Rocks which have been principally or entirely formed by chemical processes are, for the most part, of locally restricted extent, and are not at all comparable to the great masses of mechanical and organic sediments. This arises from the fact that the chemical processes occur in a conspicuous way only around the mouths of certain classes of springs (p. 191), and in closed bodies of water without outlet and subject to evaporation.

The chemical precipitates may be classed under the following heads: a, Precipitates of the alkalies and alkaline earths; b, siliceous precipitates; c, ferruginous precipitates.

A. Precipitates Of The Alkalies And Alkaline Earths

Calcareous Tufa or Sinter, Travertine, Stalactite, Onyx Marbles, are all forms of carbonate of lime deposited from solution, either around the vents of springs, or by percolating waters in limestone caverns, or in lakes and streams. These deposits are made of cal-cite (or aragonite), are often very pure, and usually white, and more or less translucent, though they may be stained by other substances dissolved with the lime. In structure they are banded and show rings of growth, which distinguishes them from the organic limestones. The so-called "Mexican onyx" or "onyx marble" is a beautifuly banded travertine derived from ancient spring deposits. Oolite is a limestone composed of minute spherules of carbonate of lime, cemented into a more or less compact mass, somewhat resembling fish-roe, whence is derived the name, meaning "egg rock." The spherules are made up of concentric layers of carbonate of lime, deposited from solution around some nucleus, it may be a particle of sand or dust, or a calcareous fragment. The beach rock of a coral reef (p. 264) is made in this fashion, and calcareous sinter often has a similar structure.

When the spheres are larger, resembling peas in size and shape, the rock is called pisolite.

Gypsum (CaS04.2H20) is deposited from solution in salt lakes find lagoons, in which evaporation balances the influx of water (p. 224). When pure, gypsum is white, but it is often coloured gray, brown, or red, by iron stains, and it may even be black. It forms compact, crystalline, or fibrous beds, looking like limestone, but much softer and not effervescing with acid; portions of the beds may consist of transparent selenite crystals. The mineral sometimes occurs in the form of anhydrite (CaS04), but it is not known under what conditions the anhydrous sulphate has been deposited from solution.

Fig. 147. - Piece of banded travertine polished, natural size.

Rock Salt (NaCl) is precipitated by evaporation from the dense brine of salt lakes and lagoons, following the deposition of gypsum, which explains the very common association of the two rocks in successive beds. The salt may be present only as an ingredient of shale (saline shale), or may form thin layers, indicating brief periods of deposition, followed by freshening of the water. Again, it may occur in enormously thick masses, the result of long-continued precipitation. One such mass, near Berlin, exceeds 4000 feet in thickness. Rock salt is often very pure, and then it is transparent and colourless; but it is frequently stained by iron, or mingled with dust blown into the lake or lagoon which deposited the salt, or mixed with clay and other mechanical sediments.

B. Siliceous Precipitates

These are much less common and extensive than the calcareous, and are formed under exceptional conditions.

Geyserite, or Siliceous Sinter, is deposited in dense and hard masses around the mouths of geysers, partly by the evaporation of the water which holds the silica in solution, and partly by the action of Algae (see p. 192). Large terraces of this rock have been built up by the geysers of the Yellowstone Park. Geyserite also occurs as an uncompacted white powder.

Chert (Flint or Hornstone) forms exceedingly dense and finegrained masses, which the microscope shows to be made up of very minute grains of chalcedony mixed with more or less amorphous silica and crystals of quartz. The mode of origin of these masses is not at all well understood, but is believed to be by precipitation from sea-water.

C. Ferruginous Precipitates

Bog and Lake Iron Ore results from the oxidation and consequent precipitation of iron circulating in solution in the soluble ferrous condition. The deposits often have a concretionary structure, not uncommonly becoming oolitic, and consist of impure limonite, sometimes mingled with siderite.