COMPUTATIONS In some analyses sodium was not determined, but the amount chemically equivalent to the excess of the acids over the sum of the calcium and magnesium is reported as "calculated." The computations and classifications of analyses in this paper follow the system which has been in use in publications of the United States Geological Survey for a number of years. They are discussed at length by Stabler 36 and by Dole " in earlier publications. Total hardness (H) is the calcium carbonate equivalent to the total calcium and magnesium and is calculated by the formula H=2.5 Ca+4.1 Mg.3 38 The scale-forming ingredients are assumed to be silica and such compounds of calcium and magnesium that the total quantity is given by the formula SiO2+2.95 Ca+1.66 Mg. The quantity of ingredients that may cause foaming in boilers is calculated at 2.7 times the combined quantities of sodium and potassium. The alkali coefficient (k) is that proposed by Stabler. It represents the depth in inches of water that would yield, upon evaporation, sufficient alkali to render a 4-foot depth of soil injurious to the most sensitive crops. CLASSIFICATION The chemical character is given in symbols showing the predominating groups of basic and acid radicles. Ca represents calcium and magnesium together, Na includes sodium and potassium, and CO, the carbonate and bicarbonate radicles. Classification for domestic use, for boilers, and for irrigation follows the usage of the publications by Dole and Stabler already cited. It is based on consideration of quality with reference to permanent living conditions. Hardness entirely unobjectionable for drinking may cause a water to be classed as poor for domestic use because it requires excessive soap consumption in laundering, and iron that will separate out and make the water turbid, or stain enamel ware and plumbing, or produce iron stains on articles washed in it may cause the water to be classed as much poorer for domestic use than it is merely for drinking. 30 Stabler, Herman, Some stream waters of the western United States, with chapters on sediment carried by the Rio Grande and the industrial application of water analyses: U. S. Geol. Survey Water-Supply Paper 274, pp. 165-181, 1911. 37 Mendenhall, W. C., Dole, R. B., and Stabler, Herman, Ground water in San Joaquin Valley, Calif.: U. S. Geol. Survey Water-Supply Paper 398, pp. 50-82, 1916. 38 The figures used with chemical symbols in this discussion represent parts per million of the radicles. Many of the wells and other sources of water represented by the analyses have so small a yield that it is unlikely that they will be used either to supply boilers or for irrigation, but the classification of existing water supplies will be of value to those who may in the future attempt to obtain larger supplies in the same localities. The main use of water in the region is for drinking by men and animals; the characteristics of the waters for this purpose are discussed in the paragraphs that follow. POTABILITY OF WATERS Aside from the possibility of contamination with disease-producing bacteria, whether or not a water is good to drink is largely a matter of opinion. To those who live in regions where very pure and lightly mineralized waters abound a small amount of salts or organic matter is noticeable and distasteful. On the desert the traveler has little choice and must drink what is available, and the permanent inhabitant is so hardened to water contaminated with mineral salts or organic matter that he accepts without question water which elsewhere would be considered unfit for human consumption. The impurities in water are suspended matter, dissolved mineral salts, and dissolved organic matter. Suspended matter consists of fine sand, silt, clay, bacteria, and rubbish of various sorts. It is common in the water of streams in flood, of charcos and rock tanks, and sometimes of wells. The careful man will boil and strain water that contains noticeable amounts of suspended matter, although such matter may not be harmful in itself, unless it includes disease-producing bacteria. The Papagos have since prehistoric time depended on the muddy water of charcos while tending their crops in the summer. They are a strong and handsome race, growing to more than average height when well fed and free from diseases introduced by the whites. Muddy water is commonly used for watering stock, and, though animals will normally choose clear water in preference, it is doubtful whether muddy water, unless it is also foul, has any deleterious effect on them under range conditions. The analyses given in this report show the amounts of different mineral constituents dissolved in the waters. The total amount expressed in the tables as total dissolved solids varies between wide limits. The lower the mineral content the more acceptable the water will be to most consumers, but the upper limit of tolerability differs with different persons. The waters of the Papago country are not excessively high in mineral content: the average of 21 wells and 2 springs is 442 parts per million total solids. Four other wells have more than 1,000 parts per million total solids. Two of these, the wells at the Black Prince mine and the Sam Clark water shaft, are sunk in rocks adjacent to mineral ground and within the limits of mining claims. High mineral content is not uncommon in such places. Garcia Well, in the San Cristobal Valley, one of the four wells mentioned above, is a dug well which at the time of sampling contained less than a foot of water but was being deepened. It is a common experience that the water first struck in a well is more salty than that lower down. Probably the withdrawal of water from the deepened well will remove the soluble matter immediately adjacent to the well, and travelers will in the future find the water much less salty than is shown in the analysis. Tule Well, on the Camino del Diablo, has water generally regarded as brackish and bad, but it was used extensively by the International Boundary Survey in 1893 and 1894 without known deleterious effects. Prospectors at the present time use the water, and though it is not suitable for persons in delicate health travelers who are sufficiently able-bodied to travel in the region will probably suffer no ill effects. Most wells in the desert are unprotected and are visited only at intervals. Much trash is blown or washed into them, and animals and insects fall in and drown. Many Papago wells are equipped with rawhide buckets for lifting the water. These buckets, made from green hides, are unsanitary. Travelers will occasionally find the water of a well putrid from the decay of material that has fallen in. This decay takes place largely through the work of bacteria in the water, and if the sun has access to the water during part of the day many microscopic plants and animals may live and thrive in it, and the water may be continually foul. Foul waters such as those described do not necessarily contain disease germs. They may produce intestinal disorders, but diseases, such as typhoid and cholera, can be produced only by the characteristic germs. Fortunately, the scant population, the great dryness of the air and soil, and the almost constant sunshine of this region are factors that work constantly to limit the number of disease germs and prevent their introduction into foul waters. Though these waters are suitable habitats for the growth of disease germs and are disagreeable in themselves, travelers will find that they do not necessarily carry disease. Precautions should be taken, however. It will be found convenient to have several receptacles for water and to fill those from which water is used for drinking only from wells that seem to be sanitary. Other receptacles from which water for washing, cooking, and filling the radiator is taken may be filled from charcos, represos, and suspected wells when the necessity arises. The boiling of water intended for drinking is a good precaution easily carried out, because of the large amount of firewood along the roads. Water from all the wells used by the Papagos, except drilled wells maintained by the United States Indian Service, should be boiled. 39 SPRINGS The springs that were studied in this region are of two typesfracture springs, which depend on water derived from rainfall and stored in the fractures that are characteristic of certain types of rocks, and fissure springs, which depend on fissures that penetrate the deeper parts of the earth's crust and allow deep-seated waters to rise to the surface. 40 The small amount of ground water and the excessive losses by evaporation wherever it approaches the surface probably account for the small number of springs and of types. Time did not permit the study of the numerous springs of the Tumacacori Mountains, where above an altitude of 4,500 feet the rainfall is sufficient to produce ground-water conditions quite unlike those of the rest of the area. FRACTURE SPRINGS The characteristics of fracture springs can be best brought out by the description of examples. Dripping Spring (Pl. XXIV, A), is on a hillside in an indentation of the northeast side of the Dripping Spring Mountains, below a small pass in whitish rock which forms the "puerto blanco" (white gateway). The approach to the spring leads through a valley that opens off of the Valley of the Ajo. High palisaded buttes stand in this valley, like guards before the mountains, and between them the road winds southward to the spring. The character of the rocks and the topography make this a picturesque and attractive spot. The water flows from cracks in the rock into a cave with an entrance 6 feet high and 4 feet wide; within the cave widens to 8 feet and is about 5 feet deep. A small concrete dam has been built across the entrance, and from this dam a pipe leads to the foot of the slope. The water is milky and opalescent and has a temperature of 69.5° F. At the end of the pipe line is a small pool fed by it, and a well dug in rock, 54 feet deep and 28 feet to water. The watering Bryan, Kirk, Classification of springs: Jour. Geology, vol. 27, p. 557, 1919. 40 Idem, p. 532. place is covered by mining claims belonging to the owners of the Dripping Spring mine, half a mile southwest. The amount of water is sufficient to supply a few men only. The rocks near the spring are a series of beds of lava and tuffs which have a general dip to the south. The tuffs make white bands on the mountain side, which are in marked contrast to the purple and brown lava, but even the tuffs have in places a reddish-brown color due to a cavernous surface rust of silica about 2 inches thick. Where this crust is broken through the softer rock inside readily weathers and falls out, forming many small caves and reentrants. Dripping Spring emerges from a light-green pitchstone dotted with small round red specks called spherulites. The pitchstone is broken into thin sheets along a system of fractures that strike N. 44° E. and dip 30° S. Along these fractures the pitchstones is altered to a greenish-yellow or pink porcelain-like rock, in which the FIGURE 41.-Diagram showing the relation of fracture systems to Dripping Spring spherulites show plainly. Crossing this system of fractures is a later system that strikes N. 26° W. Along and within these fractures red and white chalcedony has been deposited. Vugs lined with quartz crystals are also present. This alteration was probably accomplished by waters that circulated through the fissures, not only here but at other localities in the vicinity. Most of the concretions and pieces of chalcedony and agate that abound in this vicinity have been weathered from this set of fractures. These waters have long since ceased to flow, and the present spring is due to rain water that collects in the cracked and fractured rocks and comes to the surface by way of the secondary set of vertical fractures mentioned above. At certain places these fractures have been reopened by movement since the water that deposited the quartz and chalcedony ceased to flow. Sufficient rock flour or gouge was produced by the later movement to make these fault fractures, at least locally, efficient dams to the circulation of ground water. The relation of the spring to the two sets of fractures is shown in Figure |