the sedimentary material has been aggregated into variously shaped concretions. Certain kinds of sedimentary rocks are apt to occur together, such as clays and limestones, clayironstones and shales, coals and fire-clays; because the conditions under which they were respectively deposited were on the whole similar. As a rule, the finer the detritus, the wider the area over which it is spread; hence clays generally cover wider tracts than conglomerates. No inference can safely be drawn from the relative thickness of strata as to the length of time which they respectively represent; they must vary widely in this respect, and it is quite conceivable that, in many cases, the interval of time between the deposition of two successive beds of very different character and composition may have been actually longer than the period required for the deposition of the two beds. A thick series of sedimentary deposits usually indicates that the seabottom on which it was laid down was slowly sinking. In subsiding, the later deposits spread beyond the limits of the earlier ones, and thus present what is called an overlap. Where they have been laid down continuously one upon another they are said to be conformable; where they have been deposited on the disturbed and worn edges of an older series they are unconformable.

CHAPTER XIII.

SEDIMENTARY ROCKS-STRUCTURES SUPERINDUCED IN THEM AFTER THEIR FORMATION.

AFTER their deposition sedimentary materials have undergone various changes before assuming the aspect which they now wear.

CONSOLIDATION. -The most obvious of these changes is that, instead of consisting of loose materials, gravel, sand, mud, and so on, they are now hard stone. This consolidation has sometimes been the result of mere pressure. As bed was piled over bed, those at the bottom would gradually be more and more compressed by the increasing weight of those that were laid down upon them, the water would be squeezed out, and any tendency which the particles might have to cohere would promote the consolidation of the mass. Mud, for example, might in this way be converted into clay, and clay in turn might be pressed into mudstone or shale. But besides cohesion from the pressure of overlying masses, sedimentary matter has often been bound together by some kind of cement, either originally deposited with it or subsequently introduced by permeating water. Among natural cements, the most common are silica, carbonate of lime, and peroxide of iron. In a red sandstone, for example, the quartz grains may be observed to be coated over with earthy iron peroxide, which serves to unite them together into a more or less coherent stone. The effect of weathering is not infrequently to remove the binding cement, and thereby to allow the stone to return to its original condition of loose sediment.

JOINTS.-Next to their consolidation into stone, the most common change which has affected sedimentary rocks is the production in them of a series of divisional planes or fractures termed Joints. Except in loose incoherent materials, this structure is hardly ever absent. In any ordinary quarry of sandstone, limestone, or other 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 defines the faces that are in shadow, the other 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 largely

FIG. 89.-Joints in a stratified rock.

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 rocks 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 seawards from a shelving shore, and they gather at steeper angles in slopes of debris at the foot of cliffs, and 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 which, though originally deposited for the most part over the sea-bottom, now form so much of the dry land, we find them inclined at all angles, and even sometimes standing 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 dry 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