Page:Popular Science Monthly Volume 13.djvu/265

Rh slate, and shale; but the second of these three members—the Mahoning sandstone—is the principal repository of petroleum in the southwestern district. The "oil-sands" of the western district are also three in number. The first sand of this group yields a heavy lubricating oil, 30° to 35° gravity; the second, an oil about 40° gravity; the third, a light oil, 45° to 50° gravity. This third sand is the most productive, and supplies most of the oil of commerce. The "Warren oil-sand" of the northern district is very irregular in character, and the oil is found at horizons varying from 600 to 800 feet below the "third sand" of the preceding group, whose oil it, moreover, resembles. But at a depth of about 300 feet below the Warren horizon, and in the same northern district, is the Bradford oil-belt of McKean County, Pennsylvania, and Cattaraugus County, New York, the surest and safest oil-territory in all the oil-regions. The oil of the Bradford belt is of the same gravity as "third-sand oil."

The Ancient Beaches of Great Salt Lake.—The mountains round about Great Salt Lake bear plain evidences of the existence at some early period of a much larger lake in the same locality. The sides of these mountains rise, as it were by steps, to the height of 1,000 feet above the surface of the present lake, these steps marking the successive levels of the lake as it shrunk from its primeval dimensions—345 miles long, 135 miles broad—to the size it now possesses. Mr. G. K. Gilbert, of Powell's Survey, has made a very thorough study of these ancient beaches, and publishes an article on the subject in a recent number of the American Journal of Science. This ancient lake has received from geologists the name of Lake Bonneville, and the great problem was, to discover the outlet through which its waters were drained away. To this end it was necessary to find a point where the Bonneville shore-line was interrupted by a pass of which the floor was lower than the shore-line, and which led to a valley not marked by a continuation of the shore-line. These conditions are satisfied at Red Rock Pass, and, in addition, there is a continuous descent from the pass to the Pacific Ocean. All about Cache Valley the Bonneville shore-line has been traced, and it is well marked within a half-mile of the pass. The floor of the pass at the divide is 340 feet below the level of the shore-line, and its form is that of a river-channel. The gentle alluvial slopes from the mountains at the east and west, which appear once to have united at the pass, are divided for several miles by a steep-sided, flat-bottomed, trench-like passage, 1,000 feet broad, and descending northward from the divide. At the divide Marsh Creek enters the old channel from the east, and, turning northward, runs through Marsh Valley to the Portneuf River, a tributary of the Columbia. In Marsh Valley the eye seeks in vain for the familiar shore-lines of the Salt Lake Basin, and the conclusion is irresistible that here the ancient lake outflowed. On the sides of the mountains, from the highest shore-line, known as the 'Bonneville Beach,' down to the level of the modern lake, there is a continuous series of wave-wrought terraces recording the slow recession of the water. As many as twenty-five have been counted on a single slope. Some are strongly marked and others faintly, and some that are conspicuous at one point fail to appear at other points; but there is one that under all circumstances asserts its supremacy and clearly marks the longest lingering of the water—the 'Provo Beach,' which runs about 365 feet below the Bonneville Beach. When the discharge of the lake began, its level was that recorded by the Bonneville Beach. The outflowing stream crossed the unconsolidated gravels that overlay the limestone at Red Rock, and cut them away rapidly. The lake-surface was lowered with comparative rapidity until the limestone was exposed, and thenceforward the process was exceedingly slow. For a long period the water was held at nearly the same level, and the Provo Beach was produced. Then came the drying of the climate, and the outflow ceased; and slowly the lake has since shrunk to its present size.

Discolored Sea-Water.—While engaged in a survey of the Gulf of California—the Mar Vermijo, or Vermilion Sea of the early Spanish navigators—Surgeon T. H. Streets, of the navy, examined some of the water in order to ascertain the cause of the peculiar coloration. This red color occurs in patches,