recognise them, he may proceed to the examination of others, of which he will find descriptions in treatises on Mineralogy and in more advanced text-books of Geology.

Felspars. This family of minerals plays an important part in the construction of the earth's crust, for it constitutes the largest part of the crystalline rocks which, like lava, have been erupted from below; is found abundantly in the great series of schists; and by decomposition has given rise to the clays, out of which so many sedimentary rocks have been formed. The felspars are divided into two series, according to crystalline form.

Orthoclase or potashfelspar contains about 16.89 per cent of potash, crystallises in monoclinic or oblique rhombic prisms, but also occurs massive; is white, grey, or pink in colour; has a glassy lustre; can with difficulty be scratched with a knife, but easily with quartz. Associated with quartz, it is an abundant ingredient of many ancient crystalline rocks (granite, felsite, gneiss, etc.) In the form of sanidine it is an essential constituent of many modern volcanic rocks.

FIG. 55. Dendritic markings due to arborescent deposit of earthy manganese oxide.

Plagioclase. Under this name are grouped several species of felspar which, differing much from each other in chemical composition, agree in crystallising in the same type or system, which is that of a triclinic or oblique rhomboidal prism. As abundant ingredients of rocks they commonly appear as clear, colourless, or white glassy strips, on the flat faces of which a fine minute parallel ruling may be detected with the naked eye, or with a lens. This striation or lamellation is a distinctive character, which proves the crystals in which it occurs not to be orthoclase. The plagioclase felspars occur as essential constituents of many volcanic rocks, and also among ancient eruptive masses and schists. Among them are Microcline (a potash-felspar), with 15 per cent of potash ; Albite or Soda-felspar, containing nearly 12 per cent of soda (Fig. 51); Anorthite or Lime-felspar, with 20.10 per cent of lime; Soda-lime felspar, Lime-soda felspar a group of felspars containing variable proportions of soda, lime, and sometimes potash; the chief varieties are Oligoclase (Silica, 6265 per cent), Andesine (Silica, 58-61 per cent), Labradorite (Silica, 50-56 per cent).

Zeolites, a characteristic family of minerals, composed essentially of silicate of alumina and some alkali with water; often

slender needles.

marked by a peculiar pearly lustre, especially on certain planes of cleavage; usually found filling up cavities in rocks where they have been deposited from solution in water. Some of the species commonly crystallise in fine needles or silky tufts. The zeolites have obviously been formed from the decomposition of other minerals, particularly felspars. They are especially abundant in the steam-cells of old lavas in which plagioclase felspars prevail, either lining the walls of the cavities, and shooting out in crystals or fibres towards the centre (Fig. 56), or filling the cavities up entirely.

Mica, a group of minerals (monoclinic) specially distinguished by their ready cleavage into thin, parallel, usually elastic silvery laminæ. They are aluminous silicates with potash (soda), or with magnesia and ferrous oxide, and always with water. They occur as essential constituents of granite, gneiss, and many other eruptive and schistose rocks, also in worn spangles in many sedimentary strata (micaceous sandstone). Among their varieties the two most important are Muscovite (white mica, potash-mica), and Black mica (magnesia-mica, Biotite).

Hornblende or Amphibole, a silicate of magnesia, with lime, iron-oxides, and sometimes alumina, occurs in monoclinic (oblique rhombic) prisms, also columnar, fibrous, and massive. It is divisible into (1) a group of pale-coloured varieties, containing little or no alumina, white or pale green in colour, often fibrous (Tremolite, Actinolite, Asbestus), found more particularly among gneisses, marbles, and associated rocks, and (2) a dark group containing 5 to 18 per cent of alumina, which replaces the other bases; dark green to black in colour, in stout, dumpy prisms (Fig. 57), and in columnar

or

FIG. 57. Hornblende crystal.

bladed aggregates (Common hornblende). Abundant in many eruptive rocks, and also forming almost entire beds of rock among the crystalline schists.

Augite (Pyroxene), in composition resembles hornblende ; indeed, they are only different forms of the same substance, differing slightly in crystalline form, hornblende being the result of slow and augite of rapid crystallisation. Many rocks in which the dark silicate was originally augite have that mineral now replaced by hornblende, as the result of a gradual internal alteration. Like hornblende, augite occurs in two groups: (1) pale non-aluminous, found more especially among gneisses, marbles, and associated rocks; and (2) dark green or black (Fig. 50), occurring abundantly in many eruptive rocks, such as black heavy lavas (basalts, etc.)

Olivine (Peridot) (SiO241.01, MgO 49.16, FIG. 58. Olivine crys- FeO 9.83) occurs in small orthorhombic prisms tal; the light portions and glassy grains in basalts and other lavas; of composed mineral, the a pale yellowish-green or olive-green colour, shaded parts show the whence its name. These grains can often be conversion of the oli- readily detected on the black ground of the vine into serpentine. rock, through which they are abundantly dispersed. Olivine is liable to alteration, and especially to conversion into serpentine by the influence of percolating water (Fig. 58).

Chlorite (SiO2 25-28, Al2O3 19-23, FeO 15-29, MgO 13-25, H2O 9-12) is a dark olive-green hydrated magnesian silicate. It is so soft as to be easily scratched with the nail, and occurs in small six-sided tables, also in various scaly and tufted aggregations diffused through certain rocks. It appears generally to be the result of the alteration of some previous anhydrous magnesian silicate, such as hornblende.

Serpentine (Mg2Si2O7 + 2H2O) is another hydrated magnesian silicate, containing a little protoxide of iron and alumina, usually massive, dark green but often mottled with red. It occurs in thick beds among schists, is often associated with limestones, and may be looked for in all rocks that contain olivine, of the alteration of which it is often the result. In many serpentines, traces of the original olivine crystals can be detected.

CARBONATES.-Though these are abundant in nature, only three of them require notice here as important constituents of the earth's crust, those of lime, magnesia and lime, and iron.

Calcite (calcium-carbonate, carbonate of lime, CaCO3) crystallises in the hexagonal system, and has for its fundamental crystal

FIG. 59. Calcite in the form of "nail-head spar."

line form the rhombohedron, as already mentioned (p. 124). When quite pure it is transparent (Iceland spar, Fig. 45), with the lustre of glass; but more usually is translucent or opaque and white. Its crystals, where the chief axis is shorter than the others, sometimes take the form of flat rhombohedrons (nail-head spar, Fig. 59); where, on the other hand, that axis is elongated, they present pointed pyramids (scalenohedrons, dog-tooth spar, Fig. 60). The mineral occurs also in fibrous, granular, and compact forms. The decomposition of silicates containing lime by permeating water gives rise to calcium-carbonate, which is removed in solution. Being readily soluble in water containing carbonic acid, this carbonate is found in almost all natural waters, by which it is introduced into the cavities of rocks. Some plants and many animals secrete large quantities of carbonate of lime, and their remains are aggregated into beds of limestone, which is a massive and more or less impure form of calcite (pp. 154, 158). Calcite

is easily scratched with a knife, and is characterised by its abundant effervescence when acid is dropped upon it.

A less frequent and stable form of calcium - carbonate is Aragonite which crystallises in orthorhombic forms, but is more usually found in globular, dendritic, coral-like, or other irregular shapes, and is rather harder and heavier than calcite.

Dolomite assumes a rhombohedral crystallisation, and is a compound of 54.4 of magnesium-carbonate, with 45.6 of calciumcarbonate. It is rather harder than calcite, and does not effervesce so freely with acid. It occurs in strings and veins like calcite, but also in massive beds having a prevalent pale yellow or brown colour (owing to hydrated peroxide of iron), a granular and often cavernous texture, and a tendency to crumble down on exposure (p. 155).

Siderite (chalybite, spathic iron, ferrous carbonate, FeCO3), another rhombohedral carbonate, contains 62 per cent of ferrous oxide or protoxide of iron. In its crystalline form it is gray or brown, becoming much darker on exposure as the protoxide passes into peroxide. It also occurs mixed with clay in concretions and beds, frequently associated with remains of plants and animals (Sphærosiderite, Clay-ironstone, Figs. 61, 65).