WEST of the Rocky Mountains, inclosed by regions which drain to the Pacific, is the extensive area which bears the name of the Great Basin, for from it there is no outflow. This basin in form is rudely triangular, the most acute angle pointing southward, and its greatest length is about 880 miles. At the broader end the general elevation of the wide valleys or plains, which intervene between a series of parallel ridges, is about 5000 feet above the sea; at the narrower end the ground descends gradually till it is about on, or even below, the sea-level. Streams empty themselves into inland lakes in different parts of the Basin, the most important of these being familiar to everyone as the Great Salt Lake of Utah. This, however, is only the shrunken representative of a grander predecessor, a mere brine-pan compared with its fresh and far-spread waters. To a height of about 1000 feet above the present surface, the evidence of lacustrine wave-work and lacustrine sedimentation can still be traced, and to the lake thus indicated the American geologists have given the name of Lake Bonneville. This, in general outline, was rudely pear-shaped, but its shore-line was very irregular, a succession of jutting headlands and deep bays; its surface also was broken with islands. Its area measured about 19,750 miles, not much less than that of Lake Huron. This is now a region of arid deserts, spotted here and there with a salt marsh or a lagoon, and diversified by the Great Salt Lake and two others of smaller size. The greatest depth was originally 1050 feet, for the Great Salt Lake does not exceed 50 feet in any part. Then the waters of Lake Bonneville found an outlet at the northern end, not far away from the mouth of the Bear River, which is now the principal affluent of the Salt Lake. The rainfall then in the northern part of the Great Basin must have been much heavier than it is now; as it diminished, Lake Bonneville contracted in size and increased in saltness. The annual rainfall in this district is now only about 7 inches, while over the region between the Appalachians and the Mississippi it is 43 inches. In the latter the average moisture in the air is about 69 per cent. of saturation, in the former it is only 45; while the evaporation from the surface of Lake Michigan is only 22 inches per annum, for the Great Salt Lake it amounts to 80 inches. The level of the water in the latter is subject to oscillations, dependent partly on variations in the rainfall, partly on the results of extended cultivation, and appears likely in the future to fall somewhat below its present height.
By Grove Karl Gilbert. “Monographs of the United States Geological Survey,” Vol. I. (Washington, 1890.)
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