the first olive grove in the Salt River Valley were planted. Of cantaloupes and melons the annual output is nearly 6,000 carloads, and of lettuce about 10,000 carloads. Figs and dates are important products, and other small fruits are raised in great variety and large amount.

Much water for irrigation, city water systems, and individual ranches is pumped from shallow wells in the gravel and sand that underlie the Salt River Valley. These deposits contain a large volume of water, mostly the general underflow from the Salt and Gila Rivers to which is added some of the local flood water entering the valley. It is believed that although most of the rainfall is lost by evaporation or run-off, a part of it as well as considerable water that has been used for irrigation sinks into the porous material of the valley floor and in a measure replenishes the underground supply.47 The amount of underground water available varies from place to place with the thickness and character of the permeable beds, and in some localities heavy pumping has depleted the supply. It is estimated that 525 square miles in the Salt River Valley is underlain by water-bearing beds from which the water can be profitably utilized by pumping.48 About Mesa the area of water-bearing beds is 15 miles wide and some of them extend to a depth of 200 feet.

47The rate of underflow of waters in valley fill and other permeable material varies mainly with size of grains and head of the water. In sandstone a rate of a mile a year, or one-eighth of an inch a minute, is a fair average; in the sands of the rivers and wash deposits rate of 2 to 3 miles a year has been estimated. (Meinzer.)

48A detailed study of these underground water resources was made by the U. S. Geological Survey in 1900-1903. (See Water-Supply Paper 136, by W. T. Lee.)

In the Salt River Valley as in other similar districts there are two principal kinds of alluvium—the coarse river deposits of many sorts, laid down at various stages of the rivers, in old and new channels, and under different conditions of velocity; and the finer sheet wash spread by local "cloudbursts" and by the rare long-continued rains. The coarser boulders and gravel reflect the character of the country drained, the rivers bringing materials from distant regions, the smaller streams transporting them from near-by localities. The side streams, such as Queen Creek and Cave Creek, build up broad flat fans or deltas containing a large amount of sand, gravel, and boulders, the accumulation of many freshets. (Lee.)

The thickness of the valley fill is great, at least in part of the area, for a boring 1,305 feet deep at Mesa failed to reach the bedrock that constitutes the bottom of the old valley. Doubtless much time was required for the deposition of all this material, and some of the lower finer deposits may have been deposited by lakes in late Tertiary time. It is believed that during its early stages the Salt River joined the Gila River east of the Salt River Mountains, as indicated by great beds of boulders underground, and at a later stage the river shifted to its present course north of these mountains. At this time it deposited the boulder beds that yield the underflow about Phoenix. These later gravel deposits lie in an old channel roughly parallel to the present one and excavated in the finer beds which were spread widely by overflows during the earlier period of accumulation.

Care has to be taken in irrigation not to let the mineral contents of the water accumulate in the soil, especially some of the well waters, which are more highly mineralized than the river water. In some parts of the valley the soil has been poisoned in this way, but this can be avoided by suitable underdrainage to carry off water that otherwise would evaporate and leave its dissolved mineral matter.

The Salt River Mountains, which rise abruptly from the desert plain a few miles south of Phoenix, consist of chloritic schist and fine-grained biotite granite. The granite is quarried to some extent as an ornamental stones. The Sacaton Mountains and many of the peaks and ridges on the east and south sides of the Salt River Valley are made up of granite, some of which is very coarse grained, with many of the feldspar crystals as much as 2 inches in length. A few miles north of Phoenix are the Phoenix Mountains, which consist largely of quartzite and other metamorphic rocks in massive beds, several thousand feet thick in all, tilted at high angles. Some of the mountains in the Salt River region are upthrust blocks; others are remnants of older ridges nearly buried by valley deposits.

The climate of Phoenix is similar to that of most of the deserts of southwestern Arizona at elevations from 1,000 to 2,000 feet. According to the records of the United States Weather Bureau, the mean annual precipitation is about 8 inches, most of which falls in mid-summer showers. The amount varies greatly from year to year, however, in some years being less than 5 inches and in others as much as 14.41 inches (1911). The mean annual temperature is 70°, and the summers are long and warm, but the summer heat is much less oppressive than in regions with more moisture in the atmosphere. The amount of sunshine, as compared with the greatest amount possible, is 84 per cent. The mean temperature during the winter is about 40°, owing to cold nights, but most of the winter days are mild. Parts of the valley are free from killing frosts. (Continued on p. 218.)

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Last Updated: 16-Apr-2007