The lakes of the Great Basin present even greater diversity than the streams. Some of those situated principally in the mountains are of pure, limpid, wholesome water, supplied by cool, sweet brooks and rills or by the melting of the winter's snows, and overflow throughout the year. These lakes, usually of small size, are similar in all respects to the ordinary lakes of humid lands. In eastern Utah, adjacent to the west base of the Wasatch Mountains, the Provo River and other streams supply Utah Lake, the outlet of which, the Jordan River, empties into Great Salt Lake. Utah Lake is well within the Great Basin, and situated at a low elevation for the region, namely, 4,500 feet, or about 280 feet (in 1873) above the level of lake of brine into which it discharges. This is the largest of the fresh lakes in the valleys of the Great Basin, and owes its existence to the fact that a depression there occurs which is filled to overflowing by the streams from the mountains. Bear Lake, in northeastern Utah, is another exceptional example of a fresh lake of considerable size at a comparatively low altitude, in the same region. On the western rim of the Great Basin, at an elevation of 6,247 feet, and surrounded by the forested peaks of the Sierra Nevada, lies Lake Tahoe, "the gem of the Sierra," a water body of remarkable purity, which discharges through Truckee River into Pyramid and Winnemucca Lakes. These lower lakes, situated in desert valleys at an elevation of 3,780 feet above the sea, are without outlets and alkaline and bitter. The most characteristic lakes of the Great Basin, however, are those that do not overflow, and on account of concentration by evaporation are more or less highly charged with mineral matter in solution. These saline and alkaline lakes may be
divided into two classes, in reference to their duration, but the line of separation is indefinite. Certain of them have maintained their existence for many years, and probably have not been evaporated to dryness for several centuries, and may be classed as perennial lakes; others are evaporated to dryness each year, or during certain exceptionally dry and hot seasons, and may be termed ephemeral lakes. In many instances the beds of the ephemeral lakes are normally in a state of desiccation, and appear as broad, level, mud plains, usually with a white fringe of saline matter. Frequently these mud plains, or playas, as they are termed, are transformed into shallow lakes during a single storm, but the waters are absorbed by the clays beneath or evaporated within a few days or perhaps a few hours after the rain ceases. The largest and most characteristic of the perennial saline water bodies is Great Salt Lake, the counterpart in many ways of the Dead Sea. The streams discharging into this salt sea have the usual purity of river-waters, and carry but a small fraction of one per cent of saline matter in solution. The lake is supplied also in part, but to an unimportant extent, by springs, the most of which are of essentially fresh water. The source of the salts which make the waters of the lake a brine is evidently, therefore, the small percentage of mineral material brought in by the tributary streams. After reaching the lake these fresh waters, in the ordinary meaning of the term, are concentrated by evaporation. This is the explanation of the leading facts in the chemistry of all of the saline and alkaline lakes of the Great Basin region, such as Pyramid, Winnemucca, and Walker Lakes in Nevada, Mono and Owens Lakes in California, and the saline lakes of Mexico.
The volume of a lake without an outlet, or an "inclosed lake," is determined mainly by the ratio of the inflow (including the rain falling directly on its surface and the tribute from springs) and evaporation. Its volume, and consequently its area, fluctuates from season to season, and frequently varies also during periods embracing several years. With variations in volume there are fluctuations in the percentage of saline matter in solution, even if precipitation of
one or more of the contained salts does not take place during the periods of more than usual concentration. In most instances inclosed lakes are concentrated by evaporation in summer seasons, and perhaps become nearly saturated solutions, but are diluted during the rainy winter seasons. Fluctuations in volume, area, depth, salinity, etc., are thus characteristic of inclosed water bodies. They are sensitive to climatic changes which ordinary weather records fail to detect, and are modified in a conspicuous manner when the country about them becomes inhabited and irrigation is practised.
Some of the lakes of the Great Basin are dense brines from which various substances are being precipitated. The economic importance of these natural reservoirs of brine and of various soda salts is great, and will become more and more important as transportation facilities increase. Great Salt Lake, it has been estimated, contains 400,000,000 tons of common salt and 30,000,000 tons of sodium sulphate in solution. During the past ten years about 40,000 tons of common salt have been harvested from it annually. Mono Lake contains some 245,000,000 tons of saline matter in solution, of which about 92,000,000 tons are sodium carbonate and bicarbonate. Owens Lake is similar to Mono Lake in composition, and is now the basis of a large soda industry.
A marked difference between a region which drains to the ocean and one where the streams enter inclosed basins where their waters are evaporated is that in the former the waste from the land carried by the streams as an invisible load in solution or as a visible load consisting mainly of silt and sand in suspension is contributed to the ocean and widely distributed before being deposited—much of the material in solution, in fact, may be said to be a permanent contribution to the salinity of sea-water; but in most instances where streams enter inclosed basins all of the material contributed both in solution and suspension is sooner or later precipitated. The area within an inclosed basin, on which the inflowing streams lay down their loads, is as a rule less extensive than the area that is being denuded to
supply the material. The receiving basins are thus filled in or aggraded, and there is a concentration both of the mechanical wash from the land and of the substances taken in solution by the waters of streams and springs. A marked result of this process of concentration, particularly of the fine waste of the uplands and mountains, is seen in the approximately level floors of inclosed valleys. Throughout the Great Basin the valleys have been filled to a depth in many instances of hundreds of feet. Some of the lower mountain ranges in Utah have been so nearly buried beneath these valley deposits that only their summits, termed lost mountains, appear above the even surface of the desert plains. This débris, deeply filling the valleys referred to, is usually a fine yellowish dust-like material, similar in many ways, and probably in mode of origin, to the loess of China in which geologists have taken much interest. With the concentration and deposition of the fine mechanical wash of the uplands there has also been a concentration of the more soluble saline constituents of rocks, which causes the soils of arid regions to differ in an important way from those of humid lands. The leached and characteristically red-tinted soils of warm humid countries, consisting of the oxidized residue of deeply weathered rocks, are absent from arid regions; in their place we find minutely disintegrated, usually light-coloured, and not chemically impoverished soils. In warm humid regions chemical decay of the rocks is the conspicuous feature; in equally warm arid lands mechanical disintegration is carried to an extreme, without the removal of the more soluble constituents. In fact, concentration of saline matter, notably common salt, sodium sulphate, gypsum, etc., is one of the functions, so to speak, of arid climates, when the requisite evaporation basins are present. Among the important industries of the Great Basin region is the gathering and purifying of the various salts contained in the existing water bodies and in the basins of desiccated lakes.
In addition to the characteristics of the region referred to above, which are mainly the result of climatic conditions, the Great Basin has certain geological features, in
the main, so far as North America is concerned, peculiar to itself. The leading structural features of the rocks, so far as they find expression in the surface relief, is the presence of a large number of extensive faults trending in general about northeast and southwest. These faults are breaks or cracks along which the rocks have been moved up and down. One side of a fault sometimes stands higher than the opposite side, and forms a narrow and frequently high and rugged mountain range. The number of these faults within the Great Basin is as yet unknown, but they certainly number many hundreds. In a cross profile of the region between the Wasatch Mountains on the east and the Sierra Nevada on the west the number of mountain ridges due to faulting is at least a score. The precipitous western border of the Wasatch Mountains is itself a great fault scarp, as is also the eastern border of the Sierra Nevada. The faults that determine the steeper sides of these mountain ranges are not to be considered as single clean-cut gashes, but as irregular and intersecting fractures traversing a narrow belt of country. The faults referred to divide rocks of all ages, and are evidently due to the most recent disturbances that have affected the region. It is not probable that the break in any given instance was formed all at once. Such vast convulsions would be out of harmony with the rules of nature. But rather many small movements and adjustments of pressure have occurred along the same belt of fracture. This conclusion is sustained by the fact that many of the faults have experienced movements in very recent times. In places fault scarps a score or more feet in height cross the alluvial cones at the mouths of the small high-grade valleys in the mountains. These scarps in loose unconsolidated gravel and similar material, even under an arid climate, could not be expected to preserve their freshness for many years. At times the breaks cross the courses of streams and cascades, and rapids are formed by the waters flowing down escarpments thus produced in loose material. One characteristic fault scarp in Inyo Valley, California, is known to have been formed during an earthquake that shook that portion of the country
in 1872. The many small earthquakes that have been felt in the Great Basin region are believed to have been caused by slight movements along the breaks that traverse the region. This and other evidence indicates that the faults to which so much of the characteristic scenery of the Great Basin is due have grown by repeated minor displacements, and that such movements are a common cause of earthquakes.