Salt. Most of the common salt (the mineral "halite") of commercial value occurs in nature in sea or salt lake water; or in beds or strata of rock salt associated with other strata; or as natural brine in openings or pores in certain rocks. Considerable salt is obtained by evaporation of tidewater, as around San Francisco Bay, and of salt lake water, as at Great Salt Lake, Utah. It has been estimated that the Great Salt Lake, whose area is about 2,000 square miles and greatest depth 50 feet, contains several hundred million tons of common salt. This salt has been washed out of the rocks of the surrounding country and gradually accumulated in the lake because it has no outlet. Most important of all sources of salt is the rock salt which occurs in the form of strata within the earth's crust. Such strata are found in rocks of nearly all ages from the early Paleozoic to the present. They resulted from the evaporation of salt lakes or salty more or less cut-off arms of the sea, after which other strata accumulated on top of them. Thus in the Silurian system of strata underlying all of southwestern New York State there occur almost universally from one to seven beds of salt. The strata including the salt dip gently southward so that at Ithaca, New York, seven salt beds were struck in a well at a depth of about 2,200 feet. Northward the salt comes nearer and nearer the surface. One well penetrated a layer of solid salt 325 feet thick. Some of this salt is being mined much like coal, but most of it is obtained by running water into deep wells to dissolve the salt, the resulting brine being pumped out and evaporated. Under portions of southern Michigan there are both salt beds and natural brines charging certain porous rock layers. Both the salt beds (of Silurian Age) and the brines (of Mississippian Age) supply great quantities of salt from brines pumped out and evaporated. In 1918 the United States produced 51,000,000 barrels (280 lbs. each) of salt. Michigan (17,000,000 barrels) and New York were the leading States, followed by Kansas, Ohio, West Virginia, and California. Some of the uses of common salt are given in the description of halite in the preceding chapter. Gypsum. The composition and properties of this common and useful mineral are given in the chapter on "Mineralogy." Rock gypsum is the variety of great commercial importance. It is widespread, being quarried in many States, and occurs interstratified with rocks of many ages where it has originated by evaporation or partial evaporation of salt water lakes or more or less cut-off arms of the sea. Salt beds are often associated with gypsum.