Pyrenees; also with the colored Marls, alternating with layers of Rock salt and Gypsum in the Jurassic rocks of southern France.

Oolitic Dolomite.-Common in the Permian of England.

Compact Dolomite.-This has a conchoidal fracture, resembling limestone of the same name, but is readily distinguished by the tests given above for granular Dolomite.

Sandy Dolomite.-A mixture of Dolomite and Quartz sand. Argillaceous Dolomite contains clay.

Dolomite sand.-Isolated grains of Dolomite.

138. Magnesian Limestone (Dolomies Calcarifères).-Dolomite intimately mixed with limestone. The Rauwacke is a limestone rich in Magnesia; hard; compact; of a gray color, spotted with black. Often it is formed of globular masses (Botryoidal Dolomite). It characterizes the Zechstein of Mansfeld and of the Thuringen district of which the other rocks are Fetid Limestone (Stinkstein); compact or crystalline; greenish or blackish; exhaling a fetid odor when broken. There is a friable, sandy, Magnesian Limestone termed Asche. These are found also in more recent formations in Savoy and Tyrol.

Cellular Dolomite (Cargneule).-A name given in the Alps to Breccias formed of angular fragments of Dolomites or Magnesian limestones.

139. Apatite.-Calcium Phosphate. The rock also contains Calcium chloride; Fluorine or Iodine is sometimes found. It fuses with difficulty; colors the flame bluish-green when moistened with sulphuric acid. Dissolved in sulphuric or hydrochloric acid, it yields a white precipitate with Silver nitrate. Mixed in powder with oxide of Copper and treated with salt of Phosphorus, it colors the flame a bluish purple.

Crystalline form: hexagonal prism terminated by a hexagonal pyramid. Large crystals are found in the feldspathic rocks of the United States. A fibrous variety at Crown Point, Essex county, N. Y., is sufficiently abundant to be worked for fertilizing purposes. It is sometimes found in crystalline rocks.

In the Silurian argillaceous schists of Estremadura, it is found in large masses of a fibrous or compact texture, and of a white or yellowish color. It forms a large proportion of the nodules containing the débris of Ammonites at Vissant (Pas de Calais), and forms a geological bed in the clays of part of the Anglo Parisian basin. It has been found in a large number of deposits where its earthy appearance made it difficult of recognition. It sometimes resembles sandstone or millstone; sometimes resinite or calcareous concretions.

Fig. 28.

140. Guano.-Accumulations of earthy materials containing Ammonia, Phosphoric acid, Uric acid, Lime, etc. This mixture, of a yellowish, or brownish-gray color, fuses to a kind of scoriæ, and finally becomes white and earthy. It is partly soluble in hydrochloric acid. If treated with potash it releases ammonia.

Analysis of the Guano of the Isle of Chincha, according to Nesbit: organic acids with Ammonia 52.52%; Calcium phosphate 19.52%. Phosphoric acid 3.12%; Alkaline salts 3.56; Silica 1.46; water 15.82%.

Guano also contains the excrements of marine animals and birds and their remains. It covers large areas on the coast of Peru and elsewhere, and is more or less rich in Phosphates and Nitrogenized matter, according as the rains have deprived it of its original constituents.

141. Sombrerite.-Hydrated calcium phosphate from the Isle of Sombrero.

142. Anhydrite.-Calcium Sulphate (Ca SO.). Hardness, same as limestone; heated on charcoal it forms Calcium Sulphide. It has three rectangular cleavages one of which has a pearly lustre. Varieties: (1) lamellar; (2) granular. Colors: white, bluish, reddish, and smoky-gray. It forms irregular

5*

masses with Rock salt and Gypsum. Often, without changing form, it is transformed into Gypsum by hydration.

143. Barytine.-Heavy Spar. Barium Sulphate (Ba SO.). Cleavable parallel to the faces of a right prism with a rhombic base 101° 42'. (Fig. 29). Hardness 3.5. Density 4.49.

Varieties Laminated, globuliferous, fibrous, radiated, concretionary. Colors: white, yellowish, reddish-brown, sometimes Fig 29. colorless. Before the blow-pipe it decrepitates and fuses, and, if mixed with silver chloride, colors the flame yellowish-green. Heated on charcoal, it gives Barium sulphide, which is precipitated white with sulphuric acid by a solution of calcium sulphate.

144. Celestine.-Strontium Sulphate. (Sr SO.). Cleavable parallel with the faces of a right prism with a rhombic base of 1030 58'. It has a vitreous, slightly pearly lustre, sometimes a pale blue color, sometimes reddish, or white.

Before the blow-pipe it decrepitates and fuses with silver chloride.

145. Gypsum.-Hydrated Calcium Sulphate, (Ca SO, + 2H2). Slightly soluble in water and acids; has a sparry fracture. Form of cleavage: an oblique prism with a rectangular base.

One of the three cleavages is easy and parallel to the face g1. The two other planes of cleavage at 900 from the first make be

tween them an angle of 11409'. Figures 30, 31, 32, exhibit the ordinary forms of the crystal.

Generally the crystals group themselves in pairs, and the twin crystals exhibit the form of a lance-head (Fig. 33).

Fig. 33.

Gypsum has a hardness of 2. Heated in a closed tube it loses water and becomes opaque and friable. Heated on charcoal or carbonate of soda it is reduced to calcium sulphate which, if wet with acidulated water, blackens lead paper.

Common varieties: Laminated with a pearly lustre; Silky, with a silky lustre; Saccharoidal, resembling marble; Compact. The laminated varieties are often of a pure white, always dull, but possessing a soft translucency which is much prized (alabaster).

Accidental minerals, occurring with Gypsum: Rock salt, Anhydride, Mica, Boracite, Quartz, Pyrites, etc.

It is found with the Mica schists in the Alps; in the Silurian in Canada; in the Permian of Russia; in the Trias of Mansfeld. In Lorraine, the mass of Gypsum, ramifying at its extremity, has bent the stratified rock which envelopes it.

146. Glauberite.-Sodium and Calcium Sulphates. Form: monoclinic prism 83°20'. Density, 2.7. Vitreous lustre. Color: dirty gray. It fuses easily; colors the flame yellow. Treated with water, Sodium sulphate dissolves out, and Calcium sulphate is precipitated.

CHAPTER XII.

ALUMINOUS ROCKS WITH SILICA

147. Alunite. A compact rock, uniform or porphyroidal, sometimes sandy or brecciated, of a gray, yellow, or red color, and formed of the mineral Alunite. This mineral is a double Sulphate of Aluminum and Potassium. It is infusible, but soluble in sulphuric acid; in the open tube it yields water and Ammonium Sulphate, and, at a high temperature, sulphurous oxide and sulphuric acid. When it is calcined it gives, at first, a sulphurous odor, and then an Aluminic taste.

148. Aluminite (Hydrated Aluminum Silicate).-Generally in concretions of a dull white color; soft; earthy, sometimes oolitic; soluble in nitric acid. In the open tube it releases water, and, at a red heat, sulphurous acid. It is found only in nodules or veins in clay.

149. Beauxite. In rounded, granular concretions of a grayish, yellowish, or reddish-white color; distributed through a compact limestone; sometimes in oolitic or earthy masses, generally calcareous. It is an Aluminic and Ferric hydrate of a density of 2.55.