Siliceous Tufa.—Hot water, and especially hot alkaline water, circulating through the earth’s crust, is always charged with silica dissolved out of the rocks; and when such water issues on the surface in a hot spring or geyser, it is cooled by contact with the air, its solvent power is diminished thereby, and a large part of the silica is deposited around the outlet as a snowy-white porous material called _siliceous tufa_. Silica deposited in this way is, like organic silica, always in the form of opal. Siliceous tufa is distinguished from clay, slate, chalk, and limestone by the same tests as tripolite, and from tripolite itself by the absence of microscopic organisms. Novaculite.—Through the action of percolating water and pressure, siliceous tufa, like tripolite, becomes harder and denser and is thus changed to _novaculite_, which holds the same relation to chemically deposited silica that chert and flint do to organically deposited silica. The white novaculite obtained at the Hot Springs of Arkansas, and commonly known as Arkansas stone, is a typical example of this rock. The rock which, on account of the use to which it is put, is known as buhr-stone, is also an excellent example of chemically deposited silica. It is usually somewhat porous and fossiliferous. (4) _Calcareous Group._—These are the lime-rocks, including the carbonate of lime, in limestone and dolomite, the sulphate of lime, in gypsum, and the phosphate of lime, in phosphate rock. These rocks are even more closely connected in origin than in composition; and it is for this reason that rock-salt, which of course contains no lime, is also included in this group. Limestones are formed abundantly in the open sea, through the accumulation of shells and corals; but when portions of the sea become detached from the main body and gradually dry up, like the Dead Sea and Great Salt Lake, dolomite, gypsum, and rock-salt are deposited in succession as chemical precipitates. Phosphate rock may be regarded as a variety of limestone, resulting from the accumulation of the skeletons and excrement of the higher animals.