probably for the most part caused by subsidence in the earth's crust. In adjusting themselves to the new position into which a downward movement brings them, rocks must often be subject to such strains that their limit of elasticity is reached, and they break across, one portion settling down farther than the part next to it. In a normal fault, the same bed can never be cut twice by a vertical line.

In mountainous districts, however, and generally where the rocks of the earth's crust have been disrupted and pushed over each other, what are termed reversed faults occur. In these, the hade slopes in the direction of up

FIG. IOI.-Sections to show the relations of Plications (a, b) to reversed Faults (c).

throw, and a vertical line may cut the same beds twice on opposite sides of the fracture (Fig. 101). Such faults may be observed more particularly where strata have been much folded. A fold may be seen to have snapped asunder, the whole being pushed over, and the upper side being driven forward over the lower.

The amount of vertical displacement between the two fractured ends of a bed is called the Throw of a fault. In Fig. 102, for example, where bed a has been shifted from b to

d, a vertical line dropped from the end of the bed at b to the level of the corresponding part of the bed at e will give the amount of the subsidence of a', which is the throw. Faults may be seen with a throw of less than an inch

mere local cracks and trifling subsidences in a mass of rock; in others the throw may be many thousand feet. Large faults often bring rocks of entirely different characters together, as for instance, shales against limestones or sandstones, or sedimentary against eruptive rocks. Consequently they are not infrequently marked at the surface by the difference between the form of ground characteristic of the two kinds of rock. One side, perhaps, rises into a hilly or undulating region, while the other side may be a plain. Comparatively seldom does a fault make itself visible as a line of ravine or valley. On the contrary, most faults cut across valleys or only coincide with them here and there. They run in straight or wavy lines which, where the amount of displacement is great, may be traced for many miles. The Scottish Highlands, for example, are bounded along their southern margin by a great fault which places a thick series of sandstones and conglomerates on end against the flanks of the mountains. This fault may be traced across the island from sea to sea-a distance of fully 120 miles, and by bringing two distinct kinds of rocks next each other along a nearly straight line it has given rise to the boundary between Highland and Lowland scenery which, in some places, is so singularly abrupt.

METAMORPHISM. -The last structure, which will be mentioned in this chapter as having been superinduced upon rocks, is connected with the movements to which reversed faults are due. So enormous has been the energy with which these movements have been carried on, that not only have the rocks been crumpled, ruptured, and pushed over each other, but they have undergone such intense shearing that their original structure has been partially or wholly effaced. They have been so crushed that their com

ponent particles have been reduced, as it were, to powder, and have assumed new crystalline arrangements along the shearing-planes or surfaces of movement. A sandstone, for example, which in its ordinary state shows, when magnified, such a structure as is represented in Fig. 103, when it has come within the influence of this crushing process has its grains of quartz, felspar, and other materials squeezed against each other in one general direction, while a good deal of mica has been developed out of the crushed debris. This change is intensified until the component grains are hardly recognisable, and where the proportion of new mica has so increased that the rock has become a mica-schist. This alteration is known as (regional) metamorphism (see p. 266).

There are wide regions of the earth's surface where schists of various kinds form the prevailing rock. Whether they have all been produced by the shearing and alteration of previously-formed rocks has not yet been determined, if, indeed, it can ever be ascertained. But that a large number of schists are truly altered or metamorphosed rocks admits of no doubt. Sandstones, shales, limestones, quartzites, diorites, syenites, granites, in short, any old form of rock that has come within the crushing and shearing movements here referred to has been converted into schist. The gradation between the unaltered and the metamorphic condition can often be clearly traced. Granite, by crushing, passes into gneiss, diorite into hornblende-schist, sandstone into quartz-schist or mica-schist, and so on. Even where it is no longer possible to tell what the original nature of the metamorphosed material may have been, there is usually abundant evidence that the rock has undergone great compression.

Summary. In this Lesson attention has been directed to new structures produced in sedimentary rocks after their formation. Beginning with the simplest and most universal of these, we find that sediments have been consolidated into stone, partly by pressure, and partly by some kind of cement, such as silica or carbonate of lime. In the process of consolidation and contraction, they have been traversed by systems of joints, or have had these subsequently produced by the torsion accompanying movements of the crust. Though at first nearly flat, they have by these movements been thrown into various inclined positions, and more especially into undulating folds, or more complicated plication and puckering. So great has been the compression under which they have been moved, that a cleavage has been developed in them. They have also been everywhere more or less fractured, the dislocations being due either to their gradual subsidence or to excessive plication. Their most complete alteration is seen in metamorphism, where, under the influence of intense shearing, their original structure has been more or less completely effaced, and a new crystalline rearrangement has been developed in them, converting them into schists.