CHAPTER V. MINERALS AS ROCKS. We have in the first three chapters treated of those rocks which, by reason of their great extent and volume, may be regarded as the principal ingredients of the earth's crust. We have seen that they are mostly of compound character, although some few are essentially simple mineral substances. In this place we propose to enumerate those simple minerals which appear as local accumulations in different parts of the globe, forming essential members of particular formations, sometimes as stratified beds, sometimes as veins or dykes, or irregular masses; their volume being just sufficient to entitle them to be considered as members of the rock family, taking an independent part in the structure of the solid crust of the earth, although in comparison with the other rock-formations which we have hitherto treated, their bulk is for the most part very inconsiderable. A description of the formation, texture, &c., of these mineral rocks will, in most cases, be unnecessary, as they must be mineralogically determined and recognised. We shall, therefore, in each case only give the name of the mineral, adding some short remarks as to its exceptional lithological character. 68. ICE. EIS. (Germ.) Sometimes compact, sometimes granular, fibrous or laminated. We need not here describe the properties of ice, but it is not unimportant to consider the conditions under which perpetual ice occurring in large masses forms part of the solid crust of the earth. The snow which falls in the polar regions, and in mountain districts above the snow-line, only partially thaws in summer; the remainder accumulates year by year. The successive falls of snow form a series of superjacent strata, the fleecy mass becomes consolidated by pressure, and grains of ice are formed which unite into a stratified granular ice; this in the Alps is called firn or névé. The masses of névé thus formed glide gradually down over the mountain slopes and precipices into the ravines and valleys. In the course of their downward movement their stratification becomes much contorted and otherwise disturbed; they are, moreover, transformed from distinctly granular névé into indistinctly granular ice, or so-called glaciers. The glacier continues to glide with a slow movement down the valley. Its lower extremity, thus arrived in warmer regions, thaws more rapidly and equalises the accumulation of snow pressing down in fresh masses from above. Hence the general extent and size of the glacier usually remains much the same, although the individual parts are constantly changing their position. By the motion of the glacier the traces of original stratification become more and more contorted and effaced. The glacier, moreover, becomes rent with frequent fissures (crevasses), and in these the water arising from occasional thawing accumulates and freezes during night or winter into new ice, which may be distinguished from the genuine glacier ice by its more compact structure. All these phenomena are very instructive, and afford many analogies to other rock formations and transformations. From loose accumulations, by means of pressure and consolidation, masses are formed which become firmer and more solid, and at last tolerably compact. Strata are bent, pushed out of place, and overturned. The mass is torn by cracks and fissures, which are filled by water rendered fluid by heat. This freezes and constructs ice veins in ice, somewhat like granite veins in granite, only that these latter were probably filled from below, and under a much higher temperature. By a kind of weathering process even the compact venous ice in its turn becomes granular or separates into thin columnar parts, and all these changes take place before our eyes in comparatively short spaces of time. Very similar phenomena occur on a much larger scale in the polar regions; only they are less accessible, and therefore more difficult of observation. Besides these permanent masses of ice lying on the surface of the earth, there occur in the northern plains of Siberia extensive underground ice strata of great thickness, sometimes interstratified with beds of sand, or they contain sand mixed with the ice, and occasionally these strata are covered with a surface layer of soil, which during the short summer of Siberia supports vegetation. 69. OPAL. OPAL. (Germ.) As a rock, usually only forms very subordinate masses, e.g. the so-called vitrite, which occurs at Meronitz, in Bohemia, and contains numerous pyropes. If, however, we reckon under the name of opal all the various amorphous silicates enumerated by Naumann, we find amongst them several very important rocks : Varieties. : (a) SILICEOUS SINTER, or SILICEOUS TUFF.) Stratified incrustations KIESELSINTER oder KIESELTUFF. (Germ.) and porous masses; TUF SILICEUX, GEYSERITE. (Fr.) found as a deposit of hot springs in Iceland and Kamtschatka, and, according to Hochstetter, still more frequently in New Zealand. (Novarareise, 1862, vol. iii. p. 165.) (b) SEMI-OPAL. HALBOPAL. (Germ.) (c) MENILITE. MENILIT. (Germ.) { Forms independent deposits, e. g. at Bilin, in Bohemia; also irregular fillings of clefts in basaltic rocks, e. g. at Hanau on the Maine, in the dolerite. and Menilite occurs in the Paris basin in clumps and beds. It is found there in gypsum in marl (Eocene); in Auvergne, in freshwater marl (Miocene). (d) POLISHING SLATE, TRIPOLI. POLIRSCHIEFER, SAUGSCHIEFER, KLEB SCHIEFER, TRIPPEL. (Germ.) TRIPOLI. (Fr.) Consists of small shell-shaped particles of silica of a peculiar form, only to be distinguished with the aid of the microscope, so-called siliceous armour of Diatomaceæ or Infusoria; Naumann therefore calls it Diatomeenpelit. Ehrenberg reckoned that the polishing slate of Bilin in Bohemia contained in a cubic inch 41,000 millions siliceous shells of Gaillonella. Each individual is invisible to the naked eye, so that when used for polishing metallic surfaces it produces only fine invisible scratches. Distinction is made in Bohemia between the polirschiefer (soft, friable, not adhering to the tongue) and saugschiefer (adhering to the tongue and more solid, probably because it is impregnated with opal substance). Both are only known in very recent deposits; the older ones have probably been transmuted into hornstein or lydian stone. (Ehrenberg, Fossil Infusoria, Berlin, 1837, and Mikrogeologie.) (e) KIESELGUHR. KIESELGUHR. (Germ.) RANDANITE. (Fr.) The same substance as polishing slate, but more dust-like, earthy, generally white or yellow. Found in beds many feet thick in the turf deposits at Soos, near Franzensbad, Bohemia. The rock called RANDANITE by Salvetat belongs to this species; it consists of a white powder. (v. L. u. Br. Jahrb. 1848, p. 124.) 70. QUARTZ. QUARZ. (Germ.) QUARTZ. (Fr.) Occurs as an essential ingredient in many rocks, but it also occurs as an independent rock in many varieties, some of which are of considerable extent. We repeat the mention in this place of several quartz rocks which we have already noticed and included in other groups. Varieties. (a) ROCK CRYSTAL and AMETHYST. Sometimes the essential BERGKRYSTALL und AMETHYST. (Germ.) ingredient of veins and (b) COMMON QUARTZ. Forms independent bed-veins or irreQUARTZ COMMUN. (Fr.) gular masses. Quartz-schist, see p. 246, ante; Quartz-breccia, p. 305; Quartz-sandstones (siliceous sandstone), p. 296. Millstone-quartz, freshwater-quartz, or lemon-quartz, are porous varieties resembling chert, which, according to the fossils occasionally found in them, have been deposited by fresh water, as, e. g., the celebrated millstones of the Paris basin (Quartz meulier). (c) FERRUGINOUS QUARTZ. Yellow, red-brown, or black; forms transition states into jasper. Its mode of occurrence in nature is the EISENKIESEL. (Germ.) (Fr.) same as that of ordinary quartz. (d) HORNSTONE, CHERT. HORNSTEIN, HORNFELS. (Germ.) Compact, forms independent beds, veins, and masses. In Germany the name of Hornfels is given to certain rocks, the product of transmutation of argillaceous deposits, and found adjoining to plutonic rocks, to which they probably owe the change they have undergone. (e) LYDIAN STONE, or LYDITE, BLACK CHERT. KIESELSCHIEFER oder LYDIT. (Germ.) laminæ, and hence of a laminated Contains carbon which gives it a greyish colour inclining to black; usually stratified in thin texture; generally pene trated by numerous white veins of quartz; much rent by angular fissures, sometimes containing lenticular concretions, and also sometimes containing laminae of clay-slate. In fissures it contains wavellite, calaite, variscite. It occurs with tolerable frequency as a subordinate stratum in clay-slate, slate-clay, or even mica-schist. (f) JASPER. JASPIS. (Germ.) Compact, variegated, frequently striped or flamed (riband-jasper, agate-jasper). Much has been called jasper which properly belongs to the felsitic rocks, even to the felsitic tuffs. It forms subordinate layers imbedded in other rocks, and nodular concretions. Jasper may be readily distinguished from petrosilex (which it otherwise sometimes resembles) by the fusibility of the latter. (g) AGATE. The name given to certain combinations of chalcedony, carnelian, amethyst, and quartz. AGATE. (Fr.) There are many varieties :-banded agate, fortification-agate, coral-agate, &c. It frequently forms veins or fills cavities in other rocks. ACHAT. (Germ.) (h) FLINT. } Very similar to hornstone, but half amor FEUERSTEIN. (Germ.) phous, chiefly yellow, brown, grey, or black. Forms nodules, and then are fre SILEX. (Fr.) quently disposed in layers; very frequent, e. g. in chalk. 71. CORUNDUM. KORUND oder SCHMIRGEL. (Germ.) Forms fine-grained subordinate layers imbedded in crystalline schists, frequently accompanied by magnetic iron-ore. Ochsenkopff, in the Erzgebirge; Gumuchdagh, in Asia Minor; Naxos; Chester, Massachusetts. 72. FLUOR-SPAR. FLUSSSPATH. (Germ.) FLUORINE, SPATHFLUOR. (Fr.) Frequently an essential ingredient in metalliferous veins. A compact aggregate of fluor-spar forms a rock at Rottleberode and Strassberg in the Hartz Mountains. 73. ROCK-SALT. Chloride of sodium occurring as a rock is usually crystalline-granular, white, translucent or transparent, easily soluble in water, and possesses a saline taste. 2.1-2.2. Spec. grav. Pure chloride of sodium consists of 60 per cent. chlorine to 40 per cent. sodium. In nature, however, it almost always contains sulphate of lime, chloride of calcium, |