up of thoroughly rounded bowlders and pebble, there is a thick series of red sandstones, shales, and quartzites, succeeded above by earthy and shaly limestones, in the midst of which there is a stratum of well-preserved corals, among which a branching form is much like a coral found on the Verde River and referred by Professor Meek to the Devonian. (See Paleontology of Fortieth Parallel Survey, IV, 27, and pl. 11, fig. 3.)

It is interesting to note that a large quartz vein traversing the basal red sandstones and shales is auriferous.

Farther south, in the Santa Rita range, in the region of Rosemont and of Greaterville, on the east side of the mountains, limestones and quartzites are extensively developed and upheaved. The basal rock on the west is the coarse granite above described, which has a great extension to the north and northwest, and appear generally to be gold bearing, as, for example, at the Mammoth and at Goldfields. In the Greaterville region this granite has a very massive flanking on the east of a coarse conglomerate of granite fragments, indicative of a stupendous current of water from the west eastward and at the same time showing that at an early period of the history of the continent there were shallow seas and shore lines in the region of Arizona. Other and different conglomerates farther north and south bear abundant testimony to the truth of the same general statement. Arizona was largely above water in the earlier ages. This granite conglomerate seems local. It requires further study. It is overlain by quartzites and limestones, and probably unconformably, yet it is not certainly older, for masses of quartzites have been noted in its midst. It is, however, auriferous. Small seams of quartz are numerous, and in some of them coarse gold can be seen. The extensive degradation of such rock would liberate a large amount of gold, and this formation, together with other quartz veins, may be the source of the gold of the Greaterville placer. (See under Placers, chapter on gold.) Directly above the quartzite and red beds there are heavily bedded limestones, which at Greaterville and Deerings Camp are fossiliferous. I collected corals and brachiopods representing the Silurian age. A well-defined and nearly vertical fault dislocates the red beds near to Deerings Camp.

SANTA CATALINA MOUNTAINS.

This massive range of mountains bounds the view from Tucson across the broad mesa northward. It is one of the most marked and extensive of the many mountain groups which form the diversified topography of Arizona. As seen from Tucson or the plain on which the university stands, its general directions appear to be east and west, for it stretches wall like for some 10 or 15 miles approximately in that direction, but if followed around its western extremity a northerly or northwesterly trend becomes apparent, and it may be followed up the Cañada del Oro for some 30 miles in that direction to and beyond Oracle, just over the line in Pinal County. The range is rugged and precipitous, especially upon the northern face of the ridges, due to the peculiar layering of the rocks. The highest and chief ridge culminates in Mount Lemon, the summit of which is reported to be about 10.000 feet above tide, or about 7,600 feet higher than the plain of Tucson. And the high ridge reaches its greatest elevation in Mount Rice. The higher ridges and summits are favored by snow falls in winter and an abundant precipitation during the season of summer rains. These higher ridges are thus covered with vegetation and support forests of splendid furs and pines, some of the trees attaining a diameter of more than 3 feet at the base of the brink. But the precipitation is not sufficient to maintain large flowing streams descending to the surrounding plains, except for a short time during the periods of excessive rainfall on the summits or when a heavy snow fall in winter is rapidly melted by the ardent rays of an unclouded Arizona sun.

The geologic structure of the central mass of the Santa Catalinas is but little known. The southern flanks, sloping off into the great mesa of Tucson, are made up of tabular gneiss in such regular strata as to look at a distance like ordinary mechanically formed sedimentary sandstones. The planes of stratification are flat, not in curves or folds. In this respect they are peculiar and almost unique. The low angle of dip and the tabular form both indicate very little disturbance since their deposition. The formation is very thick. It is penetrated by tabular sheets of granitic rock. Large portions of the gneiss contain feldspathic nodules enfolded in the micaceous layers. giving the appearance of a porphyry like our 'bird's-eye porphyry" of the prospectors, and entitling the rock to the name of augen gneiss. Other layers are quartzose, sandy, and seemingly are quartzites or old sandstones. The whole series is ancient. I refer them to the Archæan.

66

On the extreme northeastern flank of the range there are coarse conglomerates made up of thorougly well-worn bowlders and pebbles, in which white quartz predominates, thus indicating extensive mineral shore-line eruption and deposition. The Palezoic beds are there developed; also a broad scale, with thick deposits of red sandstone and shales-the Devonian or older and fossiliferous limestones above elsewhere noticed.

Crystalline limestone in the form of statuary marble may also be found, and near Johnson's rancho, in the gulch, there are very interesting croppings of the Arizonian slates in a greatly compressed and contorted condition, showing sharp zigzag foldings and crumplings, evidently the result of intense lateral pressure. Some deposits of copper ore are being prospected in different parts of this range and in the adjoining spur or branch known as the Rincon Mountains, lying northeasterly from Tucson.

THE NATURAL BRIDGE OF ARIZONA.

The remarkable natural formation of a chasm spanned by a rock bridge is one of the wonders of the world, and it far exceeds the natural bridge of Virginia in extent and grandeur. The bridge spans Pine Creek, one of the tributaries of the Upper Verde River, in Gila County, in the extreme northwest corner of Tonto Basin and about 4 miles from the town of Pine. A description of this natural bridge was given by Patrick Hamilton in his book upon Arizona, with a full-page lithographic illustration (p. 136), but I subjoin a more recent description, giving an account of a visit to the bridge, by Mr. Harry G. Logan, one of the students of the University of Arizona.

"Pine Creek at the bridge flows in a canyon with steep sides. Arriving at the brink of this canyon we see before and below us a nearly level flat of land some 5 or 6 acres in extent, reaching across the canyon to the opposite side, where there is an abrupt wall of rocks, the face of a mountain perhaps 1,500 feet high. Standing on this level tract of land, on which there are buildings, an alfalfa field, and an orchard, it is hard to believe that we are on the top of a natural bridge across a chasm and that the stream flows below us. But, looking to the right and to the left, as we face the precipitous bluff, we see the open canyon, perhaps 300 or 400 feet wide and 250 feet deep. The height of the bridge lessens as we approach the opposite side, and at one place there is a hole in the apparently solid rock through which one can look down for 126 feet to the bottom of the canyon.

Desiring to get under this wonderful bridge, we take a trail to the left and note that the rocks lie in great benches or strata, like cave-like openings extending backwards into the buttresses of the bridge. Entering one of these openings and crawling sometimes upon our hands and knees we find a cave or vaulted chamber with beautiful translucent stalactites pendant from the ceiling and reaching nearly to the floor. These stalactites are resonant when struck, giving off musical notes. This cave is the abode of numerous bats and they made it uncomfortable for us to remain, so we descended to the bottom of the canyon where there is a pool of water 20 to 30 feet across and said to be over 50 feet deep. The opposite buttress or wall of the bridge is nearly perpendicular."

Other caves along the canyon give evidence of the flow of water carrying lime in solution, for twigs, grass, and other substances lying in the water are now covered with a crust or deposit of lime. The rocks of the canyon appear to be chiefly limestone.

Theorizing upon the origin of the bridge we may suppose that the springs and streams of calcareous water have gradually deposited lime in greater quantity above than below, and have in the lapse of time built out or cemented together the rocks and débris of the canyon until a firm cemented mass was formed under which the water cuts its way. Fossils obtained from the limestones of this locality indicate the period or age of the Lower Carboniferous limestone.

MESA FORMATION, TUCSON.

The mesa upon which the University of Arizona is located consists chiefly of horizontal deposits of washed gravel and sand derived from the breaking down of granite and gneissic rocks. The nature of the fragments indicates that they came from the canyons and cliffs of the Santa Catalinas and the Rincon Mountains. It is more than probable that a large part of the deposits came from the Sabina Canyon, the largest and most important drainage canyon from the Santa Catalina range, but at a remote period, when the rainfall was far greater than at present. In sinking a well to obtain water just north of the northwest corner of

the university tract to a depth of 85 feet the following succession of layers were cut through:

Section of the mesa formation.

Thin, sandy soil, 3 to 12 inches, then caliche and pebbles, hard cement..
Coarse gravel, granitic

Sand and water-worn bowlders.

Feet.

6

3

11

3

30

81

31

Most of the closely cemented sand and gravel not enveloped in caliche is well filled with small, sparkling crystals of cale spar, which appears to be the cementing material holding the grains of sand together.

TRIASSIC FORMATION.

I have elsewhere mentioned the probability that the red sandstones of the Salt River Valley at McDowell Mount and near Phoenix are probably of Triassic age. Dr. J. F. Blandy in 1893 published a note upon the occurrence of the Trias in southwestern Arizona (Engineering and Mining Journal, November 4, 1893), and informs me that since then he has found remnants of the Cretaceous in places, but particularly in the Date Creek Valley west of Congress.

ANCIENT CHANNEL OF SALT RIVER.

There are good evidences that at a former period of the later ages of the earth's history, but possibly before the creation of man, the Goldfield region east of Mesa was the channel through which a greater part of the ancient floods now represented by the Salt River flowed westward. The chief fact in evidence is the presence upon the higher mesas and hills of drifted river gravel, so much rounded and water worn as to prove its transportation by water currents from remote sources, and at the same time showing by the composition of this rounded gravel its source in hills and mountains far to the eastward, and at the sources of the Salt River and the Gila. Among this transported gravel we find bowlders of the same peculiar kind of slate or sericite-schist as forms the bulk of the high mountains known as the Four Peaks, which rise majestically in the west and north. It is probable that before the Salt River had cut its way to its present low channel a part at least of the drainage was through this open gateway to the west. It should also be noted that the volcanic tufa which now surmounts the hills also contain considerable quantities of similar slate, lending strength to the belief that these sediments also were laid down by great currents of water sweeping westward from the Four Peak region.

GEOLOGY OF THE SUPERSTITION MOUNTAINS.

The Superstition Mountains, rising about 20 miles east of Mesa, and lying between the Gila and Salt rivers, are not, as usually shown, one continuous range extending north and south. The name should be restricted to the east and west line of table-topped hills extending along the road from Mesa to Pinal. Instead of a continuous range beyond these hills on the north there is a broad. open basin-like valley to which the name Goldfields has been given. It is an open space between mountains on each side, and has the general elevation of 2,300 feet or 1,000 feet above Mesa City. The hills on the north side of this valley rise from 500 to 1,000 feet higher, and are formed of volcanic tufas and shyolitic lavas laid down upon a granitic foundation. So also are the summits of the hills on the south side volcanic in origin, but are harder and more porphyritic and basaltic. The general dip of all these beds of plutonic origin is to the eastward. The floor or foundation of the whole region is a coarse-grained and porphyritic granite. The distinct feldspar crystals are generally from half an inch to an