shown from the Berwickshire coast, wherein hard beds of greywacke and shale have been folded. Again, in Fig. 95, the reverse structure is exhibited, beds of grit and slate being there curved into a trough. Where rocks dip away from a central line of axis the structure is known as an Anticline; where, on the other hand, they dip towards an axis it is called a Syncline. In Figs. 94 and 95 these two structures are presented on so small a scale as to be visible in a single section. More usually, however, it is only by

observing the upturned edges of strata that anticlines and synclines can be detected. The dark part of Fig. 96 represents all that can be actually seen; but the angles and direction of dip leave no doubt that if we could restore the amount of rock which has here been worn away from the surface of the land, the present truncated ends of the strata would be prolonged upward in some such way as is indicated by the dotted lines. By observations of this truncation of strata some of the most interesting and important evidence is obtained of the enormous extent to which the land has been reduced by the removal of solid material from its surface.

Plication, Shearing. From such simple curvatures as those

depicted in the foregoing diagrams, we may advance to more complex foldings, wherein the solid strata have been doubled up and crumpled together, as if they had been mere layers of carpet. So far is this plication sometimes carried, that the lowest rocks are brought up and thrown over the highest, the more yielding materials being squeezed into the most intricate frillings and puckerings. It is in mountainous regions, where the crust of the earth has been subjected to the most intense corrugation, that

these structures are best seen. We can form some idea of the gigantic energy of the earth-movements that produced them, when we see a whole mountain-range made up of solid limestones or sandstones which have been bent, twisted, crumpled, and inverted, as we might crush up sheets of paper (Fig. 97).

So enormous has been the compression produced by important movements of the earth's crust, that the solid rocks have actually been squeezed out of shape or have undergone a process of shearing. The amount of distortion may sometimes be measured by the extent to which shells or other organic remains are pulled out in the direction of movement. In Fig. 98 the proper shape of a

trilobite (Angelina Sedgwickii) is given, and alongside of it is a view of the same organism which has been elongated by the distortion of the mass of rock in which it lies. Further results of

a

b

a

shearing will be immediately referred to in connection with the cleavage and metamorphism of rocks.

Cleavage. One of the most important structures developed by the great compression to which the rocks of the earth's crust have been exposed is known as Cleavage. The minute particles of rocks, being usually of irregular shapes, have been compelled to arrange themselves with their long axes perpendicular to the direction of pressure during the interstitial movements consequent

FIG. 97.-Section of folded and crumpled strata forming the Grosse Windgälle (10,482 feet), Canton Uri, Switzerland, showing crumpled and inverted strata (after Heim).

upon intense subterranean compression. Hence, a fissile tendency has been imparted to a rock, which will now split into leaves along the planes of rearrangement of the particles. This superinduced tendency to split into parallel leaves, irrespective of what may have been the original structure of the rock, constitutes cleavage. It is well developed in ordinary roofing-slate. Though

the leaves or plates into which a slate splits resemble those in a shale, they have no necessary relation to the layers of deposition but may cross them at any angle. In Fig. 99, for instance, the original bedding is quite distinct and shows that the strata have been folded by a force acting from the right and left of the section; the parallel highly inclined lines traversing the folds of the bedding Where the material is of exrepresent the planes of cleavage. ceedingly fine grain, such as fine consolidated mud, the original bedding may be entirely effaced by the cleavage, and the rock will only split along the cleavage-planes. Indeed, the finer the grain of a rock, the more perfect may be its cleavage, so that where alternations of coarser and finer sediment have been sub

FIG. 98.-Distortion of fossils by the shearing of rocks; (a) a Trilobite (Angelina Sedgwickii) distorted by shearing, the direction of movement indicated by the arrows; (b) the same fossil in its natural form.

jected to the same amount of compression, cleavage may be perfect in the one and rudely developed in the other, as is indicated in Fig. 99.

Cleavage may be regarded as one of the first stages in the mechanical deformation of a rock, and the production of schistose metamorphism (p. 167). Besides being compressed and having its component particles rearranged in definite planes, the rock may likewise reveal under the microscope that new minerals, such for example as crystallites or minute flakes of some mica, have been developed out of the general matrix, as may be seen in common roofing-slate. By increasing stages of crystallisation we trace gradations into phyllites and mica-schists.

Dislocation. Another important structure produced in rocks after their formation is Dislocation. Not only have they been folded by the great movements to which the crust of the earth has been subjected, but the strain upon them has often been so great that they have snapped across. Such ruptures of continuity pre

sent an infinite variety in the position of the rocks on the two sides. Sometimes a mere fissure has been caused, the rocks being simply cracked across, but remaining otherwise unchanged in their relative

FIG. 99.-Curved and cleaved rocks. Coast of Wigtonshire. The fine parallel oblique lines indicate the cleavage, which is finer in the dark shales and coarser in the thicker sandy beds.

situations. But, in the great majority of instances, one or both of the walls of a fissure have moved, producing what is termed a Fault. Where the displacement has been small, a fault may appear as if the strata had been sharply sliced through, shifted, TANIU

and firmly pressed together again (a in Fig. 100). Usually, however, they have not only been cut, but bent or crushed on one or both sides (b); while not infrequently the line of fracture is represented by a band of broken and crushed material (Fault-rock, c). The fracture is seldom quite vertical; almost always it is inclined