Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/312

Rh 298 But a third set may usually be noticed cutting across the columns, though less continuous and dominant than the others. When these transverse joints are few in number or occasionally absent, columns many feet in length can be quarried out entire. Such iiionoliths have been from early times employed in the construction of obelisks and pillars. In rocks of ﬁner grain than granite, such as many diorites and dolerites, the numerous perpendicular joints give the rock a prismatic character. The prisms however are unequal in dimensions, as well as in the number and pro- portions of their sides, a frequent diameter being 2 or 3 feet, though they may sometimes be observed three times thicker, and extending up the face of a cliff for 300 or 400 feet. It is by means of joints that precipitous faces of rock are produced and retained, for, as in the case of those in stratiﬁed masses, they serve as openings into which FIG. 16.-—Joints in granite, Killin_ey iin1,—17i1bii:i. (G. V. Du .'oycr.) frost drives every year its wedges of ice, whereby huge slices are stripped off. They likewise give rise to the formation of those fantastic pinnacles and fretted buttresses so generally to be observed among igneous rocks in which they occur. But undoubtedly the most striking series of joints to be found among igneous rocks is in the regularly colunniar, or as it is often called, basaltic structure. This structure has been already (ante, p. 249) described in connexion with modern volcanic rocks. .It may be met with in rocks of all ages. It is as well displayed among the felsites of the Lower Qld Red Sandstone, and the basalts of the Carboni- ferous Limestone in central Scotland, as among the Tertiary lavas of Auvergne or the Vivarais. _ _3. I n F_'oliatecl Ifoc/cs.-—The schists likewise possess their joints, which approximate in character to those among the inassive igneous rocks, but they are on the whole less dis- tinct and continuous, while ‘their effect in dividing the rocks into oblong masses is considerably modiﬁed by the trans- verse lines of foliation. These lines play somewhat the same part as those of stratification do among the stratiﬁed rocks, though with less deﬁniteiiess and precision. Ill. INcLi.'.i'.rioN or ROCKS. The most casual observation is sufficient to satisfy us that the rocks now visible at the earth’s surface are seldom in their original position. lVe meet with sandstones and conglomerates composed of water-worn particles, yet form- ing the angular scarps of lofty mountains ; shales and clays full of the remains of fresh-water shells and land—plants, yet covered by limestones made up of marine organisms, and these limestones rising into great ranges of hills, or undu- lating into fertile valleys, and passing under the streets of busy towns. Such facts, now familiar to every reader, and even to inany observers who know little or nothing of systematic geology, point unmistakably to the conclusion that the rocks have in many cases been formed under water, sometimes in lakes, more frequently in the sea, and that they have been elevated into land. But further examination discloses other and not less GEOLOGY [iv. STI-'.UCTURAL. convincing evidence of movement. Judging from what takes place at the present time on the bottoms of lakes and of the sea, we confidently infer that when the strata now constituting so much of the solid framework of the land were formed, they were l:iid down either horizontally or at least at low angles. When, therefore, we ﬁnd them iii- clined at all angles, and even standing on end, we conclude that they have been (listurbetl. Over wide spaces they have been upraised bodily with little alteration of their original horizontality ; but in most places some departure from that original position has been effected. The inclination thus given to rocks is termed their dip. Its amount is expressed in degrees measured from the plane of the horizon. Thus a set of rocks liali"-way between the horizontal and vertical position would be said to dip at an angle of -15°, while if vertical they would be marked with the angle of 90°. The edges of strata, where they come up to the surface, are termed their outcrop or brzsset. When they crop out, that is, rise to the surface, along a perfectly level piece of ground, the outcrop runs at a right angle to the dip. But any inequalities of the surface, such as valleys, ravines, hills, and ridges will cause the outcrop to describe a circuitous course, even though the dip should remain perfectlysteady all the while. If a line of precipit- ous gorge should run directly with the dip, the outcrop will there be coincident with the dip. The occurrence of a ~ ' Al/I‘.-l - ,?‘Hlfl" l :1 -. " .,. _,- yr.-,4""’_.-‘“'.J='_ ‘ ' i it 2; pl: .. . ’ ..I ' I - , . .57: i ,;|-,‘u -. ~' I 5.‘... -_o, r,')— -.‘ Fia. 17.—'ertic:il strata, originally dcposEt-d liorizciitally or at low angles. gently shelving valley in that position will cause the out- crop to descend on one side and to mount in a correspond- ing way oii the other, so as to form a V-shaped indentation in its course. A ridge, on the other hand, will produce a deﬂexion in the opposite direction. Hence a series of parallel ridges and valleys running in the same direction as- tlie dip of the strata underneath would cause the outcrop to describea widely serpentinous course. Agaiii, should the rocks be vertical, the outcrop will necessarilycorrespond with the dip, and continue to do so irrespective altogether of any irregularities of the ground. The lower therefore the angle of inclination the greater is the effect of surface inequalities upon the line of outcrop; the higher the angle the less is that inﬂuence, till when the beds stand on end it ceases. A line drawn at a right angle to the dip is called the slri/re of the rocks. From what has just been said this line must coincide with outcrop when the surface of the ground is quite level, and also when the beds are vertic-ll. At all other times they are not strictly coincident, but the outcrop wanders to anrl fro across the strike according to the changes in the angle of inclination and in the form of the ground. The strike may be a straight line or may curve rapidly in every direction, according to the behaviour of the dip. If, for instance, a set of beds dips for half a mile continuously to the north, the strike will run for that distance as astraight east and west line. If the dip gradu- ally changes to north-west and west, and then by south- west to south, it is obvious that the strike must curve round by north-east, north, and north-west till it once more
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