Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/242

Rh 228 usually more or less soda, sometimes as much as 10 per cent., and (2) the Triclinic or Plagioelase felspars (with oblique cleavage angles, or less than 90°), including a S0t a group with 8 to 12 per cent. of soda, and a lime group with 6 to 20 per cent. of lime. The felspars form a large part of most igneous roc ‘s. By their decay they form clay, and in that condition enter largely into the composition of the argillaceous stratified rocks, such as shale, mudstone, slate, &c. Ilornblende is a meta-silicate of magnesium, with lime, iron, or znangancse, and frequently alumina. The white non-aluminous varieties (tremolite, aetinolitc, anthophyllite, asbestos) chiefly occur as constituents of such metamorphic rocks as crysta1l1nc_l1mcstone, gnciss, &c. The black or dark green aluminous varieties entcr_as essential constituents into the composition of many rocks, as dioritc and hornblende slate. _ . _ _ .-lugite (resembling hornblende in composition) is d1_v1s1blc into two groups. The pale non-aluminous varieties (diopsidc, sahhtc, coccolite, &c.) occur under conditions like those of t ie pale horn- blcndes. The dark aluminous or common augite is abundant as an ingredient of some igneous rocks, as basalt. Allied to augitc are diallage (important as a constituent of diallage-rock_or gabbro), hypersthcne, and bronzite. Uralitc is the name of a mineral of fre- quent occurrence among Paheozoic rocks, having the external form of augite and the cleavage of hornblende. Iiz-inc (an ortho-silicate of magnesium, with part of the mag- nesium replaced by iron or manganese) is a conspicuous ingredient among the basalt rocks. It appears also to have been the original magnesian constituent of many rocks now altered into serpentine. Nr-plwline (a silicate of alumina and soda with a little potash) takes the )llJ.C0 of fclspar in some lavas. It likewise occurs among the ejected blocks of Somma, and, in the form of claeolite, among the ancient crystalline rocks of 1'orway. Lcucite (K2Al.,Si_,O,2) is a characteristic ingredient of many 'l‘ertiary and recent lavas. It has not been met with among any of the Paleeozoic or Secondary igneous rocks, nor ever in association with quartz. Ilaizync and Nosean are two minerals allied to garnet, found in some Tertiary lavas. 1|[ica.—Under this general term are included several species of minerals distinguished by their basal cleavage into thin lamina.- and by their splendent or silvery lustre. The non-magnesian micas include muscovite or potash-mica, the most abundant of all, and lepidolite or lithia-mica; of the magnesian micas the most important is biotite. Muscovite enters into the composition of granite, gneiss, mica-schist, micaceous sandstone, and many other rocks. Biotite is likewise abundantly distributed among the older crystalline rocks. Lepidomelane is a black mica often found in ﬁne-grained granites. Other species are margarodite—an abundant constituent of many unctuous schists formerly called talc-schists, and haughtonite— which, according to Heddle, is the common mica of the granites in the Scottish Highlands. Garnet (an aluminous ortho-silicate with lime, magnesia, iron, or manganese) occurs in rhombie dodecahedrons or allied forms, and also massive in many metamorphic rocks, as mica-schist, eelogite, &c. Epidote (a variable silicate of lime, alumina, iron, or manganese) occurs in yellow or greenish translucent crystals or crystalline masses in many of the older crystalline rocks, though seldom as an abundant constituent. It is probably always an alteration-product. Tourmaline, in its common black variety, schorl, forms with quartz the rock known as schorl-rock, and occurs in some granites, gneisses, schists, and other crystalline rocks. Zircon (silicate of zirconium) is found as a constituent of zircon- syenite, and more sparingly in other crystalline rocks. The hydrous silicates have resulted from the alteration of the anhydrous forms. As constituents of rocks they may be grouped into two series:——(1)the aluminous, including the zeolites, and (2) the magnesian, embracing talc, chlorite, serpentine, and their allies. Zeolites form at numerous genus of minerals distinguished usually by their boiling up before the blowpipe, owing to the escape of their water of crystallization, by their frequent pearly lustre, inferior hardness, and their occurrence in cavities and veins where they have been deposited from solution. They are found as abundant secondary products in many amygdaloids, also in altered limestoncs and other metamorphic rocks. Serpentine (SiO,,44 '14 ; MgO,42'97; H,O,12'89) isa dull impure, usually green, granular to compact, more rarely foliated, mineral, with a hardness of 3 to 4 or even sometimes 5. Like the other hydrous magnesian silicates it has a soapy or greasy feel. It occurs abundantly in many altered rocks as a pscudomorph after some of the anhydrous magnesian silicates, also as a massive rock forming huge beds often associated with metamorphosed limestones. I}/ilorite is a general term including several minerals which agree in possessing agreenish colour, soapy feel, hardness of only 2 to 2'5, and speciﬁc gravity of 2'65 to 2'85. It occurs in chlorite slate and in many rocks as an alteration-product. Talc (SiO.,,59 to 63; MgO,30 to 33; I120, from a trace u to 7 per cent.) occurs in hexagonal plates or scales, cleaving reailily into flexible non-elastic laminae, but most commonly ganular and GEOLOGY -of granite and other rocks. [11, GEOGNOSY. massive, white to nile leek or apple-green, with marked pc:n'l_' lustre on cleavagc~p]l-ancs. It is met with in tale-slate, also fre- quently in crystalline rocks as a result of the alteration of horn- blcnde, augite, or other anhydrous magnesian silicate. Dclcssitc and Saponitc are soft’ green hydrous magncsian silicates found as products of alteration in basalt-rocks. Carbon occurs chiefly as beds in the form of coal, lignite, peat, &c. Graphite, however, is often met with in black or steel-grey splcndent scales and granular masses in mctamor vhosed rocks. Anthracite also takes sometimes the form of black glancing grains or of a diffused ﬁne black dust through certain palazozoic formations. Carboimlcs play an important part both as individual minerals and as rock-masses. The three most important are calcite, dolo- mite, and sidcrite. Calcite (carbonate of lime) is one of the most abundant minerals. It occurs crystallized as a secondary product in most rocks which have undergone decomposition, especially where. they contain sili- cates into the composition of which lime enters. It is also found massive as limestone, forming beds having sometimes an aggregate thickness of many hundred feet and an extent of thousands of square miles. Dolomite (carbonate of lime and magnesia) is likewise both a product of alteration and an original formation. ln the former condition it is met with crystallized as bitter-spar in many meta- morphic rocks as well as in veins and cavities of unaltered forma- tions. It occurs also as an amorphous granular substance, some- times replacing calcite, and sometimes in vast beds or masses of original deposit. Sidcrite, Chalybitc, or Spafhic Iron (carbonate of iron) occurs both crystallized and massive. In the crystallized form it is compara- tively unimportant as a constituent of rocks, being then found chiefly in veins and cavities where other alteration-products have been deposited. But in its massive condition it is found mixed with clay and other impurities, and forming beds and nodules which are among the most important ores of iron. Sulphur, though seldom occurring in large masses, is widely diffused as an accessory ingredient of rocks. It occurs crystallized or ﬁnely granular in mineral veins, in nodules of limestone, and other eoneretions, and in beds of limestone and marl. It also takes the form of a crust in the sublimations of volcanic vents. Its fre- quent association in Tertiary strata with the remains of lacustriue shells, insects, and plants shows that it has in these cases been formc(l at ordinary temperatures from aqueous solutions. Sulphidcs, combinations of sulphur with the metals, iron, copper, lead, zinc, and a few others, have a wide distribution among rocks. Vherc aggregated into masses they form mineral veins. lt is the iron sulphides which deserve chiefly the attention of the pctrographcr. They occur in two varieties—pyr2'tc, r-rystallizing in isometric forms, a11d marcasitc, in rhombie forms. The former has a remarkably extensive diffusion throughout rocks of all ages, usually as minute crystals and thin streaks, but often in (‘0llC1‘L‘ll01l.' and 1norc massive veins. Marcasitc also is abundantly distributed though less so than pyrite. From its greater liability to 0.'idati0n the strata through which it is diffused are apt to yield rapidly to the action of the weather, sulphuric acid and different alum com- pounds being produced. S-ulpImtes.——’l‘lic most generally occurring sulphates in rocks are gypsum and barytes. G3/psmn (hydrous sulphate of linn) in minute monoclinic prisms aml macles may be obtained by the cvaporation of sea-water, and in larger crystals of the same form it is found in many stratified formations. It likewise occurs as :1 secondary product in laminar or ﬁbrous veins through rocks of igneous origin. Beds of gypsum, resulting from aqueous deposition, frequently appear intcrstratificd with rock-salt and the associated products of evaporation. The anhydrous sulphate, anhydritc, like- wise occurs among rock-salt deposits, but has a much more limited diffusion than gypsum. Btu-_1/tes (sulphate of baryta) almost always occurs in veins or threads running through rocks. It is a common vein-stone in association with metallic ores. Ilalitc or Rock-salt (chloride of sodium) is more widely diffused than was formerly supposed. Microscopic research has shown its presence in the form of cubes in the mi1n1tc cavities in the quartz It occurs as scattered crystals, generally replaced by clay or some other substance, in many stratified forma- tions. lts chief habitat, however, is in the various saliferor.s deposits where it takes the form of solid beds of salt. Fluorite or Fluor-spar (fluoride of lime) is essentially a vein-stone, associated with metallic ores, especially with sulphides of lead and zinc. It occurs also in scattered cubes through various crystalline rocks, such as granite, gneiss, porphyritc. Apatite (phosphate of lime, with fluorine and often chlorine) has been shown by microscopic investigation to have a very wide dis- tribution among crystallinc rocks. lt occurs in fine needles or stoutcr hexagonal prisms in a large number of crystalline rocks, as gmnitc, quartz-trac iyte, sycnite, diorite, basalt, and many others. It also oc- curs massivc as beds among the more ancient geological formations. Iron o.rides.—Tliese are abundantly distributed through locks of