FLUORITE GROUP-(fluorides) fluorite, yttrocerite, fluocerite, cryolite. HEAVY-SPAR GROUP-(carbonates) witherite, barytocalcite, bromlite, strontianite; (sulphates) barite, celestite.

Gypsum Group—(sulphates) gypsum, selenite, satin-spar, alabaster, glauberite, anhydrite, polyhalite.

HALITE GROUP-(chlorides) halite (rock-salt), sylvite, sal-ammoniac, carnallite; (carbonates) natron, trona, thermonatrite; (nitrates) nitre, nitratine, nitro-calcite, nitro-magnesite; (sulphates) epsomite, löwite, thenardite, mirabilite, kalinite (alum), alumite, apjohnite, halitrichite; (borates) borax, sassolite, boracite, hydro-boracite. ORGANIC SALTS-(oxalates) Whewellite, Humboldtite;. (mellitates) mellite.

The INFLAMMABLES.

SULPHUR GROUP-Sulphur, selen-sulphur.

CARBON GROUP-Diamond.

CARBONACEOUS GROUP-Graphite, anthracite, common coal, jet, lignite, peat.-(Coal in its numerous varieties.)

HYDROCARBONS—(simple) — naphtha, petroleum, maltha, elaterite, asphalt, albertite, ozocerite, hatchetine; (oxygenated) succinite (amber), ambrite, copalite.

MIXED ROCKS.

The "Mixed Rocks" constitute, as has been already stated, the main bulk of the earth's crust. They may consist of two or more mineral ingredients, and are often of very varied and irregular composition. Without rigid adherence to mineralogical exactitude, they may be arranged, according to their predominating or more obvious ingredients, into the following groups. We give such explanations as may enable the nonmineralogical reader to understand their distinctive characteristics.

(Arenaceous or Fragmentary Group.)

Sand is in general a loose aggregation of water-worn particles, arising from the disintegration of pre-existing rocks or other mineral matter. It occurs in many varieties, as quartz-sand, shell-sand, coral-sand, iron-sand. The finely comminuted particles of volcanic matter are spoken of as volcanic sand.

Gravel is the term applied to water-worn fragments of rocks when the particles or pebbles vary from the size of a pea to that of a hen's egg. There are many varieties, according to the nature of the rocks from which these may be derived, as flint-gravel, quartz-gravel, &c.

Shingle is the geological term for water-worn rock-fragments larger and less rounded than those of gravel. Shingle beaches are common on the more exposed portions of sea-coasts.

Rubble is a convenient and expressive term, applicable to accumulations of angular rock-fragments indiscriminately thrown together, and such as may arise from river-floods, ice-drift, or the action of frost on cliffs and precipices.

Boulder is a term applied to the larger water-worn blocks of stone found on the soil or amid the superficial material. They usually owe their origin

to the ice-drifts of the glacial period, but occasionally also to wave-action, as the "Boulder Beach" of Appledore.

Block is the term applied to the more angular masses; hence such phrases as "blocks and boulders," "perched blocks," &c.

Sandstone is simply consolidated sand, the particles having been compacted by pressure, or cemented together by lime, clay, iron-oxide, or other material.

Grit is the term applied to a sand-rock, when the particles are hard and irregular—that is, "sharper" than in ordinary sandstones.

Conglomerates (sometimes termed Pudding-stones) are aggregates of gravel and pebbles of all sizes-in other words, consolidated gravel. According to the size of the fragments, geologists speak of "pebbly conglomerates and "bouldery conglomerates."

Breccias (Ital. breccia, a crumb), are agglutinations of angular fragments, which have not suffered attrition, as in the pebbles of conglomerates.

(Argillaceous or Clayey Group.)

Clay is a fine impalpable sediment from water, and consists wholly, or almost so, of alumino-silicious particles. It is usually tough and plastic, and is of various colours, according to the presence or absence of organic matter and metallic oxides.

Fire-clay is a variety usually obtained from the coal formation, and is so called from its power of resisting the strongest action of heat-a property it acquires from its freedom from alkaline earths, such as soda, potash, and lime.

Fullers' Clay or Earth is a hydrous silicate of alumina, employed, from its absorbent nature, in the scouring or fulling of greasy woollens; hence the name.

Mud is the familiar as well as technical term for the fine impalpable matter worn and borne down by water, and deposited in seas, lakes, and estuaries. It is often a very miscellaneous admixture, partly of mineral and partly of vegetable and animal origin.

Silt is the general term for the miscellaneous matter deposited in lakes, estuaries, bays, river-reaches, and other still waters. It may consist of

intermingled mud, clay, and sand, or of distinct layers of these.

Shale is merely consolidated mud, assuming a structure less or more laminated, and very variable, of course, in composition.

Mudstone is a convenient term employed by geologists to designate an earthy clayey rock, void of shaly lamination, and often of compact and homogeneous texture.

Slate is often applied indiscriminately to all hard, laminated, argillaceous rocks, that can be readily split up; hence slaty sandstone, mica-slate, clayslate, &c. It would be better, however, to restrict the name to the clayslates or roofing-slates.

Claystone, the name applied by the older mineralogists to the softer and earthy varieties of felstone or felsite, and now almost obsolete.

(Calcareous or Lime Group.)

Limestone is the general term for all rocks, the basis of which is carbonate of lime-that is, lime in union with carbonic acid. Calcareous rocks are all less or more acted upon by the ordinary acids, effervescing on the application of these liquids.

Marble is an architectural rather than a geological term, and is applied to the compact, crystalline, mottled, and veined varieties of limestone susceptible of a fine polish.

Chalk is a familiar as well as a technical term for the softer and earthier varieties of limestone. The chalks appear in various colours.

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Calc-tuff and Calc-sinter are precipitates or deposits from calcareous waters, and appear as porous, incrusting, stalactitic, and stalagmitic masses. Marl is a loose application for all friable compounds of lime and clay. The marls of fresh-water lakes are spoken of as clay-marls," "marlclays," and "shell-marls," as one or other ingredient predominates. Gypsum is a sulphate of lime, which when calcined forms the well-known plaster of Paris or stucco. It occurs massive-crystalline, granular, or fibrous, and when crystallised is known as selenite.

Alabaster is the term applied to fine translucent varieties of carbonate of lime and of sulphate of lime, the former being known as calcareous, and the latter as gypseous, alabaster.

Magnesian limestone is a compound of carbonate of magnesia and carbonate of lime; but as many limestones contain a small portion of magnesia, the term is generally restricted to those containing from 18 or 20 per cent and upwards.

Dolomite (after the French geologist Dolomieu) is a granular or crystalline variety of magnesian limestone.

(Silicious or Flinty Group.)

Quartz, properly speaking, is fine silica; rock-crystal is the name given to clear, transparent, crystallised varieties; and coloured varieties are known as amethyst, cairngorm, topaz, &c.

Quartz-rock is massive quartz of various colours, and occurs in veins or stratiform masses.

Quartzite is the term applied to granular varieties, and to sandstones apparently reconverted by heat or chemical change into quartz.

Jasper, Agate, Carnelian, Hornstone, Lydian stone, &c., are compact silicious rocks and minerals of various colours, exhibiting smooth or conchoidal fractures.

Flint is nodules of impure silica of various colours, and usually found in chalk and limestone strata.

Chert is the name given to highly silicious limestones or admixtures of flint and limestone, and occurs in concretions, nodules, and rock-masses. Calcedony, Opal, Silicious-sinter, &c., are silicious minerals, generally produced by infiltration of water holding silica in solution, and appearing as incrustations of greater or less thickness.

(Carbonaceous and Bituminous Group.)

Coal is a well-known substance, and may be briefly described as mineralised vegetable matter, containing more or less of earthy impurities. It occurs in many varieties, as caking or coking coal, splint or slaty coal, cubic or rough coal, cannel-coal, &c., which are all bituminous, giving off smoke and flame in burning; and also as anthracite or stone-coal, which is non-bituminous, and burns without smoke or flame.

Lignite, also known as wood-coal, board-coal, and brown coal, is a variety of recent formation, and in which the woody structure is still apparent. Indeed, the transition from peat to lignite, from lignite to coal, and from coal to anthracite, is often so apparent, that there can be no doubt that they are all merely vegetable masses in different stages of mineralisation.

Jet is a compact, lustrous variety of coal, susceptible of a high polish, and on that account usually worked into personal ornaments.

Graphite (familiarly known as plumbago and black-lead, from its appearance, though entirely devoid of lead) is almost pure carbon, containing only slight traces of iron and earthy impurities.

Bitumen is an inflammable mineral substance (hydrocarbon), found either in a free or in a combined state. As free bitumen, it occurs limpid, as naphtha; liquid, as petroleum or rock-oil; slaggy, as maltha or mineral pitch; and solid, as asphalt. It can be discharged from coals, coaly shales, and other substances, by the application of heat; hence such substances are said to be "bituminous," or more properly "bituminiferous."

(Saline or Salt-like Group.)

Common Salt (chloride of sodium) is found in incrustations in desiccated sea-beaches, and in the sites of dried-up lakes. It occurs abundantly in the solid crust as rock-salt, and is held in solution by all sea-water and brine springs.

Nitrates of Soda and Potash (natron, trona, saltpetre, &c.) occur as incrustations and efflorescences in many plains, marshes, and lakes in hot countries. Such deposits or salinas are often of considerable thickness and extent.

Alum (sulphate of alumina and potash), though chiefly extracted for commercial purposes from certain shales and schists, is also found in nature in the saline or crystallised state.

Borax (borate of soda), another saline product, boracic acid being abundantly discharged by the thermal springs of some volcanic regions.

Borate of Lime, another saline substance occurring in radiated nodules, is a product of salinas, such as those of Bolivia and Peru.

Sulphur is found massive and in crystals in almost all volcanic districts. It is also found largely in combination with many of the earths and metals.

(Simple Minerals and their Rock Compounds.)

Felspar (a chemical admixture of silica, alumina, and potash or soda) is a softer mineral than quartz. The larger and softer crystals occurring in granite are of felspar; they can be scratched by the knife when quartz resists it, and can also be distinguished by the flat glassy aspect of their cleavage.

Compact Felspar, Felstone, or Felsite, is a massive, amorphous, felspathic rock, forming dykes and mountain-masses.

Porphyry and Felspar Porphyry are rocks mainly composed of compact felspar, with interspersed crystals of felspar.

Mica (Lat. mico, I glisten) is a soft, sectile mineral, readily splitting up into thin transparent plates, and is a chemical compound of silica, magnesia, and potash. The glistening scaly crystals in ordinary granites are

mica.

Mica-schist and Mica-slate are schistose or slaty rocks, largely composed of micaceous particles-the former splitting irregularly, the latter with greater flatness and regularity.

Hornblende, Hornblende-rock, Hornblende-schist.-As a mineral, hornblende is of a dark or dark-green colour, with a horny glistening lustre (hence the name), and occurs largely as a constituent of certain greenstones and granites. When massive, it constitutes hornblende-rock; when fissile, hornblende-schist.

Hypersthene is a greenish-black or greenish-grey mineral, having somewhat of a metallic lustre, nearly allied to hornblende, and occurring largely in igneous rocks, or forming independent rock-masses.

Actynolite (Gr. actin, a thorn), another mineral closely allied to hornblende of a glassy lustre, and deriving its name from the thorn-like shape and disposition of its crystals. It occurs massive, as Actynolite-rock-and fissile, as Actynolite-slate.

Augite, a black and harder mineral than hornblende, forming the princi pal constituent of the basalts and clinkstones.

Asbestos or Amianthus, so well known from its fine fibrous texture, may be regarded as a variety of actynolite. It occurs in flexible fibres, in rigid masses, and in tough aggregates known as "mountain wood," "mountain cork, ," "mountain leather," &c., from its resemblances to these substances.

Chlorite (Gr. chloros, greenish-yellow) is a mineral of a greenish hue, and generally of a foliated texture, in which condition it forms the principal ingredient in the rocks known as chlorite-slate and chlorite-schist.

Talc, a whitish-green magnesian mineral, closely allied to and resembling mica. It is transparent in thin plates, but is generally massive, sectile, soft, and non-elastic. It enters largely into the earlier schists, known as talc-schists and talcose-schists.

Steatite, Stea-schist, Soapstone, Potstone. -All rocks containing steatite, which may be regarded as a variety of talc, have a greasy or soapy feel, hence the name, from stear, fat or grease. Some from this feel are termed soapstone; others, from their sectility and power of resisting heat, are known and used as Potstones.

Serpentine, so called from its variegated or mottled hues, like the skin of a serpent, is one of the magnesian rocks, occurring largely in primitive districts, and employed as an ornamental stone.

(Igneous or Pyrogenous Rocks.)

Granite and Syenite.-Ordinary granite is a granular-crystalline compound of quartz, felspar, and mica, and variously coloured from the presence of iron in the felspar, or from the hues of the mica. There are many vari

eties of granite, differing in size of grain, colour, and compactness. When hornblende takes the place of mica, or when present in addition, the rock is usually known as Syenite, from Syene in Upper Egypt, where it was early quarried.

Trap-Rocks (from Swedish trappa, a stair, owing to the step-like or terraciform aspect they give to the hills composed of them) include a great variety of igneous rocks all less crystalline than the granitic, and all more compact and less vesicular than volcanic products. These are the basalts, clinkstones, greenstones, felstones, pitchstones, amygdaloids, tuffs, and ashy agglomerates.

The basalts, clinkstones, and greenstones are generally hard, closegrained, subcrystalline rocks, often assuming columnar and subcolumnar structures. They consist of varying admixtures of felspar, augite, and hornblende. The felstones, amygdaloids, and trap-tuffs are softer and less crystalline rocks-the felstones compact or earthy; the amygdaloids having their vesicular cavities filled with agate, carnelian, calc-spar, &c; and the tufas evidently consolidated ejections of dust and ashes.

The Volcanic Rocks consist of lavas, obsidians, pumice, scoriæ, ashes, lapilli, sulphurous muds, &c., and occur, according to their age, from rocks differing little from greenstones and basalts to loose accumulations of dust and cinders.

The Trachytes are rough-grained (Gr. trachys, rough) subcrystalline varieties of felspathic lava.

The Lavas proper occur in many varieties-porous, vesicular, compact, basaltic, subcrystalline; glassy, as obsidian-and light and cellular, with silky-fibrous texture, as pumice.

Scoria, lapilli, bombs, dust, sand, &c., are the familiar names for the loose and fragmentary ejections.

(The Metallic Group.)

The metals are found either native-that is, in a pure state-or combined with mineral matter in the state of ores. Gold, silver, platinum, copper,