vitreous lustre, and is readily recognisable from other similar substances by its great weight; it does not effervesce with acids. Barytes is usually met with in veins traversing rocks, especially in association with metallic ores.

Phosphates. Only one of these requires to be enumerated in the present list of minerals-the phosphate of lime or Apatite.

APATITE (tricalcic phosphate, phosphate of lime, p. 160) crystallises in hexagonal prisms which, as minute colourless

FIG. 63. Group of fluor-spar crystals.

needles, are abundant in many crystalline rocks; it also occurs in large crystals and in amorphous beds associated with gneiss. It is soluble in water containing carbonic acid, ammoniacal salts, common salt, and other salts. Hence its introduction into the soil, and its absorption by plants, as already mentioned (p. 160).

Fluorides. The only member of this family which occurs conspicuously in the mineral kingdom is calcium fluoride or FLUOR-SPAR (Fluorite, CaF2) which, in the form of colourless, but more commonly light green, purple, or yellow cubes, is found in mineral veins not infrequently accompanying lead-ores (Fig. 63).

Chlorides. Reference has already been made to the only chloride which occurs plentifully as a rock-mass, the chloride of sodium, known as Halite or Rock-salt (NaCl, chlorine 60.64, sodium 39.36). It crystallises in cubical forms, and is also found massive in beds that mark the evaporation of former salt-lakes or inland seas (p. 207).

Sulphides. - Many combinations of sulphur with the metals occur, some of them of great commercial value; but the only one that need be mentioned here for its wide diffusion as a rock-constituent is the iron-disulphide (FeS2), in which the elements are combined in the proportion of 46.7 iron and 53.3 sulphur. This substance assumes two crystalline forms: (1) PYRITE which occurs in cubes and other forms of the first or monometric system, of a bronze-yellow colour and metallic lustre, so hard as to strike fire with steel, and giving a brownish-black powder when scratched. This mineral is abundantly diffused in minute grains, strings, veins, concretions (Fig. 64, c), and crystals in many different kinds of rocks; it is usually recognisable by its colour, lustre, and hardness; (2) MARCASITE (white pyrite) crystallises in the tetragonal system, has a paler colour than ordinary pyrite, and is much more liable to decomposition. This form, rather than pyrite, is usually associated with the remains of plants and animals imbedded among rocks. The sulphide has no doubt often been precipitated round decaying organisms by their effect in reducing sulphate of iron. By its ready decomposition, marcasite gives rise to the production of sulphuric acid and the consequent formation of sulphates. One of the most frequent indications of this decomposition is the rise of chalybeate springs (p. 79).

CHAPTER XI.

THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES IN THE EARTH'S CRUST.

FROM the distribution of the more important elements in the earth's crust and the mineral forms which they assume, we have now to advance a stage farther and inquire how the minerals are combined and distributed so as to build up the crust. As a rule, simple minerals do not occur alone in large masses; more usually they are combined in various proportions to form what are known as Rocks. A rock may be defined as a mass of inorganic matter, composed of one or more minerals, having for the most part a variable chemical composition, with no necessarily symmetrical external form, and ranging in cohesion from loose or feebly aggregated debris up to the most solid stone. Blown sand, peat, coal, sandstone, limestone, lava, granite, though so unlike each other, are all included under the general name of Rocks.

In entering upon the study of rocks, it is desirable to be provided with such helps as are needed for determining leading external characters; in particular, a hammer to detach fresh splinters of rock, a pocket-knife for trying the hardness of minerals, a small phial of dilute hydrochloric acid for detecting carbonate of lime, and a pocket lens. The learner, however, must bear in mind that the thorough investigation of rocks is a laborious pursuit, requiring qualifications in chemistry and mineralogy. He must not expect to be able to recognise rocks from description until he has made good progress in the study. He will find much advantage in procuring a set of named specimens, and making himself familiar with such of their characters as he can himself readily observe.

Great light has in recent years been thrown upon the structure and history of rocks by examining them with the microscope. For this purpose, a thin chip or slice of the rock to be studied is ground smooth with emery and water, and after being polished with flour-emery upon plate-glass, the polished side is cemented with Canada balsam to a piece of glass, and the other side is then ground down until the specimen is so thin as to be transparent. Thin sections of rock thus prepared reveal under the microscope the minutest kinds of rock-structure. Not only can the component minerals be detected, but it is often possible to tell the order in which they have appeared, and what has been the probable origin and history of the rock. Some illustrations of this method of investigation will be given in a later part of the present chapter. It will be of advantage to begin by taking note of some of the more important characters of rocks, and of the names which geologists apply to them.

Sedimentary-composed of sediment which may be either a mechanically suspended detritus, such as mud, sand, or gravel; or a chemical precipitate, as rock-salt and calcareous tufa. The various deposits which are accumulated on the floors of lakes, in river-courses, and on the bed of the sea, are examples of sedimentary rocks.

Fragmental, Clastic-composed of fragments derived from some previous rock. All ordinary detritus is of this nature. Concretionary-composed of mineral matter which has been aggregated round some centre so as to form rounded

a, b, “Fairy stones;" c, Pyrite, showing internal radiated structure. or irregularly-shaped lumps. Some minerals, particularly pyrite (Fig. 64, c), marcasite, siderite, and calcite, are frequently found in concretionary forms, especially round some organic relic, such as a shell or plant (Fig. 61). In alluvial clay, calcareous concretions which often take curious imitative shapes, are known as " fairy stones" (Fig. 64, a, b, see p. 235).