Most clays originate by the weathering of rocks, particularly igneous and metamorphic rocks rich in the mineral feldspar. As a result of the decomposition of the feldspar, much clay is formed, the main substance of which is kaolin. Both feldspar and kaolin are described in the preceding chapter. When the resulting clay rests upon the rock from which it has been derived it is called residual clay. Much of the clay is, however, carried away, mainly by streams, and deposited in lakes or the sea, or on river flood plains. Some clay deposits are of wind-blown origin, and still others are formed by the grinding action of glaciers. Clays are very widespread, and they are directly associated with rocks of all geologic ages.
Among the many important uses of clay are the following: manufacture of common brick, fire brick, pottery, chinaware, porcelain ware, tiles, terra cotta, and Portland cement.
Lime and Cement. Limestone, which is one of the most common and widespread of all stratified rocks, forms the basis for the manufacture of the important substances lime (or “quicklime”) and Portland cement. Lime results when pure limestone (carbonate of lime) is “burned” or heated to a temperature high enough to drive off the carbonic acid gas. The greatest use of lime is for mixing with water and sand to make mortar. A few of its other numerous uses are in plastering; whitewashing; purifying certain steel; in making gas, paper, and soap; and as a fertilizer.
Certain limestones containing clay of the right kind and proportion are called natural cement rocks because, after being “burned,” they develop the property of “setting,” like cement when mixed with water. The “setting” of a cement is due to the fact that certain chemical compounds formed during the heating crystallize when mixed with water, and the hard, tiny interlocking crystals of the newly formed silicate minerals give rigidity to the mass. Of recent years Portland cement has largely superseded the natural rock cements. “Portland cement is the product obtained by burning a finely ground artificial mixture consisting essentially of lime, silica, alumina, and some iron oxide, these substances being present in certain definite proportions.” (Ries.) The necessary ingredients are generally obtained by grinding and burning carefully selected mixtures of limestone in some form, and clay or shale. The great and growing uses of cement need not be detailed here.
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.
For about ten years the average yearly production of gypsum in the United States has been approximately 2,500,000 tons, or about ten times that of the nearest foreign competitor (Canada). New York, Iowa, Michigan, and Ohio are the chief producers. In New York the rock gypsum (usually four to ten feet thick) lies between shale and limestone strata of Silurian age, and it is quarried from the central to the western part of the State. In Michigan the rock gypsum beds, commonly five to twenty feet thick, lie in Mississippian strata in the southern portion of the State. A great bed of exceptionally pure rock gypsum underlies about twenty-five square miles of Webster County, Iowa, in rocks of late Paleozoic Age. The Kansas gypsum deposits extend across the central part of the State in rocks of Permian Age.