Dolomite.—If for calcite, which is the sole essential constituent of all limestone, we substitute the allied mineral dolomite, we have the rock dolomite. As might be inferred from its composition, dolomite is very closely related to limestone, although there are some important differences. Physically, the two rocks differ about as the two minerals do. Dolomite is harder than limestone, and being also less soluble, it resists the action of the weather more. Dolomite, if pure, effervesces feebly, or not at all, with cold dilute acid. Here, however, we have to recognize the fact that dolomite is rarely pure; but there exists, in consequence of the admixture of calcite, a perfectly gradual passage from pure dolomite to pure limestone, and parallel with this every degree of vigor in the reaction with acid. Hence, it is entirely an arbitrary matter as to where we shall draw the line between dolomitic limestone and calcareous dolomite. Dolomite is a very much less abundant rock than limestone, and, unlike limestone, it rarely contains many fossils, and is never of organic origin; _i.e._, there are no organisms which secrete the mineral dolomite to form their hard parts or skeletons. Like gypsum and rock-salt, dolomite is probably never deposited in the open ocean, but only in closed basins. Like limestone, it occurs with both the compact and the crystalline textures. Gypsum.—When pure, this rock (specimen 36) is identical with the mineral gypsum (specimen 17), except that it is rarely crystalline. It is usually, however, not only perfectly compact, but more or less dark-colored from the admixture of clay and other impurities. Its most notable characteristics are its softness, the absence of the argillaceous odor, except where it contains much clayey impurity, and its non-effervescence with acids. The first two usually serve to distinguish it from slate, while the acid test separates it readily from limestone and all other carbonate rocks. The deposition of gypsum is purely chemical, and it occurs under about the same physical conditions as the deposition of salt; _i.e._, in drying-up portions of the sea. Hence we usually find gypsum associated with beds of rock-salt; and, since drying-up seas are few in number, and small compared with the whole extent of the ocean, we can easily understand why neither rock-salt nor gypsum are abundant rocks, except in a few localities. Rock-Salt.—This interesting and useful rock, as we have already learned, is deposited in a purely chemical way, and only in drying-up portions of the sea, like the Dead Sea, Great Salt Lake, etc. In some parts of Europe there are beds of solid rock-salt over a hundred feet thick.