Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/251

Rh 1-‘RAGMENTAL ROCKSJ Blown sand is sand which has been produced by previous wave- actioii, and is blown into long ridges or dunes by prevailing winds. It varies in composition as ordinary sandstones do, being sometimes entirely siliceous, sometimes calcareous where derived from triturated shells or other calearcous organisms. Layers of ﬁner and coaiser particles often alternate as in water-formed sandstone. Grasses and other plants bind the surface of the shifting sand, but are apt to be covered by fresh eiicroaclinieiits of the loose material, and then by their decay they give rise to dark peaty layers in the sand. Calcareous blown sand is compacted into hard stone by the action of raiii-water which alternately dissolves a little of the lime and re-deposits it on evaporation as a thin crust cementing the grains of sand to- go-tlicl'. (L'hj_I/'-tlcbris consists of angular rubbish disengaged by frost and ordinary atmospheric waste from the face of clills, crags, and steep slopes. It slides down the declivities of hilly regions, and accumu- latcs at the base of slopes and prccipices, until washed away by rain or by brooks. It naturally depends for its composition upon the nature of the solid rocks from which it is derived. The material constituting glacier moraines is of this kind. 1t':,Lin-wa.sh is a loam or earth which accumulates on the lower parts of slopes or at their base, and is due to the gradual descent of the finest particles of disintegrated rocks by the transporting action of rain. Brie/c-c(u't/I, is the name given in the soutli-cast of England to thick masses of such loam which are extensively used for making bricks. Subsoil is the broken-up part of the rocks immediately under the soil. Its character of course is determined by that of the rock out of which it is formed by subaerial disintegration. Soil is the product of the subacrial decomposition of rocks and of the decay of plants and animals. Primarily the character of the soil is determined by that of the subsoil, of which indeed it is merely
 * 1 further disintegration. The formation of soil is treated in part

iii., pages 265, 269. Uouglomcmtc (I’iuldin_r/stone) is a name given to any rock formed of consolidated gravel or shingle. The Component pebbles are rounded and water-worn. They may consist of any kind of rock, though usually of some hard and durable sort, such as quartz or quartz-rock. A special name may be given according to the nature of the pebbles, as quartz-coiigloinerate, liinestoiie-conglomerate, granite- coiigloiiieratc, &c. The paste or cementing matrix may consist of a hardened sand or clay, and may be siliceous, calcareous, argillaceous, or fcrrugiiious. In the coarser conglomerates, where the blocks may exceed 6 feet in length, there is often very little indication of ~tiat.iticatioii. Except where the ﬂatter stones show by their general pai tllcllslll the rude lines of deposit, it may be only when the mass of conglomerate is taken as a whole, in its rclation to the rocks below and hb0-'c it, that its claim to be considered a stratiﬁed rock will be conccilcd. I.’rcec1'a is a rock in which the stones are angular and not rounded, and usually with less trace of stratiﬁcatioii than in conglomerate. lnteriiiediate stages between this rock and the preceding, where the -.[0llCS are partly angular and partly subaiigular and rounded, are known as brccciatcd conglomerate. .S'r.m.rZstonc is a rock formed of consolidated sand. The component _£1l".llllS are for the most part of quartz—a most durable mineral, which must here be regarded as the residue left after all the more decomposable minerals of the original rocks have been carried away in solution or in suspension as tine iimd. The colours of sand- stones arise, not so much from that of the quartz, which is com- 2-iionly white or grey, as from the ﬁlm or crust which often coats the grains and holds them together as a cement. As already stated 1=‘0ll is the great colouring ingredient of rocks. In sandstones it gives rise to red, brown, yellow, and green hues, according to its ii--__;ree of oxidation and hydration. In ordinary red sandstones, for I-Xillllplt‘. caeh grain of sand is coated with red earthy haematite. In yellow sandstone the oxide has become hydrous in the form of limonitc. There is as much variety of composition among sandstones as among conglomerates. Though they consist for the most part of siliceous grains, they include others of clay, felspar, mica, 01‘ other mineral ; and these may increase in number so as to give a special character to the rock. Thus sandstones may be argillaceous, fels- pathic, micaeeous, calcareous, &c. By an increase in the argillaceous Constituents, a sandstone may pass into one of the clay-rocks, just as modern sand on the sea-ﬂoor shades imperceptibly into mud. Hn the other hand, by an augmentation in the size of the grains a sandstoiie may become a grit, or a pebbly or conglomeratic sandstone, and pass into a ﬁne conglomerate. A piece of ﬁne-graiiicd sand- stone seen under the microscope looks like a coarse conglomerate, so that the difference between the two rocks is little more than one of relative size. Among the varieties of sandstones may be mentioned Flagstone, a tliiii-bedded sandstone capable of being split into slabs or ﬂags ; lv'recst_onc, a sandstone which can be cut freely in any direction (the term is popularly applied to some limestones and other rocks); and Ihzhrstone, a highly siliceous, exceedingly compact, though cellular, GEOLOGY 237 rock (with Chara seeds, &c.), found alternating with unaltered Tertiary strata in the l’aris basin, and forming from its hardness and roughness an excellent material for the grindstones of ﬂour-mills. Greg/wackc is a compact aggregate of rounded or subangular grains of quartz, slate, felspar, or other minerals or rocks cemented by a paste which is usually siliceous but may be argillaceous, felspathic, or calcareous. Grey, as its name denotes, is the prevailing colour; but it passes into brown, brownish-purple, and sometimes, where anthracite occurs, into black. The rock is distinguished from ordinary sandstone by its darker hue, its hardness, the variety of its component grains, and above all by the compact cement in which the grains are iiiibedded. In many varieties so pervaded is the rock by the siliceous paste that it possesses eat tou liness, and its grains seem to graduate into each other as wel as into t e surrounding matrix. Such rocks when ﬁne-grained can hardly, at ﬁrst sight or with the unaided eye, be distinguished from some compact igneous rocks, though a microscopic examination at once reveals their frag- ineiital character. In other cases, where the greywaeke has been formed mainly out of the debris of granite, quartz-porphyry, or otlicr felspathic masses, the grains consist so largely of felspar, and the paste also is so felspathic, that the rock might be mistaken for soine close-graincd granular porphyry. Greywacke occurs exten- sively among the Palzeozoic formations in beds alternating with shalcs and conglomerates. It represents the sand of the Palzeozoic sea-floor, retaining often its ripple-inarks and sun-cracks. The metamorphism it has undergone has generally not been great, and for the most part is limited to induration, partly by pressure and partly by permeation of a siliceous cement. Quart:-roclc (Quart:-itc) is a close-graincd granular aggregate of quartz cemented by a highly siliceous matrix. Originally it con- sisted of a tolerably pure quartz-sand, which has been metamor- phosed by pressure and the transfusion of a siliceous cement into an exceedingly hard mass. This cement was probably produced by the solvent action of heated water upon the quartz grains, which very generally seem to shade off into each other, or into the intervening silica. It is owing no doubt to the purely siliceous character of the grains that the blending of these with the surrounding cement is more intimate than in greywacke, so much so that the rock often assumes an almost ﬂinty homogeneous texture. That quartz-rock as here described is an original sedimentary rock and not a chemical deposit is shown, not only by its granular texture, but by the exact resemblance of all its leading features to ordinary sandstonc—false-bedding, alternation of coarser and ﬁner layers, worm-burrows, and fueoid-casts. It occurs in the form of large masses interstratiﬁed with limestones, slates, and schists. It is also met with locally as an altered form of sandstone, where this rock is traversed by igneous dykes and indurated into quartz-rock for a distance of a few inches or feet from the intrusive mass. Bands of highly silieated sandstones, having the lustrous aspect, ﬁne grain, and great hardness of quartz-rock, occur among the unaltered shales and other strata of the Carboniferous system. In such cases, the supposition of any general metamorphism being inadmissible, we must suppose either that these quartzose bands have been indurated, for example, by the passage through them of thermal silieated water, or that the quartz-rock is there an original formation. (1).) 0lay—rocl.'s.——Thes3 are composed of the ﬁner argil- laceous sediments or inuds derived from the waste of pre- viously formed rocks. Perfectly pure clay, hydrated silicate of alumina, may be seen where some granites and other felspar-beariiig rocks decompose. But, as a rule, the clay is mixed with various impurities. Pipe-clay is white, nearly pure, and free from iron. Fire-clay is a deposit largely found in connexion with coal-seams, contains little iron, and is nearly free from lime and alkalics. Some of the most typical ﬁre-clays are .those long used at Stoui-bridge, Vorcestcrshii-e, for the manufacture of pottery. The best glass- house pot-clay, that is, the most refractory, and therefore used for the construction of pots which have to stand the intense heat of a glass-housc, has the following composition :——silica, 73'82 ; alumina, 15'88; protoxide of iron, 2‘95; lime, trace; magnesia, trace; alkalies, '90; sulphuric acid, trace; chlorine, trace; water, 6'45; speciﬁc gravity, 2'51. A very siliceous closc-grained or ﬂinty variety, termed Gan-nistc-r, occurs in the Lower Coal-nicasurcs of the north of England, and is now largely ground down as a material for the hcarths of iron furnaces. .l»’ricIc-clay is properly rather an industrial than a geological term, since it is applied to any clay, loam, or earth, from which bricks or coarse pottery are made. It is an impure clay, containing a good deal of iron,_‘1tl1 other ingredients. An analysis gave the following composition of a brick-clay :—silica, 49'44 ; alumina, 34 '26; scsquioxide of iron, 7'74; lime, 1'48 ; magnesia, 5'14; water, 1'94. _ Jlludstonc is a ﬁne, usually more or less sandy, argillaceous rock, having no ﬁssile character, and of somewhat greater hardness than any form of clay. The term Clay-rock has been applied by sonio