Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/275

Rh PRESSURE AND CONTRACTIONJ perature of the melting point of cast-iron, it rapidly attacks silica, and deposits the mineral in snow-white crystals as it cools. § 2. 19_'[/‘eats of Pressztre and 0'ontractz'on. Besides the inﬂuence of pressure in raising the melting point of subterranean rocks, and in permitting water to remain fluid among them at temperatures far above the boi1ing—point, even at a red or perhaps a white heat, we have to consider the effects produced by the same cause upon rocks already solidiﬁed. The simplest and most obvious result of pressure upon such rocks is their consoli- dation, as where a mass of loose sand is gradually compacted into a more or less coherent stone, or where a layer of vegetation is compressed into peat, ligiiite, or coal. In many cases the cohesion of a sedimentary rock is due merely to the pressure of the superincumbent strata. But it usua.lly happens that some cementing material has contri- buted to bind the component particles together. Of these natural cements the most frequent are peroxide of iron, silica, and carbonate of lime. Pressure equally distributed over a rock presenting every- where nearly the same amount of resistance will promote consolidation, but may produce no further internal change. If, however, the pressure becomes extremely unequal, or if the rock subjected to it can find escape from the inﬂuence in one or more directions, there will be a disturbance or rearrangement of the particles, which by this means are made to move upon each other. Five consequences of these movements may be noticed here. (1.) Cleava_r]e.—Wl1eii a mass of rock, owing to subsidence or any other cause, is subjected to powerful lateral compres- sion, its innate particles, which in all rocks have almost invariably a longer and shorter axis, tend, under the intense strain, to rearrange themselves in the line of least resist- ance, that is, with their long axes perpendicular to the direction of the pressure. The result of this readjustment is that the rock affected by it acquires a facility for splitting along the lines in which its component particles have placed themselves. Fine-grained argillaceoiis rocks show most characteristically this internal change ,' but in coarse materials it becomes less conspicuous, or even disappears. Rocks which have been thus acted 011, and have acquired this superinduced ﬁssility, are said to be cleaz'e«_l, and the ﬁssile structure is termed clerwage. This has been proved experimentally by Sorby, who produced perfect cleavage in pipeclay through which scales of oxide of iron had previously been mixed. Dr Tyndall superiiiduced cleavage on bees-wax and other substances by subjecting them to severe pressure. Cleavage among rocks occurs on a great scale in countries where the strata have been greatly plicated, that is, where they now occupy much less horizontal surface than they once did, and consequently where, in accommodating themselves to their diminished area, they have had to undergo much powerful lateral compression. The structure of districts with cleaved rocks is described in part iv. (2.) Further evidence of the compression to which rocks have been subjected is furnishecl by the way in which con- tiguous pebbles in a conglomerate may be found to have been squeezed into each other, and even sometimes to have been elongated in a certain general direction. It is doubt- less the coarseness of the grain of such rocks which permits the effects of compression to be so readily seen. Similar effects must take place in ﬁne-grained rocks, though they escape observation. Organic remains both of plants and animals may often be found to have undergone consider- able distortion from this cause. M. Daubrée has imitated e_xperinientally the indentations produced by the coi1- tiguous portions of conglomerate pebbles} 1 Comptc: Rendus, xliv. 823. GEOLOGY 261 (3.) The ingenious experiments of ‘.I. Tresca on the flow of solids have proved that, even at ordinary atmospheric temperatures, solid resisting bodies like lead, cast-iron, and ice, may be so compressed as to undergo an internal motion of their parts which is closely analogous to that of ﬂuids. Thus, a solid jet of lead has been produced by placinga piece of the metal in a cavity between the jaws of a power- ful compressing machine. Iron, in like manner, has been forced to flow in the solid state into cavities and take their shape. On cutting sections of the metals so compressed, their particles or crystals are found to have ranged them- selves iii lines of ﬂow which follow the contour of the space into which they have been squeezed. Such experiments are of considerable geological interest, for they show that in certain circumstances, under great pressure, the unequally mixed particles of rocks within the earth’s crust may have bccn forced to move upon each other, and thus to acquire a “ ﬂuid-structure ” resembling that which is seen in rocks which have possessed true liquidity. No large sheet of rock can be expected, however, to have undergone this internal change; the effects could only be produced excep- tionally at places where there was an escape from the pres- sure as, for instance, along the sides of fissures, or in other cavities of rocks. The explanation caimot be applied to the case of rocks like schists, which display a kind of rude foliation or ﬂuid-structure over areas many thousands of square miles in extent. (4.) Plicatio-n.——Ret'erence has already been made to the fact that, owing to the more rapid contraction of the inner portion of the globe, the outer layer or crust is from time to time forced to adjust itself to this change by subsidinv. As a consequence of the subsidence, the descending area requires to occupy less horizontal space, and must therefore suffer powerful lateral compression. The rocks are thus crumpled up, as, in the classic experiment of Sir James Hall, folds of cloth are folded when a weight is placed upon them and they are squeezed from either side. The mere subsidence of such a curved surface as that of our globe must thus necessarily produce much lateral compres- sion. Mr J. M. Wilson has calculated that, if a tract of the earth’s surface, 345 miles in breadth, be depressel one mile, it will undergo compression to the extent of 121 yards ; at two miles the compression will be 189 yards; at eight miles 598 yards. The observed amount of com- pression in districts of contorted rocks, however, far exceeds these ﬁgures. Another cause of the compression and contortion of rocks is the injection into them of igneous masses from below, but this is probably a minor source of disturbance. The character of plicated rocks is described in part iv. p. 300. (5.) Faults.—Closely connected with the disturbances which have produced contortions come those by which the crust of the earth has been fractured. But in this case the movement is one of elevation rather than of subsidence ; for, instead of having to occupy a diniinished diameter, the rocks get more room by being pushed up, and as they cannot occupy the additional space by any elastic expansion of their mass, they can only accommodate themselves to the new position by a series of dislocations. Some portions will be pushed up farther than others, and this will happen more particularly to those which have a broad base. These will rise more than those with narrow bottoms, or the latter will seem to sink relatively to the former. Each broad- bottomed segment will thus be bounded by two sides slop- ing towards the upper part of the block. This is found ‘to be almost invariably the casein nature. A fault or dis- location is nearly always inclined from the vertical, and the side to which the inclination rises, and from which it “hades,” is the uptlirow side. The details of these features of geological structure are discussed in part iv., section v.