In by far

at angles varying up to 70° or more from the vertical. the largest number of faults, the inclination of the plane of the fissure, or what is called the Hade of the fault, is away from the side which has risen or toward that which has sunk. In the examples given in Fig. 100, a, b, this relation is expressed; but in nature it often happens that the beds on two sides of a fault are entirely different (Fig. 100, c), and consequently that the side of upthrow or downthrow cannot be determined by the identification of the two severed positions of the same bed. But if the hade of the fault can be seen, we may usually be confident that the strata on the upper or hanging side belong to a higher part of the series than those on the lower side. Faults that follow this rule (normal faults) are by far the most frequent. They occur universally, and are 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

122

b

each other, what are termed reversed faults occur. In these, the hade slopes in the direction of upthrow, 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 6 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 d, 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.

In regions of the most intense terrestrial disturbance, tracts of rock many square miles in area and hundreds or thousands of feet in thickness, have been torn away and pushed upward and forward until they have come to rest on rocks originally much higher in geological position. Such displaced cakes or slices of the earth's crust sometimes rest upon an almost horizontal or gently inclined platform of undisturbed materials. Vertical or

contorted strata are thus placed above others which may be flat or but little inclined. The plane of separation between the moved and unmoved masses is really a dislocation, but to distinguish it from faults, which are generally placed at steep angles, it is called a Thrust-plane. Structures of this kind on a colossal scale are traceable for about 100 miles in the north-west of Scotland.

Metamorphism. The last structure, which will be mentioned in this chapter as having been superinduced upon rocks, is connected with the movements to which plication, cleavage, and 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, as was pointed out at p. 168, their original structure has been partially or wholly effaced. They have been so crushed that their component 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 flattened and squeezed against each other in one general direction as in cleavage, while out of the crushed debris a good deal of new mica has been developed. This change may be intensified until the component grains are hardly recognisable, and the proportion of new mica has so increased that the rock has become a micaschist. Other new minerals, such as garnet, may likewise make

structure.

their appearance, until the rock assumes an entirely crystalline Such an alteration of the internal structure of a rock is known as Metamorphism. Where the change arises from mechanical movements combined with chemical rearrangement, it usually affects a wide district, and is then spoken of as regional metamorphism.

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. 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 (see pp. 167-170).

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.

CHAPTER XIV

ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHITECTURE OF THE EARTH'S CRUST

NOT only have sedimentary formations since their deposition been hardened, plicated, fractured, and sometimes even turned into crystalline schists, but into the rents opened in them new masses of mineral matter have been introduced which, in many regions, have entirely changed the structure of the crust below and the appearance of the surface above. Broadly speaking, there are two ways in which these new masses have been wedged into their places. First of all, eruptive material in a molten, or at least in a viscous or plastic condition, has been thrust upward into the cool and consolidated crust of the earth; and in the next place, various ores and minerals have been deposited from solution in cracks and fissures, which they have entirely filled up. Το each of these two kinds of later rocks attention will be given in this chapter.

Eruptive Rocks.

The rise of eruptive matter thrust upwards from lower depths within the planet is one of the causes by which the structure of the crust has been most seriously affected. In Chapter IX reference was made to some of the features connected with the protrusion of molten rocks in the production of volcanoes, and more particularly to those subterranean changes which, when all the outer and ordinary tokens of a volcano have been swept away, remain as evidence of former volcanic action, even in districts where every symptom of volcanic activity has long vanished. We have now to inquire, generally, in what forms eruptive matter has been built into the earth's crust, and what