sedimentary rock, or along a natural cliff of the same materials, a little attentive observation will show that the bare wall of rock forming the back of the quarry or the face of the cliff has been determined by one or more natural fissures in the stone, and that there are other fissures running parallel with it through every outstanding buttress of rock. Moreover, we may observe that these vertical or highly inclined lines of fissure are cut across by others, more or less nearly at a right angle, and that the sides of the buttresses have been defined by these transverse lines, just as the main face of rock has been formed by the

first set. Such lines of division are Joints. In close-grained stone they may be imperceptible until it is quarried or broken, when they reveal themselves as sharply defined, nearly vertical fractures, along which the stone splits. There are usually at least two series of joints crossing each other at right angles or obliquely, whereby a rock is divided into quadrangular blocks. In the accompanying diagram (Fig. 89) a group of stratified rocks is seen to be traversed by two sets of joints, one of which (dipjoints, "cutters" of the quarrymen) defines the faces that are in shadow, the other (strike-joints, “backs” of the quarrymen) those that are in light. By help of these divisional planes, it is possible to obtain large blocks of stone for building purposes. The art of the quarryman, indeed, largely consists in taking advantage of these natural lines of fracture, so as to obtain his materials with the least expenditure of time and labour, and in large masses. In nature also the existence of joints is a fact of the highest importance. Reference has already been made to the way in which they afford a passage for the descent of water from the surface. It is in great measure along joints that the underground circulation of water is conducted. At the surface, too, where rocks yield to the decomposing influence of the weather, it is by their joints that they are chiefly split up. Along these convenient planes of division, rain-water trickles and freezes; the walls of the joints are separated, and the space between them is slowly widened, until in the end it opens into yawning rents, and portions of a cliff are overbalanced and fall, while detached pinnacles are here and there isolated. The picturesqueness of the scenery of stratified rock is, in great measure, dependent upon the influence of joints in promoting their dislocation and disintegration by air, rain, and frost.

In many cases, joints may be due to contraction. A mass of sand or mud, as it loses water and as its particles are more firmly united to each other, gradually occupies less room than at first. In consequence of the contraction strains are set up in the stone, and relief from these is eventually found in a system of cracks or fissures. In other instances, joints have been produced by the compression or torsion to which large masses of rock have been exposed during movements of the earth's crust.

Original Horizontality. — As laid down upon the margin or floor of the sea, on the bottoms of lakes, and on the beds or alluvial plains of rivers, sedimentary accumulations are in general nearly flat. They slope gently, indeed, seawards from a shelving shore, and they gather at steeper angles on slopes of debris at the foot of cliffs, or down the sides of mountains. But, taken as a whole, and over wide areas, their original position is not far removed from the horizontal. If we turn, however, to the sedimentary rocks that form so large a part of the earth's crust, and so much of the dry land, we find that although originally deposited for the most part over the sea-bottom, they are now inclined at all angles, and even sometimes stand on end. Such situations, in which their deposition could never have taken place, show that they have been disturbed. Not only have they been upraised into land, but they have been tilted unequally, some parts rising or sinking much more than others.

Dip. The inclination of bedded rocks from the horizon is called their Dip. The amount of dip is reckoned from the plane of the horizon. A face of rock standing up vertically above that plane is said to be at 90°, while midway between that position and horizontally it lies at an inclination of 45°. The angle of dip is accurately measured with an instrument called a Clinometer, of which there are various forms. One of the simplest kinds is a brass half-circle graduated into 90° on each side of the vertical, on which a pendulum is hung as in Fig. 91. The instrument is held between the eye and the angle to be measured, and the upper

FIG. 90.-Dip and Strike. The arrow shows the direction of dip; the line
ss marks the strike.

edge is made to coincide with the line of the inclined rock. The pendulum, remaining vertical, points to the angle of inclination from the horizon. A little practice, however, enables an observer to estimate the amount of dip by the eye with sufficient accuracy for most purposes. The direction of dip is the point of the compass toward which a stratum is inclined (shown by the arrow in Fig. 90), and is best ascertained with a magnetic compass.

But

here again a little experience in judging of the quarters of the sky without an instrument will usually enable us to tell the direction of dip with as much precision as may be required.

Strike. A mathematical line running at a right angle to the direction of dip is called the Strike (s s in Figs. 90, 92). Where a series of strata dips due north or due south the strike is east and west; but the direction of strike changes with that of the dip. Suppose, for example, that certain strata dip due east, then veer round by south-east to south, and so on by west and north, back to east again. The strike following this change would describe a circle. In fact, the beds would be included in a basin-shaped or dome-shaped arrangement and the strike would be the lip of the basin or rim of the truncated dome. vary from place to place, still, if it direction along the line of certain whole uniform.

Though the dip may slightly remains in the same general strata, their strike is on the

Outcrop. The actual edge presented by a stratum at the surface of the ground is called its Outcrop. On a perfectly level

surface, strike and outcrop must coincide; but as ground is seldom quite level they usually diverge from each other, and do so the more in proportion to the lowness of angle of dip and the inequalities of the ground. This may be illustrated by a diagram such as that given in Fig. 92, which represents a portion of the edge of a table-land, deeply trenched by two valleys that discharge their waters into the plain below (P). The arrows point out that the strata dip due N. at 5°. On the level plain, the outcrop and the strike (s s) of the beds are coincident and run due E. and W. But as the surface rises towards the high ground and the deep valleys, the outcrop (oo) is observed to depart more and more from the strike till in some places the two lines are at right angles ; yet, as the dip remains the same, the strike is likewise unchanged, the sinuosities of the outcrop being entirely due to the irregularities of the surface of the ground.

Curvature. It requires no long observation to perceive that in being tilted from their original more or less level positions, stratified rocks have been thrown into curves. Suppose, for instance, that in walking along a mile of coast-line, where all the successive strata of a thick series are exposed to view, we should

A

B

FIG. 93.-Inclined strata shown to be parts of curves.

observe such a section as is drawn in Fig. 93. Beginning at A, we find the beds tilted up at angles of 70° which gradually lessen, till at B they have sunk to 15°. As there is no break in the series, it is evident that the lines of bedding must be prolonged downward, and must once have been continued upward in some such way as is expressed by the dotted lines. The visible portion which is here shaded must thus form part of a great curvature of the rocks.

But the actual curvature may often be seen on coast-cliffs, ravines, or hillsides. In Fig. 94, for example, a simple arch is