of sandstone 20 feet thick may have accumulated round a submerged tree in a few years. On the other hand, a corresponding depth of fine laminated clay may have required tenfold more time for its deposition. But the same thickness of rock composed of alternations of shale and limestone might represent a still longer period. For it is obvious that the change from one kind of sediment to another must often have been brought about by an extremely gradual modification of the geography of the region from which the supply of sediment was derived. Hence the interval between two beds or groups of beds, differing much from each other in mineral composition, may have been considerably longer than the time required for the actual deposition of the strata of either or both beds or groups of beds.

On any probable estimate, the deposition of sedimentary rocks, to a depth of many thousand feet and over areas many thousands of square miles in extent, must have demanded enormous periods of time. Side by side with the growth of mechanical sediments, there must have been a corresponding wasting of land. Every bed of conglomerate, sand, or mud represents at least an equivalent amount of rock worn away from the land and transported as sediment to the floor of the sea. During such prolonged ages as these changes required, there was ample time for the outburst of many successive volcanoes, for the passage of many earthquake-shocks, and for the subsidence or upheaval of many parts of the earth's crust.

Proofs of Subsidence.-A mass of sedimentary material of

FIG. 86.-Hills formed out of horizontal sedimentary rocks.

great thickness which, from the remains of sun-cracks and other evidence, was obviously deposited in shallow water near land can only have been accumulated on an area that was gradually sink

ing. Suppose, for instance, that a hill formed out of such strata rises a thousand feet above the valley at its foot (Fig. 86), and that proofs of deposition in shallow water can be detected from the lowest beds all the way up to the highest. The lowest beds having once been close to the surface, as shown by the sun-cracks and other evidence, could only be covered with hundreds of feet of similar strata by a gradual sinking of the ground, during which fresh sediment was poured in, so that, although the original bottom sank a thousand feet, the water may never have become sensibly deeper, the rate of deposit of sediment having, on the whole, kept pace with that of the subsidence.

Overlap. During such tranquil movements, as the area of land lessens and that of the sea increases, the later sedimentary accumulations must needs extend beyond the limits of the older ones. Suppose, for instance, that such a sloping land-surface as that

represented in the section (x, Fig 87) were slowly to subside beneath the sea, the first-formed strata (a) will be covered and overlapped by the next series (6), and these in turn, as the seafloor sinks, will be similarly concealed by the following group (c). This structure, termed Overlap, may usually be regarded as evidence of a gentle subsidence of the area of deposit.

Conformability, Unconformability.-When stratified deposits are laid down regularly and continuously upon each other, with no interruption of their generally level position, they are said to be conformable. In the section Fig. 80, for instance, the series of sediments there represented has evidently been deposited under the same general conditions. The nature of the sediment has of course varied from time to time; limestones, shales, and sandstones have alternated with each other; but there has been no marked interruption or disturbance in their sequence. Suppose, however, that owing to subterranean movements, a series of rocks (a in Fig. 88) is shifted from its original position, and after being uplifted, is exposed to the wearing action of the sea, rivers, air, rain, frosts, and the other agents concerned in the degradation of

the surface of the land. If a new series of deposits (6) is laid down upon the denuded edges of these rocks, the bedding of the whole will not be continuous. The younger strata will rest successively upon different parts of the older group, or, in other words, will be unconformable. Such a relation or unconformability (unconformity) implies a terrestrial disturbance, and usually also the lapse of a long interval of time between the respective periods of the older and younger rocks, during which denudation of the older strata took place. It serves to mark one of the breaks or gaps in geological history. Unconformabilities differ much from each other in regard to the length of interval which they denote. In some cases, the blank may be of comparatively slight moment; in others, it is so vast as to include the greater part of the time represented by the stratified rocks of the earth's crust.

By means of unconformabilities the different ages of mountainchains are determined. If, for example, a mountain showed the structure represented in Fig. 88, its upheaval must obviously have taken place between the deposition of the two series of rocks. Suppose the series a to represent Lower Silurian, and 6 Carboniferous rocks, the date of the mountain would be between the Lower Silurian and Carboniferous periods. If, in another mountain, series were unconformably overlain by a younger series, say of Jurassic age, this mountain would thereby be shown to have undergone a subsequent uplift in the long interval between the Carboniferous and the Jurassic periods.

Of

Summary. In this Lesson some of the more characteristic original features of sedimentary rocks have been considered. these features, one of the most distinctive is the arrangement into layers of beds, each of which is the record of a portion of geological history, the oldest being below and the youngest above. The smallest subdivision of these records is a lamina or thin leaf, such as those into which shales may be split. A stratum or bed,

which may contain many laminæ or none, is a thicker layer separable with more or less ease from those below and above it. Though strata lie on the whole parallel with each other, they often show oblique current-bedding, especially in sandstones. Traces of shore-lines and of surfaces laid bare by the retirement of the water in which they were deposited, are found in sun-cracks, rainpittings, and footprints. Not infrequently, instead of being evenly spread out in layers, 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, clay-ironstones 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 one group has been deposited on the disturbed and worn edges of an older series the two are unconformable to each other.

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