REFERENCE LIST OF THE MORE COMMON MINERALS.
Actinolite—a magnesium-calcium-iron amphibole (q.v.); commonly bright green to grayish green; crystals usually slender or fibrous.
Agate—a banded or variegated chalcedony (quartz, q.v.).
Alabaster—a fine-grained variety of gypsum (q.v.), either white or delicately colored.
Albite—a soda feldspar (q.v.), an aluminum-sodium silicate; H. 5–6; cleavage perfect in two planes; luster vitreous or pearly white; occasionally bluish gray, reddish, greenish; sometimes opalescent.
Amethyst—a variety of quartz of purple or bluish-violet color, due probably to manganese.
Amphibole—the type of an important group of rock-forming minerals known as the amphibole or hornblende group; a ferromagnesian silicate, monoclinic, H. 5–6; luster vitreous to pearly; fibrous varieties often silky; black, ranging through various shades of green to light colors; embraces the magnesium-calcium varieties, tremolite and nephrite; the magnesium-calcium-iron variety actinolite; the aluminous-magnesium-iron-calcium variety hornblende, and others.
Analcite—analcine, one of the zeolites; a hydrous aluminum-sodium silicate; luster vitreous, colorless, white; occasionally grayish, greenish, yellowish, reddish, transparent to opaque.
Andesine—a plagioclase feldspar (q.v.); a sodium-calcium-aluminum silicate, intermediate in composition between albite and anorthite; H. 5–6; white, gray, grayish, yellowish, flesh red; luster subvitreous, inclining to pearly.
Andalusite—an aluminum silicate; luster vitreous; whitish, rose red, flesh red, variety pearly gray, reddish brown, olive-green; H. 7.5, infusible; impurities sometimes so arranged in the interior as to exhibit a colored, crossed, or tesselated appearance in cross-section (chiastolite).
Anhydrite—a calcium sulphate; H. 3–3.5; luster pearly to vitreous; white, sometimes bluish or reddish; differs from gypsum in absence of water and in its greater hardness.
Anorthite—a plagioclase feldspar (q.v.); a calcium-aluminum silicate; varies much by impurities and admixtures; H. 6–6.5; pearly or vitreous luster; white, grayish, reddish.
Anthracite—hard coal; hydrocarbon with impurities; supposed to be derived from bituminous coal by metamorphism.
Antimony—a native metal, tin-white, brittle; rather rare in native form.
Apatite—essentially calcium phosphate with chlorine or fluorine; hexagonal; H. 5; luster vitreous or subresinous; colors usually greenish to bluish, characterized by a hexagonal form.
Aragonite—a calcium carbonate; differs from calcite in cleavage, and in being orthorhombic; H. 3.5–4; luster vitreous or resinous; white, also gray, yellow, green, and violet.
Asphaltum—asphalt; mineral pitch, bitumen; a natural mixture of different hydrocarbons; odor bituminous; melts at 90 to 100 degrees C.; burns with a bright flame; graduates into mineral tars and through these into petroleum; probably the residue of the latter.
Augite—one of the pyroxenes (q.v.); an aluminum-calcium-magnesium-iron silicate; H. 5–6; monoclinic, crystals usually thick and stout; sometimes lamellar; also granular; black, greenish black, deep green; an important rock-forming mineral.
Beauxite—essentially hydrated alumina; occurs in concretionary grains of clay-like form, whitish to brown; valuable as a source of aluminum.
Beryl—a beryllium-aluminum silicate; hexagonal; prismatic; H. 8; luster vitreous or resinous; marl-green, pale passing into whitish; closely resembles apatite, but distinguished by superior hardness and in composition.
Barite—barites, heavy-spar, barium sulphate; orthorhombic, H. 3–3.5; luster vitreous to resinous, sometimes pearly; white, inclining to yellow, gray, blue, red, or brown; very heavy, sp. sr. 4.3–4.7.
Biotite—black mica, a potash-aluminum-magnesium-iron silicate; monoclinic; easy basal cleavage into thin laminæ; sometimes occurs as a massive aggregation of cleavable scales; H. 2.5–3; luster splendent on cleavage surface; black to dark green; cleavage surfaces smooth and shining; a very common constituent of crystalline rocks.
Bitumen—the same as asphaltum (q.v.).
Bismuth—a metal of whitish color and rather brittle nature; occurring occasionally native, usually as an ore.
Bronzite—a variety of enstatite (q.v.); grayish green to olive-green and brown with luster on cleavage surface often adamantine, pearly or bronze-like and submetallic.
Calcite—calcspar; calcium carbonate; rhombohedral, perfect rhombohedral cleavage; often taking the forms known as dogtooth spar, nail-head spar; frequently stalactitic and stalagmitic; H. 2.5–3.5; luster vitreous; white, occasionally pale shades of gray, red, green, blue, violet, yellow, brown; strong double refraction; embraces variety called Iceland spar; a very common mineral; the essential basis of limestone.
Cassiterite—tin stone; an oxide of tin; tetragonal; luster adamantine, usually splendent; brown or black, sometimes red, gray, white, or yellow; an important source of tin.
Catlinite—essentially a hardened red clay, rather a rock than a mineral; much prized by Indians for pipes.
Chalcedony—a cryptocrystalline variety of quartz having a wax-like luster, either transparent or translucent; white, grayish, pale brown to dark brown, black, sometimes delicate blue, occasionally other shades; frequently occurs as the lining or filling of cavities, taking on a botryoidal or mamillary form.
Chiastolite—andalusite (q.v.).
Chlorite—the type of an important group of secondary minerals usually characterized by a green color, softness and smoothness or unctuousness of feeling; they are usually aluminum-magnesium-iron silicates, with chemically combined water; derived from several other species, as pyroxene, amphibole, biotite, garnet, etc.; embraces a number of species, among which are clinochlore, penninite, prochlorite, and delessite.
Chromite—chromic iron; essentially an iron chromate; isometric; luster submetallic; iron black to brownish black; opaque; sometimes magnetic; resembles magnetite.
Chrysolite—olivine; essentially a magnesium-iron silicate; orthorhombic; H. 6–7; luster vitreous; green, commonly olive-green, sometimes yellow, brownish, grayish green; highly infusible; a common constituent of certain basic igneous rocks; the name olivine is more commonly used by geologists.
Chrysotile—a delicately fibrous variety of serpentine (q.v.).
Corundum—alumina; an oxide of aluminum; H. 9; rhombohedral; large crystals usually rough; luster vitreous; color blue, red, yellow, gray, and nearly white; purer forms of fine colors are sapphires; the red variety is ruby, the yellow, oriental topaz, the green, emerald, and the purple, amethyst; dark colors, with iron oxide, emery.
Delessite—a ferruginous chlorite, usually olive-green or blackish green; occurring commonly in the cavities of amygdaloids.
Diallage—a variety of pyroxene (q.v.); H. 4; characterized by thin foliæ; usually grayish green to grass-green, or deep green; luster on cleavage surface pearly, sometimes metalloid or brassy; an essential mineral in the gabbros, as sometimes defined.
Elæolite—a variety of nephelite (q.v.); occurring in large coarse crystals or massive, with greasy luster, from which the name is derived; a characteristic constituent of elæolite syenite.
Enstatite—one of the pyroxenes; essentially a magnesium silicate; orthorhombic; H. 5.5; luster a little pearly on cleavage surface; metalloidal in the bronze variety (bronzite); grayish white, yellowish white, greenish white to olive-green and brown; very infusible; a common mineral in certain basic crystalline rocks.
Epidote—a complex aluminum-calcium-iron silicate of varying composition; monoclinic; H. 6–7; luster vitreous, pearly, or resinous; color usually pistachio-green, or yellowish green to brownish green; can usually be detected by its peculiar pistachio hue, which is seldom found in other minerals; common in many crystalline rocks, usually as a secondary product.
Feldspar—a group of minerals of the first importance in rock formation, embracing orthoclase, microcline, albite, oligoclase, andesine, labradorite, anorthite, and numerous variations; aluminum silicates, with either potassium, sodium, or calcium or two or more of these; crystallizes in both the monoclinic and triclinic systems; possesses very distinct cleavage in two directions; H. 6–6.5; range in color from white through pale yellow, red, or green, and occasionally dark; triclinic feldspars frequently called plagioclase (see individual feldspars).
Fluorite—fluorspar; calcium fluoride; isometric, usually cubic; H. 4; luster vitreous, sometimes splendent; white, yellow, green, rose, crimson red, violet, sky-blue, and brown; yellow, greenish, and violet most common; occurs usually in veins or cavities in beautiful crystalline form.
Galenite—galena; lead sulphide; isometric, usually cubic; perfect cubic cleavage; luster metallic; lead-gray; a common ore of lead; occurs in veins and layers, also as linings of cavities.
Garnet—a complex silicate of varying composition, embracing aluminum, calcium, magnesium, chromium, iron, and manganese, but usually only two or three of these are present in abundance, and the varieties are characterized by the leading constituent; isometric, usually in dodecahedrons or trapezohedrons; H. 6.5–7.5; luster vitreous to resinous; commonly red or brown, sometimes yellow, white to blue, green or black; common in mica schist, gneiss, hornblende schist; also in granite, syenite, and metamorphosed limestone.
Geyserite—a concretionary deposit of silica in the opal condition; formed about geysers; white or grayish.
Glauconite—green-sand, a hydrous potassium-iron silicate usually impure, amorphous, or earthy; dull olive-green or blackish, yellowish, or grayish green; opaque, commonly occurs as grains or small aggregations.
Graphite—plumbago, black lead; a form of carbon, usually impure; rhombohedral, but rarely appearing as a crystal; more often as thin laminæ of greasy feel; yields a black adhesive powder; hence its common use for lead pencils; occurs in granite, gneiss, mica schist, crystalline limestone; sometimes results from alteration of coal by heat; occasionally occurs in basaltic rocks and meteorites.
Gypsum—a hydrous calcium sulphate; monoclinic; perfect cleavage into smooth polished plates; occurs in a variety of forms, including fibrous and granular; H. 1.5–2; luster pearly and shiny; white, sometimes gray, flesh-red, yellowish, and blue; impure varieties dark; crystallized varieties include selenite, satinspar, alabaster, etc.; easily recognized by its softness and want of effervescence with acids; occurs in beds; calcined and ground constitutes plaster of Paris.
Haüynite—a complex sodium-aluminum silicate and calcium sulphate; crystals dodecahedrons; luster vitreous or somewhat greasy; bright blue, sky-blue, or greenish blue, or green; occurs in certain igneous rocks, commonly associated with nephelite and leucite.
Hematite—ferric oxide, Fe2O3, iron-sesquioxide; rhombohedral, more commonly columnar, granular, botryoidal, or stalactitic; luster metallic, sometimes earthy; iron-black, dark steel-gray, red when earthy; gives red streak or powder; a leading iron ore, 70 percent. metallic iron when pure; the chief source of the red color of soils and rocks generally.
Hornblende—an amphibole; name sometimes used as a synonym for amphibole; sometimes to designate a variety under amphibole (q.v.).
Hyalite—a variety of silica in the opal condition; clear and colorless like glass, consisting of globular concretions or crusts.
Hypersthene—one of the pyroxenes; a ferromagnesian silicate; orthorhombic; H. 5–6; luster somewhat pearly on cleavage; surface often iridescent; dark brownish green, grayish, or greenish black and brown; a frequent constituent of crystalline rocks.
Iceland spar—a form of transparent calcite (q.v.).
Ilmenite—menaccanite; a titanium iron oxide; rhombohedral; resembles hematite; luster submetallic; iron-black; powder black or brownish red; occurs frequently in crystalline rocks associated with magnetite.
Iron pyrites—pyrite (q.v.).
Kaolin—kaolinite; essentially a hydrous aluminum silicate; usually in clay-like or earthy form; white or grayish white; often tinged with impurities; commonly arises from decomposition of aluminous silicates, especially the feldspars; basis of pottery and china.
Labradorite—a plagioclase feldspar; essentially an aluminum-calcium-sodium silicate; composition intermediate between that of albite and anorthite; triclinic; H. 6; luster pearly or vitreous, gray, brown, or greenish; sometimes colorless or white; frequently shows play of colors; important constituent of various crystalline rocks, especially of the basic class; usually associated with a pyroxene or amphibole.
Lepidolite—lithia mica; essentially like muscovite (q.v.) except that potash is replaced by lithia.
Leucite—essentially an aluminum-potassium silicate, allied to the feldspars; H. 5–6; luster vitreous, white, ash-gray, or smoke-gray; occurs in certain volcanic rocks, particularly lavas of Vesuvius.
Limonite—brown hematite, ocher;—a hydrous iron oxide; commonly earthy; also concretionary, stalactitic, botryoidal, and mamillary, with fibrous structure; H. 5–5.5; luster silky, sometimes submetallic, but commonly dull and earthy; brown, ocherous yellow; streak and powder yellowish brown; constitutes ocher, bog-ore, ironstone, etc.; is the chief source of the yellow color of soils and rocks; arises from the alteration of other iron ores.
Magnesite—magnesium carbonate; rhombohedral; white, yellowish, grayish white to brown; fibrous, earthy, or massive; found in altered magnesium rocks.
Magnetite—magnetic iron ore; iron oxide, Fe3O4; octahedral or dodecahedral; strongly magnetic; H. 5.5–6.5; abounds in igneous and metamorphic rocks.
Marcasite—white iron pyrites; iron sulphide; same composition as pyrite, which it closely resembles; H. 6–6.5; luster metallic, pale gray, bronze, or yellow; prone to decomposition; disseminated through various rocks, particularly plastic clays containing organic matter.
Martite—iron sesquioxide; originally magnetite, which by oxidation has assumed the composition of hematite.
Mica—the type of an important group of rock-forming minerals well known for their perfect cleavage into thin elastic laminæ; among the leading varieties are the common potassium mica (muscovite), the sodium mica (paragonite), the lithium mica (lepidolite), the magnesium-iron mica (biotite), the magnesium mica (phlogopite), and the iron-potash mica (lepidomelane).
Menaccanite—ilmenite; titanium iron ore (q.v.).
Microcline—a triclinic feldspar, closely resembling orthoclase in appearance and having the same composition.
Muscovite—common or potash mica; essentially an aluminum-potassium silicate; H. 2–2.5; monoclinic; remarkable for its basal cleavage; splits easily into exceedingly thin, flexible, elastic laminæ; luster vitreous, more or less pearly or silky; colorless or variously tinged brown, green, or violet; a common mineral in crystalline rocks, particularly in the granites or gneisses.
Nephelite—nepheline; essentially an aluminum-sodium silicate with potash; allied to the soda-feldspars; hexagonal; usually in thick prisms; H. 5.5–6; luster vitreous to greasy, white or yellowish, varying to greenish, bluish, and red; occurs in volcanic rocks; the variety elæolite characterizes the elæolite syenite.
Nosite—nosean; a complex sodium-aluminum silicate and sulphate, like haüynite, but with little calcium; common in phonolites.
Oligoclase—a plagioclase feldspar; essentially an aluminum-calcium-sodium silicate which may be regarded as a mixture of albite and a small amount of anorthite; triclinic; luster vitreous, pearly, or waxy; whitish grading into greenish and reddish; H. 6–7; common in crystalline rocks.
Orthoclase—a potash feldspar; essentially a potassium-aluminum silicate; varying by the replacement of the potassium by sodium and less frequently by other substitutions; monoclinic; occurring in distinct crystals and also in cryptocrystalline forms; cleavage planes perfect with pearly luster on cleavage surface; white, gray, and flesh-red, occasionally varying to greenish white and bright green; H. 6–6.5; difficultly fusible; sanidine a glassy variety; felsite a cryptocrystalline form; a very common mineral, especially in the granites and gneisses.
Olivine—chrysolite (q.v.).
Omphacite—a variety of pyroxene of grass-green color and silky to fibrous luster; allied to diallage.
Opal—silica with a varying amount of water; differs from quartz in a lack of crystallization and in lower degree of hardness; amorphous, massive; sometimes reniform, stalactitic, or tuberous; also earthy; H. 5.5–6.5; luster vitreous, inclining to resinous; white, yellow, red, brown, green, gray, blue, generally pale; colors arise from admixtures; sometimes play of colors as in precious opal.
Ozocerite—a native paraffine, mineral wax.
Petroleum—naphtha; a native mineral oil; a hydrocarbon, commonly believed to arise from organic matter, both animal and vegetable, but held by some to be due to deep-seated chemical and thermal action.
Pictotite—a variety of spinel, containing chromium.
Pisolite—a concretionary variety of calcite.
Picrolite—a variety of serpentine.
Piedmontite—a manganese epidote.
Plagioclase—a general term embracing the triclinic feldspars whose two cleavages are oblique to each other; embracing albite, oligoclase, andesine, labradorite, and anorthite (q.v.).
Plumbago—graphite (q.v.).
Psilomelane—essentially a hydrous manganese oxide occurring in massive, botryoidal, reniform, and stalactitic forms; luster submetallic; iron-black, passing into dark steel-gray; H. 5–6; the common ore of manganese.
Pseudomorph—a false form, i.e., having the form of one mineral and the composition of another; usually arises from the replacement of a mineral, particle by particle, by a solution of another substance, leaving the original form unchanged.
Pyrite—iron pyrites, fool’s gold, iron sulphide; isometric; commonly in cubes; H. 6–6.5; luster metallic, splendent, or glistening; pale brass-yellow; occurs widely disseminated throughout a large class of rocks; usually harder and lighter in color than copper pyrites, and deeper in color than marcasite, which has the same composition.
Pyroxene—the type of a large and important group of rock-forming ferromagnesian minerals; varies in composition and embraces a large number of varieties; usually a magnesium-iron-calcium silicate; crystals usually thick and stout, but varying greatly; sometimes lamellar and fibrous; H. 5–6; luster vitreous inclining to resinous; green of various shades verging towards light colors, occasionally more often to browns and blacks; among the minerals belonging to the pyroxene group are augite, bronzite, diallage, diopside, enstatite, hypersthene, and others.
Quartz—crystallized silica; rhombohedral; crystals commonly six-sided prisms capped by six-sided pyramids; without cleavage; H. 7; scratches glass; usually transparent, glassy, colorless when pure, shaded by impurities to yellow, red, brown, green, blue, and black; varieties, amethyst, purple, or violet; false topaz, yellow, rose-quartz, smoky, milky, cat’s eye, opalescent; aventurine, spangled with scales of mica; chalcedony is a cryptocrystalline variety; carnelian, a red chalcedony; chrysoprase, an apple-green chalcedony; prase, a leek-green variety; agate, a variegated or banded chalcedony; moss-agate, a chalcedony containing moss-like or dendritic crystallizations of iron or manganese oxide; onyx, a chalcedony in layers; sardonyx, like onyx in structure, but includes layers of sard (carnelian); jasper, an opaque-colored quartz, usually red or brown; flint, an opaque impure chalcedony; chert, an ill-defined term applied to an impure flinty rock; hornstone, a translucent, brittle, flinty rock.
Rutile—titanium oxide; tetragonal, crystals commonly in prisms; H. 6–6.5; luster metallic, adamantine; reddish brown, passing to red; sometimes yellowish, bluish, violet, and black; occurs in crystalline rocks and is a common secondary product in the form of microlites.
Sanidine—a glassy variety of orthoclase feldspar.
Satinspar—a variety of selenite or gypsum.
Selenite—a distinctly crystallized transparent form of gypsum.
Serpentine—a hydrous magnesium silicate; usually in pseudomorph forms; also fibrous, granular, cryptocrystalline, and amorphous; H. 2.5–4; luster subresinous to greasy, pearly or earthy, resinous or wax-like; feel, smooth and somewhat greasy; leek-green to blackish green and siskin green verging into brownish and other colors; apparently derived most commonly from chrysolite or olivine and also from other magnesian minerals; sometimes constitutes the bulk of rock masses.
Siderite—iron carbonate; rhombohedral; H. 3.5–4.5; luster vitreous, more or less pearly, ash-gray, yellowish or greenish, also brownish; occurs as extensive iron deposits and in crystalline rocks.
Smaragdite—a form of amphibole or hornblende (q.v.).
Spherosiderite—a globular form of siderite.
Spinel—a magnesium-aluminum oxide; crystals, octahedrons; red of various shades, passing into other colors; spinel-ruby is a variety.
Staurolite—a complex hydrous iron-magnesium-aluminum silicate; orthorhombic; disposed to cruciform shapes; occurs in schists and gneisses.
Steatite—soapstone, a variety of talc (q.v.); a hydrous magnesium silicate.
Sulphur—a well-known element occurring native in volcanic regions; also formed by the decomposition of sulphides, particularly pyrites.
Talc—a hydrous magnesium silicate; usually in foliæ; granular or fibrous forms; also compact; easy cleavage into thin flexible laminaæ, but not elastic; feel greasy; luster pearly on cleavage surface; apple-green to silvery white; H. 1–2; a secondary product from the alteration of magnesian minerals; distinguished by its soft, soapy feel, soapstone being one variety; whitish form is known as French chalk.
Titanite—calcium-titano-silicate; monoclinic; luster adamantine to resinous; brown, gray, yellow, green, and black; H. 5–5.5; occurs in various crystalline rocks.
Topaz—an aluminum silicate, with part of the oxygen replaced by fluorine; orthorhombic; H. 8; luster vitreous; colorless, straw-yellow verging to various pale shades, grayish, greenish, bluish, and reddish; distinguished by its hardness and infusibility; occurs in crystalline rock.
Tremolite—a calcium-magnesium amphibole; a common constituent of certain crystalline rocks.
Viridite—a general term used for green products of rock alteration, usually hydrous silicates of iron and magnesia; mainly chlorite.
Wad—bog manganese; a variety of psilomelane (q.v.).
Zeolite—a group of minerals derived from the alteration of various aluminous silicates.
Zircon—zirconium silicate; H. 7.5; luster adamantine; pale yellowish, grayish, yellowish green, brownish yellow, and reddish brown; infusible; occurs characteristically in square prismatic forms; found in crystalline rocks and granular limestone.
REFERENCE LIST OF THE MORE COMMON ROCKS.[206]
Adobe—a fine silty or loamy deposit formed by gentle wash from slopes and subsequent lodgment on flats; especially applied to silty accumulations in the basins and on the plains of the western dry region.
Agglomerate—an aggregate of irregular, angular, or subangular blocks of varying sizes, usually of volcanic origin, distinguished from conglomerate in which the constituents are rounded.
Alluvium—sediment deposited by streams.
Amygdaloid—a vesicular igneous rock whose cavities have become filled with minerals; the fillings are called amygdules, because sometimes almond-like in form.
Andesite—an aphanitic igneous rock consisting essentially of the plagioclase feldspar andesine (sometimes oligoclase) and pyroxene (or some related ferromagnesian mineral); sometimes cellular, porphyritic, or even glassy; usually rich in feldspar microlites.
Anorthosite—a rock consisting mainly of the feldspar labradorite.
Aphanite—a rock whose constituents are so minute as to be indistinguishable to the naked eye; rather a condition of various rocks than of any specific rock.
Aqueous rocks—a general term applied to rocks deposited through the agency of water.
Arenaceous rocks—either those which are mainly sand or those in which sand is a notable accessory.
Argillite—a clayey rock; usually applied to hard varieties only.
Arkose—a sand or sandstone formed of disaggregated granite or similar rock in which a notable part of the grains are feldspar or other silicate; sand when undefined, is understood to be quartzose.
Augitite—a rock mainly made up of augite.
Basalt—a dark, compact basic igneous rock consisting of a mass of minute crystals sometimes with more or less glassy base, often containing also visible crystals; composed of plagioclase and pyroxene, with olivine, magnetite, or titaniferous iron as common accessories; a basic lava in which the crystallization has taken place rapidly; usually rich in crystallites or microlites; graduates into dolerite and basic andesite.
Bituminous coal—common soft coal, intermediate between lignite and anthracite; contains much bituminous matter, i.e., hydrocarbons.
Bowlders—rounded masses of rock, particularly those that have been shaped by glaciers.
Breccia—a rock composed of angular fragments, contrasted with pudding-stone or conglomerate, in which the fragments are rounded.
Buhrstone—a compact, flint-like silicious rock full of small cavities, so named from use as millstones.
Calc-sinter (calcareous tufa)—a loose cellular deposit of calcium carbonate made by springs; travertine is the better term, as tufa should be left for volcanic elastics.
Cannel coal—a very fine-grained homogeneous bituminous coal, giving off much gas and burning with a candle-like flame.
Chalk—a fine-grained soft rock composed essentially of calcium carbonate derived from minute marine organisms.
Chlorite schist—a schistose rock in which chlorite is a predominant mineral; usually greenish, whence the name.
Clastic rock—formed from the débris of broken-down rocks; the same as fragmental or detrital rock.
Clay—a term commonly applied to any soft, unctuous, adhesive deposit, but in strict use confined to material composed of aluminum silicate; many so-called clays are chiefly silicious silts or loams.
Clay ironstone—a clayey rock heavily charged with iron oxide, usually limonite; commonly in concretionary form.
Clinkstone—a name applied to phonolite because of its metallic clinking sound when struck; composed of orthoclase, with nephelite and one or more of the ferromagnesian minerals as accessories.
Chert—an impure flint, usually of light color, occurring abundantly in concretionary form as nodules in certain limestones.
Coal—a carbonaceous deposit formed from the remains of plants by partial decomposition.
Concretions—aggregates of rounded outlines formed about a nucleus; the material is various: clay, iron ore, calcite, silica, etc.
Conglomerate (pudding-stone)—a rock formed from rounded pebbles, consolidated gravel.
Coquina—a rock formed almost wholly of small and broken shells; especially applied to a shell limestone of Florida.
Dacite (quartz-andesite)—an andesite (q.v.) with quartz.
Diabase—a dolerite (q.v.) which has undergone alteration; consists essentially of plagioclase feldspar and augite, with magnetite or titaniferous iron as a common accessory; one of the greenstones.
Diatom ooze—a soft silicious deposit found on the bottom of the deep sea, made largely or partly of the shells of diatoms; similar deposits are formed from the shells of radiolaria.
Diorite—an igneous rock usually of dark-greenish color, consisting of plagioclase feldspar and hornblende; often speckled from the commingling of light feldspar and dark hornblende.
Dolerite—a fine-grained igneous rock composed of plagioclase feldspar (labradorite or anorthite) and augite (or related ferromagnesian mineral, as enstatite, olivine, or biotite), with magnetic or titaniferous iron as common accessories; crystals usually of medium size, assuming the ophitic structure; embraces many of the greenstones; graduates into basalt on the one hand and gabbro on the other.
Dolomite—a magnesian limestone.
Drift—in common American usage, a mixture of clay, sand, gravel, and bowlders formed by glacial agencies.
Eolian rocks—deposits formed by wind, embracing especially dunes and one variety of loess.
Felsite (felstone)—a light-colored aphanitic rock composed of feldspar often with quartz, in which the crystallization is very imperfect or obscure, giving a close-grained texture with conchoidal fracture and flinty aspect; certain varieties are called petrosilex and hälleflinta.
Flint—a compact dark chalcedonic or lithoid form of quartz.
Freestone—a sandstone of uniform grain without special tendency to split in any direction.
Fulgurites—glassy tubes, produced through fusion by lightning in penetrating sand, earth, or rock.
Gabbro (euphotide)—a crystalline rock composed of the plagioclase feldspar, labradorite (or anorthite), and diallage (or a related ferromagnesian mineral), with magnetite or titaniferous iron as a common accessory.
Gangue—a term applied to the crystalline material in which ores are imbedded.
Gannister—essentially a quartz silt or pulverized quartz used for lining iron furnaces.
Garnetite—a rock composed largely of garnets.
Geest—residual earth or clay left by the decomposition of rocks, especially limestones.
Geyserite—the silicious sinter deposited about hot springs.
Globulites—minute spherical bodies embraced in volcanic glass.
Gneiss—a foliated granite, consisting typically of quartz, feldspar, and mica; the feldspar typically orthoclase.
Granite—a granular crystalline aggregate of quartz, feldspar, and mica; the feldspar typically orthoclase; popularly and properly used for any distinctly granular crystalline rock.
Granitell—a name used to designate a quartz-feldspar rock.
Granitite—a biotite granite with quartz.
Granulite—a fine-grained granite with little or no mica.
Greensand—a sand or sandstone containing a notable percentage of grains of glauconite.
Greenstone—a comprehensive term used to designate igneous and metamorphic crystalline rocks of greenish hue and of intricate and often minute crystallization; they are mostly dolerites, diabases, and diorites; a convenient term for field use where the constituents cannot be determined, and for general use when the variety is unimportant.
Greisen—an aggregate of quartz and mica, i.e., a granite without feldspar.
Greywacke—a sand rock in which the grains are basic silicates instead of quartz.
Hälleflinta—a compact flint-like felsitic rock.
Hornblendite—a rock essentially composed of hornblende.
Hornstone—a very compact, silicious rock of horn-like texture, allied to flint; term also applied to flinty forms of felsite.
Hypogene rocks—those formed deep within the earth under the influence of heat and pressure.
Ironstone—a rock composed largely of iron, usually applied to clayey rocks having a large iron content.
Infusorial earth (tripolite)—an earthy or silt deposit consisting chiefly of the silicious shells of diatoms.
Itacolumite—a flexible sandstone whose pliability is due to an open arrangement of sand grains which are held together by scales of mica.
Jasper—a reddish variety of chalcedonic quartz.
Keratophyre—a felsite with a large percentage of soda.
Kersantite—a mica dolerite consisting chiefly of plagioclase, augite, and biotite.
Lapilli—small fragments of lava ejected from volcanoes; volcanic cinders.
Laterite—a red, porous, ferruginous residual earth of India and other tropical countries.
Lava—a molten rock, especially applied to flows upon the surface, whether from vents or from fissures; also applied to the solidified product.
Lignite (brown coal)—a soft, brown, impure coal.
Limburgite—a compact basic igneous rock of the basaltic class, composed essentially of augite and olivine, with magnetite iron and apatite as common accessories.
Limestone—a rock composed primarily of calcium carbonate, though magnesium sometimes replaces a part of the calcium. (See dolomite and marble.)
Liparite (rhyolite)—an acidic igneous rock of aphanitic or glassy texture, characterized by flowage lines and various microscopic crystals; rhyolite is the more common American name.
Loess—a very fine porous silicious silt containing some calcareous material which often collects in nodules (Löss Kindchen) or in vertical tubules; characterized by a peculiar competency to stand in vertical walls; held by some to be eolian, by others to be fluvial or lacustrine, and by still others to be partly eolian and partly aqueous.
Marble—typically a granular crystalline limestone or dolomite produced by metamorphic action; but the term is variously applied to calcareous and even to other rocks that are colored ornamentally and susceptible of polish.
Marl—an earth formed largely of calcium carbonate, usually derived from the disintegration of shells; or the calcareous accretions of plants, notably the stoneworts; term also sometimes applied to glauconitic and other fertilizing earths.
Melaphyre—a term of varying usage; most commonly applied perhaps to an altered basalt (q.v.), especially an olivine-bearing variety.
Meta-diabase—a term sometimes used for a metamorphic diabase; in like manner meta is prefixed to dolerite, syenite, etc.; not in general use.
Meta-igneous rock—a metamorphosed igneous rock.
Metamorphic rock—a rock which has been altered, particularly one which has been rendered crystalline, or recrystallized by heat and pressure.
Meta-sedimentary rock—a metamorphosed sedimentary rock.
Microgranite—a very fine-grained granite.
Microlites—incipient crystals found in glassy lavas; usually needle-shaped, or rod-like; occurring singly and in aggregates.
Millstone—see buhrstone.
Minette (mica-syenite)—a rock consisting essentially of orthoclase and mica, or a syenite in which mica replaces hornblende or predominates over it.
Monzonite—a granitic rock composed of orthoclase and plagioclase in nearly equal proportions, with ferromagnesian minerals; a rock intermediate between syenite and diorite.
Mudstone—solidified mud or silt, shale.
Nephelinite—a rock composed essentially of nepheline and augite, with magnetite and other accessories.
Nevadite—a variety of rhyolite of granitoid aspect due to an abundance of porphyritic crystals.
Nodules—concretionary aggregations of rounded form.
Norite—a fine-grained rock consisting of plagioclase and hypersthene.
Novaculite (honestone, oilstone)—a very fine-grained, hard sandstone or silt-stone, used for whetstones.
Obsidian—a typical form of volcanic glass usually of the acidic class.
Onyx—a variety of chalcedonic quartz having colored bands alternating with white; the “Mexican onyx” is a crystalline calcium carbonate, variegated with delicate colors due to iron and manganese.
Oolite—a limestone or dolomite composed of small concretions resembling the roe of fish.
Ooze—an exceedingly soft watery deposit of the deep sea; characterized usually by microscopic shells from which it is mainly derived, as diatom ooze, globigerina ooze, etc.
Orthophyre (orthoclase porphyry)—a rock consisting of crystals of orthoclase in an aphanitic base.
Peastone (pisolite)—a very coarse variety of oolite.
Peat—the dark brown or black residuum arising from the partial decomposition of mosses and vegetable tissue in marshes and wet places.
Pegmatite—a term of ill-defined usage applied to rocks whose grain varies from coarser to finer, and often takes on peculiar aspects due to the simultaneous crystallization and mutual intergrowths of the crystals; graphic granite is a distinct type of pegmatite in which quartz and orthoclase crystals grew together along parallel axes so that cross-sections give figures resembling certain Semitic letters ([Fig. 345]).
Peridotite—a very basic igneous rock composed chiefly of olivine with augite or related ferromagnesian minerals, with magnetite and chromite as accessories.
Pelites—a general term embracing clay rocks.
Perlite (pearlstone)—a form of glassy lava made up in part of small spheroids formed of concentric layers which have a lustrous appearance like pearls.
Petrosilex—an old name for felsite or hälleflinta.
Phonolite (nephelite-trachyte, clinkstone)—a compact resonant igneous rock formed of sanidine and nephelite with accessories.
Phyllite (argillite)—a variety of indurated, partly metamorphosed, clay silt in which finely disseminated micaceous scales are abundant and lustrous; intermediate between typical clay slate and mica-schist.
Pitchstone—a dark vitreous, acid, igneous rock of less perfect glassy texture than obsidian and more resinous and pitch-like.
Plutonic rocks—igneous rocks formed deep within the earth under the influence of high heat and pressure; hypogene rocks; distinguished from eruptive rocks formed at the surface.
Porphyrite—a term sometimes used for an altered form of andesite, usually porphyritic in structure.
Porphyry—a rock consisting of distinct crystals embedded in an aphanitic ground-mass.
Propylite—an altered form of andesite and similar igneous rocks.
Protogine—a hydrated micaceous or chloritic variety of granite or gneiss.
Pumice—a glassy form of lava rendered very vesicular through inflation by steam.
Pyroclastic rocks—fragmental or clastic rocks produced through igneous agencies, embracing volcanic ashes, tuffs, agglomerates, etc.
Pyroxenite—an igneous rock consisting essentially of pyroxene.
Quartzite—a rock consisting essentially of quartz, usually formed from quartzose sandstone by cementation or metamorphic action.
Regolith—a name recently suggested by Merrill to embrace the earthy mantle that covers indurated rocks, chiefly residuary earths; mantle-rock.
Rhyolite—an aphanitic or glassy igneous rock showing flowage lines, usually applied only to the acidic varieties.
Sandstone—indurated sand usually composed of grains of quartz, but not necessarily so; sometimes formed of calcareous grains or of grains of the various silicates.
Schist—a crystalline rock having a foliated or parallel structure, splitting easily into slabs or flakes, less uniform than slate; they are mainly composed of the silicate minerals.
Scoriæ—light, cellular fragments of volcanic rock, coarser than pumice; cinders.
Septaria—concretions the interior of which have parted, and the gaping cracks become filled with calcite or other mineral deposited from solution ([Figs. 375–77]).
Serpentine—a rock consisting largely of serpentine; derived in most cases by alteration from magnesian silicate rocks.
Shale—a more or less laminated rock, consisting of indurated muds, silts, or clays.
Slate—an argillaceous rock which is finely laminated and fissile, either due to very uniform sedimentation or (more properly) to compression at right angles to the cleavage planes; e.g., common roofing-slate ([Fig. 362]).
Soapstone (steatite)—a soft unctuous rock, composed mainly of talc.
Stalactites—pendant icicle-like forms of calcium carbonate deposited from dripping water.
Stalagmite—the complement of stalactites formed by calcareous waters dripping upon the floors of caverns.
Steatite—see soapstone.
Syenite—a granitoid rock composed of orthoclase and hornblende, or other ferromagnesian mineral; the name was formerly applied to a granitoid aggregate of quartz, feldspar, and hornblende.
Tachylite (hyalomelane, basaltic glass)—a black glass of basaltic nature corresponding to the acidic glasses, obsidian and pitchstone.
Till (bowlder clay)—a stony or bowldery clay or rock rubbish formed by glaciers.
Trachyte—a name formerly applied to a rock possessing a peculiar roughness due to its cellular structure; but at present mainly confined to a compact, usually porphyritic igneous rock, consisting mainly of sanidine associated with varying amounts of triclinic feldspar, augite, hornblende, and biotite.
Trap—a general term for igneous rocks of the darker basaltic types.
Travertine—a limestone deposited from calcareous waters, chiefly springs; usually soft and cellular, and hence also called calcareous tufa, calc sinter.
Tuff (tufa)—a term including certain porous granular or cellular rocks of diverse origins; the volcanic tuffs embrace the finer kinds of pyroclastic detritus, as ashes, cinders, etc.; the calcareous tufa embrace the granular and cellular deposits of springs; the better usage limits the term to volcanic clastics.
Water-lime—an impure argillaceous limestone possessing hydraulic properties.
Wacke—a dark earthy or granular deposit formed from basic tuffs or from the disaggregation of basaltic and similar rocks; a term which may well come into more general use to distinguish the silicate sands that arise from the disaggregation, but only partial decomposition, of basic rocks, as arkose does, the like products of the acidic or granitoid rocks, and as sandstone does, the granular products of complete chemical decomposition.
ORE-DEPOSITS.[207]
Ore-deposits are but a special phase of the rock-forming processes already discussed. They have peculiar interest because of their industrial value. An ore is simply a rock that contains a metal that can be profitably extracted, though for convenience the term is used more broadly to include unworkable lean ores and ore material. The metal need not preponderate or form any fixed percentage of the whole, for the criterion is solely economic and not petrologic. A gold ore rarely contains more than a very small fraction of one percent. of the precious metal, while high-grade iron ore yields sixty-odd percent. of the metal. In iron ore, the metallic oxide or carbonate makes up nearly the whole rock; in gold ore, the metal is the merest incidental constituent, from the petrologic point of view.
Concentration.—The essential fact in the formation of ores is the unusual concentration of the metal. There are vast quantities of all the metals disseminated through the rock substance of the earth and even throughout the hydrosphere, but they do not constitute ores because they have no economic value. They become ores when concentrated in accessible places to a workable richness. The degree of concentration required is measured by the value of the metal. The essential elements for consideration are, therefore, (1) the original distribution of the metallic materials through the rocks, (2) their solution by circulating waters (or, rarely, by other means), (3) their transportation in solution to the place of deposit, (4) their precipitation in concentrated form, and (5) perhaps their further concentration and purification by subsequent processes.
Exceptional and doubtful cases.—There are a few cases where ore-deposits are made by volcanic fumes or vapors, but these may be neglected here. Formerly, ores were often attributed to vapors supposed to arise from the hot interior, but this mode of origin seems incompatible with physical conditions. Ores have been attributed to water originally contained as steam in lavas, and to waters escaping from the interior of the earth, these waters being supposed to be especially mineralized. Direct evidence on this point is obviously beyond reach. Segregation in the molten state is recognized as a source of ores, but its function is probably confined chiefly to partial enrichment as stated below. There are other occasional methods, but the chief process of concentration, immeasurably surpassing all others, consists in the leaching out of ore materials disseminated through the country rock and their redeposition in segregated forms, as an incident of the recognized system of water circulation.
Original distribution.—The original distribution of ore material through the primitive rocks is beyond the ken of present science, for even the nature of the true primitive rocks is unknown. For present purposes it is sufficient to regard all rocks concerned in ore-deposition as either igneous or sedimentary, and to inquire, as a first step, how far ordinary igneous and sedimentary processes contribute to the segregation of ore material, leaving for a second stage of inquiry the subsequent processes of concentration.
Magmatic segregation.—In a few instances workable masses of ore seem to have arisen from lavas by direct segregation in the molten state, without the aid of subsequent concentration by water action, on which most ores are dependent. It is not improbable that the segregation of metallic iron and nickel, and perhaps other metals, in the deeper parts of the earth may be a prevalent process, giving rise to masses like the native iron found in basalt in Greenland. This iron closely resembles the nickel-irons of meteorites, which may be illustrations of similar action in small planetary bodies that have been disrupted. Metallic masses so segregated presumably gravitate toward the planetary center and hence, whatever their inherent interest, have little relation to a subject whose basal criterion is economic. It is not at all improbable, however, that in the magmatic differentiation of the lavas that come to the surface, there is some metallic segregation that may make the enriched parts effective ground for the concentrating processes of water circulation, and so determine the location of ore-deposits. Igneous rocks are not equally the seats of ore-deposits, even when the circulatory conditions seem to be equally favorable. These conditions may not really be equally favorable, but there is good ground to believe that some igneous masses constitute a richer field for concentration than others. No definite rule, however, for distinguishing rich varieties of rock from lean ones has been determined. The basic igneous rocks are, on the whole, perhaps somewhat richer in ores than the acidic class, but there is no established law. Many acidic rocks bear more and richer ores than many basic ones. The view here entertained is that both classes are subject to regional enrichment through conditions connected with their origin, as yet little known.
Marine segregation and dispersion.—In the formation of the sedimentary rocks from the primitive and igneous rocks there was notable metallic concentration in some cases, and even more notable depletion in others. The ground-waters of the land, after their subterranean circuits, carried into the water-basins various metallic substances in solution. These were either precipitated early in the marine or lacustrine drift of the waters, or became diffused throughout the oceanic body. In the main they appear to have been widely diffused, and either to have remained long in solution, or to have been very sparsely deposited through the marine or lacustrine sediments. As a rule, these sediments seem to contain less of valuable ore material than igneous rocks, and this is rational, for, as we shall see, the ground-water circulation of the land tends to concentrate and hold back a part of the metallic content of the land rocks so that only a residue reaches the sea. But there are important exceptions to this general rule of sedimentary leanness.
The iron-ore beds of Clinton age ranging from New York to Alabama, and appearing also in Wisconsin and Nova Scotia, form a stratum in the midst of the ordinary sediments, and contain marine fossils. The great ore beds of Lake Superior were originally of similar type, and so are most other important iron deposits. It cannot be said, in most cases, that these iron deposits are marine as distinguished from lacustrine or lodgment deposits, but they are at least sedimentary. The ferruginous material was originally disseminated widely through antecedent land rocks, but was concentrated in the course of the sedimentary processes.
Limestone appears to have been sometimes enriched locally in lead and zinc, and more rarely in copper, in the course of its sedimentation. The lead and zinc regions of the Mississippi basin have been regarded as dependent on such regional enrichment as a primary condition. This localized enrichment has been attributed to solutions brought into the sea from neighboring metal-bearing lands and precipitated by organic action in the sea-water,[208] this organic action being more effective in some areas than in others because of the unequal distribution of life and the concentration of its decaying products. It is assumed that such precipitates were at first too diffuse to be of value, and further concentration was required to bring them together into workable deposits; but the further processes appear to have been effective only where the preliminary enrichment had taken place. At any rate, the workable deposits are singularly localized, while the concentrative processes are very general.
Metallic material is sometimes partially concentrated in sandstones and shales in the process of sedimentation, though more rarely. The copper-bearing shale (Kupferschiefer) of the Zechstein group in Germany, so extensively worked along the flanks of the Harz Mountains, is a striking example.
It is in every way reasonable to suppose that land-waters, on reaching the margins of the water-basins, must occasionally find conditions favorable for the precipitation of their metallic contents, and that the ratio of these precipitates to other material might be relatively high in the more favorable situations, and that this enrichment of the country rock may be a condition precedent to a sufficient subsequent concentration to yield workable accumulations.
It is, therefore, inferred that while the processes of sedimentation tended on the whole to leanness, they gave rise to (1) some very important ore-deposits, notably the chief iron ores, the greatest of all ores in quantity and in real industrial value, and (2) a diffuse enrichment of certain other areas which made them productive under subsequent concentrative processes, while the sedimentary formations in general were left barren.
Origin of ore regions.—From these considerations it appears that for the fundamental explanation of “mining regions” we must look mainly (1) to magmatic differentiation, so far as the country rock is igneous, and (2) to sedimentary enrichment, so far as the rock is secondary. The determining conditions in both cases are obscure and unpredictable, but the recognition of such regions, and of the function of preliminary diffuse regional enrichment, contributes to a comprehensive view of the complex processes of ore concentration. The subsequent processes consist in the further concentration of the ore material into sheets, lodes, veins, and similar aggregations by ground-water circulation, or else in the purification of the ores by the removal of useless or deleterious material, or in both combined.
Surface residual concentration.—The simplest of all modes of concentration takes place in the formation of mantle-rock. An insoluble or slightly soluble metallic substance sparsely distributed through a rock may be concentrated to working value by the decay and removal of the main rock material, leaving the metallic material in the residuary mantle. The tin ores of the Malay peninsula[209] are especially good examples. The crystals of tin oxide were originally scattered sparsely through granite and limestone, but by their decay and partial removal it has accumulated in workable quantities. Certain gold fields and certain iron ores have acquired higher values in the same way. Such residuary material may be further concentrated by wash into gulches or alluvial flats, in the course of which the lighter parts of the mantle-rock are largely carried away, and the heavier, including the metal or its compounds, are mainly left behind. Gold placers are the best example. The mining of placers by hydraulic processes is but a further extension of the natural process of concentration.
Such concentrates in past ages have in some cases been buried by later deposits, and hence certain ancient sandstones, conglomerates, and mantle-rocks have become ore-bearing horizons. The Rand of South Africa appears to be of this type.
Purification and concentration.—A somewhat different mode of concentration and purification has affected certain of the great iron deposits. As already explained, the iron compounds were originally dissolved from the iron-bearing constituents of the primitive or of igneous rocks, or their derivatives, and were deposited in beds as chemical stratiform deposits. In some cases they were sufficiently pure, as first precipitated, to be worked profitably, but in most cases they were seriously affected by undesirable mineral associates. When, however, such impure deposits are subjected for long periods to the percolation of waters from the surface under favorable conditions, the impurities are often dissolved and the ores concentrated. The great Bessemer ore-deposits of Lake Superior are examples. Originally impure carbonates or silicates, they have been converted into rich and phenomenally pure ferric oxides along certain lines of ground-water circulation, and in certain areas of free leaching. Van Hise has shown the definite relation between the water circulation and the production of the high-grade ores.[210] Vast quantities of unconcentrated lean ores lie in the tracts not thus purified and enriched by circulating waters. This does not appear to be simply residual concentration. The waters seem to have added ferric oxide brought from above, while they carried away the “impurities,” silica, carbon dioxide, etc. Perhaps this is an instance of mass action in which the ore present aided in causing additions to itself.
Concentration by solution and reprecipitation.—By a process almost the opposite of residual concentration, ore material is often leached out of the surface-rock by water circulating slowly through its pores, cleavage planes, and minute crevices, and is carried on with the circulation until it reaches some substance which causes a reaction that precipitates the ore material. This substance may be a constituent of some rock which the circulating water encounters, such as organic matter. More commonly, the precipitation seems to be due to the mingling of waters charged with different mineral substances, the mingling inducing reaction and the precipitation of the ore. Precipitation, however, does not necessarily follow such commingling. The junctions of underground waterways are sometimes characterized by barrenness instead of richness. In the expressive phraseology of the miners, a tributary current sometimes “makes” and sometimes “cuts out.” In chemical phrase, when the mingling waters reduce the solubility of the appropriate substance sufficiently, an ore-deposit is formed; when they increase its solubility, they promote barrenness. Changes of pressure and temperature may enter into the process, and mass action may lend its aid when once a deposit is started.
More concretely stated, the general process of underground ore formation appears to be this: the permeating waters dissolve the ore material disseminated through the rock and carry it thence into the main channels of circulation, usually the fissures, broken tracts, porous belts, or cavernous spaces. If precipitating conditions are found there, deposition takes place. The precipitating conditions may be merely changes of physical state, such as cooling or relief of pressure, but probably much more generally they consist in the commingling and mutual reaction of waters that have pursued different courses and become differently mineralized, as implied above. In these cases the metal-bearing current may be scarcely more important than the precipitating current.
Since the solvent action is a condition precedent to deposition, the location of the greatest solvent action first invites attention. At present it must be treated in general terms, for it is not known what solutions must be formed beyond the fact that they must include the ore material. Probably they must include much besides. Furthermore, it is not known that deep-seated rocks carry more ore material than similar rocks at or near the surface or at any other horizon. Fantastic conceptions of deep-seated metallic richness are to be shunned as quite beyond practical consideration. The water circulation is probably very slight below a depth of two or three miles at most, and above that depth there is little ground to suppose that the rocks of one horizon are inherently more metalliferous than others of their kind. There is no assignable reason why the igneous rocks at the surface are not as rich in ore material as the igneous rocks two or three miles below, since all are probably eruptive and of much the same nature on the whole, being in many cases parts of the same eruptions.
Location of greatest solvent action.—Solvent action is probably most intense where the temperature and pressure are highest, that is, in the deeper reaches of water circulation; but the amount of water passing in and out of the deeper zone is but a small fraction of that which courses through the upper horizons, and the total solvent action is quite certainly much greater in the upper zone than in the lower. At the same time the solutions in the upper zone are quite certainly more dilute than those below. The horizon of greatest solution lies between the surface and a level slightly below the ground-water surface, or, in other words, in the zone where atmosphere and hydrosphere coöperate. Surface-waters are charged with atmospheric and organic acids and other solvents, and their general effect upon the rocks is markedly solvent down to or often below the permanent water-level. In this zone concentration by residual accumulation may take place, as already noted, if the metallic compounds resist solution; otherwise this zone is depleted of its ore material by solution, and preparation is made for deposition elsewhere.
Solution also continues to take place varyingly as the water descends below this zone of dominant solution, and extends probably to the full depth of water circulation, but in the deeper circuit, precipitation also takes place and the action becomes complex. With the waters taking up and throwing down material at the same time, it is difficult to estimate the balance of results.
When waters that have been mineralized near the surface descend, they often take on a precipitating phase at no great depth below the upper level of the ground-water; thus sulphides that were oxidized and dissolved near the surface are reprecipitated, often at horizons not greatly below the permanent water-level. Waters that dissolve metallic substances in the upper levels often become charged with sulphuretted hydrogen and other precipitants within a few scores or a few hundreds of feet of the surface, as deep wells abundantly prove. The freshness of surface which metallic sulphides often exhibit at these levels is fair ground for inferring recency of deposition and absence of solvent action. Actual demonstrations of depositions in progress are not wanting.
Short-course action.—The concentration which thus takes place by solution in the upper zone, followed closely by reprecipitation within a few score or a few hundred feet, may well be termed the short-course mode of ore concentration. It finds its most important illustration in what is commonly known as the “secondary enrichment” of ore-deposits. The ores in the outcropping edge of the vein or lode are dissolved by the surface-waters, carried a short distance down the ore tract and redeposited, causing enrichment at that point. This is only a special case of what takes place generally at this horizon. It is effective in this case because it has a previous partial concentration to work upon. Secondary enrichments of this kind often contain most or all the workable values of the ore tract. If instead of a previous concentration in a vein, lode, or similar ore tract, there had been partial concentration in the country rock by sedimentation, as in the case of iron-ore beds and perhaps lead-, zinc-, and copper-impregnated sediments, the short-course method may give working values not before possessed. In some of the more obscure cases of previous partial concentration in the crystalline and other rocks, it is probably this short-course action that brings the concentration up to working value. It is probably effective also in concentrating the metallic contents of certain igneous rocks that were rich in metallic material when extruded. How far this is true has been, and still remains, a mooted question.
Long-course action.—After the surface-waters have once passed through a cycle of dissolving and precipitating action, as they are apt to do within the first few hundred feet of their courses below the water-level, they are liable to pass through a succession of dissolving and depositing stages, each reaction resulting in a state that makes a new reaction possible. This is especially true if the waters pursue deep courses. Strictly speaking, the precipitations usually concern only a part of the substances dissolved. New substances are often taken up in the very act of throwing down those already held, and the way thus prepared for further changes. If the water pursues a deep and devious course, it may receive additions by solution and suffer losses by precipitation at many points in its course, both descending and ascending. The changes are very complex, and in the case of a deep or long circuit where various rocks, pressures, and temperatures are encountered, the history becomes one long succession of complexities, the full nature of which is not yet revealed.
In the deeper circuits, each individual current usually takes on a descending, a lateral, and ascending phase, the three being necessary to complete a circuit. The chemical conditions of the waters in the three phases are probably not sharply distinguished from one another, and hence there seems to be no defined horizon of concentration comparable to that near the water-level already described. The chief distinctions in the deeper regions relate to pressure, temperature, length or depth of penetration, and duration of contact. It seems safe to assume, as a general truth, that, other things being equal, the solutions become more complex and more nearly reach general saturation the farther and the deeper the waters penetrate.
It has long been a mooted question whether ore-deposits are due chiefly to descending, to lateral, or to ascending currents. The question in its usual form is too undiscriminating for advantageous discussion, but if the ore-deposits due to surface or short-course concentrations and reconcentrations be set aside, as in some sense a separate class, the relative functions of the descending, the lateral, and the ascending portions of the deeper circulations become a measurably definite question. Two great working factors enter into the comparison: (1) much greater circulation in the upper zone, where lateral movement most prevails; (2) much greater heat and pressure in the lower zone, where the circulation must be chiefly vertical.
Heat and pressure in general favor solution, and hence so far as this factor goes, descending water is likely to be increasing its mineral content, rather than diminishing it by deposition. But this is only general; particular elements of the solution may be deposited. In ascending, as the same water must later, it is predisposed to deposition from loss of solvent power through reduction of pressure and temperature. The theoretical balance is here clearly in favor of preponderant deposition by the ascending portion of the current. So far as precipitation is dependent on the mingling of differently mineralized waters, descending and ascending currents seem to be situated much alike, in general, for both are subject to accessions and mutual unions.
The amount of water that circulates in the deeper horizons is much less than that nearer the surface. Allowing a few hundred, or at most one or two thousand feet for the special short-circuit zone next below the water-level (it is known to reach 1000 to 1500 feet in some cases), the water circulating through the next 1000 or 2000 feet is probably several times greater than all that circulates at greater depths, and this greater circulation above doubtless offsets, in greater or less measure, the intensified action of the deeper circulation. Much of the upper and more rapid circulation is lateral, being actuated by the sloping surface of the ground-water, which in turn is determined by topography, precipitation, and other surface conditions. Theoretical considerations, therefore, favor the view that lateral flow is an important factor in the concentration of ore material. But as descending and lateral currents almost inevitably meet and mingle with ascending currents, it is difficult to distinguish, in the ore-deposits, the special functions of each phase of action. It is even more difficult to determine whether the different phases are not alike essential to the mutual reactions on which the deposition depends. It may be as necessary to have a precipitant as to have a metallic constituent in solution to be precipitated, and what is more, this precipitating agency may be a substance of no economic value in itself and of no obvious relations to the substances that form the ores. If the deposition is due solely to a physical state, as relief of pressure or lowering of temperature, these considerations do not hold.
Summary.—The general results are probably these: In the deeper circuits, more ore material is brought upward and deposited than is carried downward and deposited, so that metallic values are shifted toward accessible horizons. In the lateral currents, more metallic values are shifted toward the trunk-lines of circulation—the great crevices and other waterways—than are carried from these into the rock and distributed, and lateral segregation results. At the same time the atmospheric waters acting at or near the surface concentrate ore values downwards. The sum total of these processes is to promote the development of the higher ore values in accessible horizons, and along the main lines of circulation.
The influence of contacts.—As ore-deposits depend on a dissolving state followed by a depositing state of the waters, and perhaps on a complex succession of these alterations, it is obvious that conditions which favor changes of state and the commingling of different kinds of water are apt to be favorable to ore production. At any rate it is observed that many important ore-deposits occur at the contact between formations of different character. The contact of igneous rock with limestone is a rather notable instance. It is not to be inferred that such contacts are generally accompanied by workable ore-deposits, but merely that a notable proportion of workable ore-deposits occur at such junctions. It is rational to suppose that where the chemical nature of the two formations is in contrast, the waters that percolate through the one are likely to be mineralized very differently from those that course through the other, and hence that on mingling at the contact, reactions are specially liable to take place, and that when a valuable metallic substance is present it is liable to be involved and by chance to suffer precipitation. Reactions are the more probable because the contact is likely to be a plane of crustal movement, and hence more or less open and accompanied by fractures, zones of crushed rock and other conditions that facilitate circulation and offer suitable places for ore formation.
The effect of igneous intrusions.—A special case of much importance arises when lavas are intruded into sediments that have previously been partially enriched in the ways above described. The igneous intrusion not only introduces new contact zones, and more or less fracturing, but it brings into play hot waters with their intensified solvent work, their more active circulation, and the reaction between waters of different temperatures. The special efficiency of these agencies is believed to be the determining factor in many cases.
The influence of rock walls.—The rock walls themselves are thought sometimes to be a factor in ore-precipitating reactions. By mass action, they may withdraw a constituent of the solution and destroy its equilibrium in such a way as to cause the precipitation of the metallic constituent. Once deposited on the walls ores aid, by mass action, the further accretion of ores.
The special forms which ores assume in deposition, as beds, veins, lodes, stockworks, disseminations, segregations, etc., are chiefly incidental to the local situation in which the essential chemical or physical change takes place.