Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/315

Rh CURVATURE or nocKs.] along the axis of that anticline ﬁnds himself getting into progressively higher strata, as the fold sinks down. On the other hand, in advancing southwards along the synclinal axis, he loses stratum after stratum and gets into lower portions of the series. When a fold diminishes in this way it is said to “nose out.” In ﬁg. 21 there is obviously a general inclination of the beds towards the north, besides the outward dip from the anticline and the inward dip from the syncline. Hence the anticline noses out to the north and the syncline to the south. - It occasionally happens that the maximum movement either of upheaval or subsidence has taken place not along a line of axis but at some one point. Hence arise, on the one hand, dome-shaped elevations of strata where the dip is outward from a centre (quaquaversal), round which the beds are disposed in successive parallel layers or rings, and, on the other hand, circular basin—shaped depressions, towards the centre of which there is a general inclination of the rocks. So great has been the compression to which rocks have been subjected during the process of curvature that the folds may often be found inverted. This has taken place I-‘i(:. ‘2-‘3.—Section of inclined axes, showing consequent inversion of strata. abundantly in regions of great plication. The Silurian up- lands of the south of Scotland, for instance, have the arches and troughs tilted in one direction for miles together, so that in one half of each of them the strata lie bottom upwards. It is in large mountain—chains, however, that inversion can be seen on the grandest scale. The Alps furnish numerous striking illustrations. On the north side of that chain the older Tertiary rocks have been so completely turned over for many miles that the lowest beds now form the tops of the hills, while the highest lie deep below them. Indivi- dual mountains, such as the Gliirnisch, present stupendous examples of inversion, great groups of strata being folded over -and over above each other as we might fold carpets.

1-‘IG. .‘4.—Curved and contorted rocks, near Old Head of Kinsalle. (Du Noyer.) Where curvature has been carried so far, we may nearly always discover localities at which it has been so intenr-iﬁed that the strata have been corrugated and crumpled till it becomes almost impossible to follow out any particular bed through the disturbance. On a small scale instances of such extreme contortion may now and then be found at landslips, where ﬁssile shales have been pressed forward by advancing heavy masses of 1nore solid rock. But it is of course among the more plicated parts of mountain-chains that the struc- ture receives it best illustrations. Few travellers who have passed the upper end of the Lake of Lucerne can have failed to notice the remarkable cliffs of contorted rocks near Fluelen. But innumerable examples of equal or even GEOLOGY -3 .201 superior grandeur may be observed among the more precipit- ous valleys of the Swiss Alps. No more impressive testi- mony could be given to the potency of the force by which mountains were upheaved. V. I).1.sLocA'r1oxs or Ilocxs. The movements which the crust of the earth has under- gone have not only folded and corrugated the rocks, but have fractured them in all directions. These dislocations may be either simple ﬁssures, that is, rents without any vertical displacement of the mass on either side, or fclzzlts, that is, rents where one side has been pushed up or has sunk down. It is not always possible in a shattered rock to discriminate between joints and true ﬁssures. The joints indeed have sometimes served as lines along which ﬁssuring has taken place. It is common to meet with traces of fric- tion along the walls of ﬁssures even when no proof of actual vertical displacement can be gleaned. The rock is more or less shattered on either side, and the contiguous faces present numerous slickensided surfaces. Mineral deposits may also commonly be observed encrusting the cheeks of a ﬁssure, or ﬁlling up, together with broken fragments of rock, the space between the two walls. In a large proportion of cases, however, there has been displacement as well as fracture, and the rents have become faults as well as ﬁssures. Faults on a small scale are sometimes sharply—deﬁned lines, as if the rocks had been ,////////////, ' I'////.. ‘/ I ’./:/ 9‘ Aw, " ”_’_ [I-‘re. 2.'z.—Section of clean-cut iault. sliced through and ﬁtted together again after being shifted (ﬁg. In such cases, however, the harder portions of the dislocated rocks will usually be fonnd slickensided. More frequently son1e disturbance has occurred on one or both sides of the fault. Sometimes in a series of strata the beds on the side which has been pushed up are bent down I-‘Ia. 2G.—Section of strata. bent at a line oi’ fault. against the fault, while those on the opposite side are bent up (ﬁg. 26). Most commonly the rocks on both sides are considerably broken, jumbled, and crumpled, so that the line of fracture is marked by a belt or wall—like mass of fragmentary rock. Where a dislocation has occurred through materials of very unequal hardness, such as solid