An unconformity may thus be visible at a glance in a quarry or in a cliff-section; but in many cases it may be necessary to trace out the junction for some miles, the degree of tilting of the lower series of beds being but slight. However, the irregular and worn surface of that series will often be a guide; and a still surer piece of evidence will be furnished by the discovery of pebble-beds in the upper series, formed of materials derived from the lower series. It is obviously not necessary, in that case, that the contact between the two rocks should actually be visible at any point. If one rock contains pebbles worn from another, unconformity, whether local or far-reaching, is proved satisfactorily, though the true plane of junction may never be discovered.

Unconformity commonly originates in the raising of strata until they come under the influence of ordinary agents of denudation. They thus form a local land-surface, while deposition goes on elsewhere. At some later time, submergence takes place, and strata are laid down in the excavated hollows of the hills, and finally even across their ridges. There has been, however, a marked interval between the two series, of which no record, beyond, perhaps, a few terrestrial deposits, has been preserved. To fill up this gap, we must go elsewhere, to some region where elevation did not take place at the same time. Hence, could we know the whole world, we could compile a complete history from its stratified deposits, every local unconformity being represented by a conformity somewhere else.

Occasionally, as the Challenger observers have found, even in water 1700 fathoms deep, the soft but partly consolidated sea-floor may be disturbed by currents and may become broken up, irregular lumps of the beds that were formed under conditions more favourable to deposition becoming rolled along and banked together, finally to be covered by a new stratum. This is a special case of unconformity, the cause of which has been a change in the direction of currents, and not an upheaval of the sea-floor. In this way, in the "Carboniferous limestone" of County Dublin, lumps of a similar rock occur, evidently derived from the underlying beds of the same series; remains of corals, sea

1 See W. F. Hume, "The Genesis of the Chalk," Proc. Geol. Assoc., vol. xiii, p. 231.

lilies, &c., may be found within the lumps, and other examples of the same species may have used these rolled blocks as a basis for their growth. In such a case the unconformity must be fairly local in character. The whole series of strata is, indeed, conformable, when judged by the regularity of its bedding; and no long interval is represented by the phenomena of the blocks derived from earlier layers.

Sometimes two or more unconformities may occur in a small space of country. Thus on the shore at Skerries, in the north of County Dublin, the sea has exposed a series of altered shales, which we will call A; a limestone series, B; a gravel series, C; and it is also here and there depositing the present beach, D. The tilting of the beds A and B does not correspond, so that we suspect an unconformity, although the water covers the actual junction. The case is proved, however, by the occurrence of a magnificent old beach in the series B, formed of great flaky pebbles worn from A. A little farther south, certain beds of B have been churned up, as described on p. 217, giving us a second unconformity, but a purely local and trivial one. In the little cliff above, on the other hand, a very obvious unconformity is seen between B and the gravels C, the latter lying across the edges of the beds B, while the surface between has been ground down and striated by glacial action. C contains scratched blocks derived from both B and A, together with a number of materials borne by ice and rivers from distant areas. Finally, the modern beach D is being formed from the decay of all the preceding deposits, and in places resembles, repeating the details with the most interesting accuracy, the very ancient beach exposed in contact with it on the surface of the series B. Thus in three hundred yards or so we have three principal unconformities, and one trifling one, the latter being due to local changes in the conditions under which the strata B were being deposited.

In an unconformable junction, the lowest bed of the upper series crosses, as we have said, the worn-down edges of the lower series; and this bed is said to successively overstep 1 those beneath it, one after another (fig. 20). Again, in the basin-like hollow of a sea or lake, particularly if its floor is sinking, each successive stratum of the upper series

1 This term is due to Mr. J. G. Goodchild; see also Jukes-Browne, "Physical Geology," 2nd edition, p. 548.

extends farther across the lower and unconformable series than does the stratum which lies immediately below it and in conformity with it. Thus the first stratum of the upper series occupies only a small area on the floor of the hollow; the next, resting conformably on it, spreads more widely; and this increase in the area of successive layers becomes the more marked, if the shore against which they are deposited is a gently shelving one. If the coast drops downward, on the other hand, in a vertical wall, as in some Norwegian fjords, this difference between the layers is of course abolished. The spreading of any one layer of a conformable series beyond the limits of the underlying layer or layers of the same series is spoken of as overlap (fig. 17).

FIG. 20.-UNCONFORMABLE JUNCTION, WITH OVERLAP AND OVERSTEP. The series A-D is unconformable to the series 1-6, &c. D oversteps successively 1, 2, 3, &c. C overlaps D, and is overlapped by B, which is in turn overlapped by the youngest bed, A.

Thus in any one district the relative ages of rocks, whether sedimentary or igneous, can be fairly made out, and we can even learn a good deal as to events which took place between the formation of one series and another. But what we want is to be able to go up to a rock-mass in any region, and to discover from its special characters the period at which it was constructed. It is improbable that we shall ever be able to do this in the case of igneous rocks, seeing that lavas and intrusive masses do not seem to have changed their nature from the time of the earliest eruptions until now; and all attempts to classify such rocks according to their geological age have hopelessly broken down. Similarly, sandstones and clays and limestones seem to have existed in all times, and a limestone may be deposited in one part of a sea while a sandstone is being deposited simul

taneously in another. Even the crystalline rocks called schists and gneisses (Chap. X) may belong to very different ages.

Thus we say that, in the case of any rock, "lithological characters," i.e., the nature and structure of the stone itself, are useless in assigning it to its place in the annals of the earth. We have now, however, another and a remarkable guide, which we can employ in any quarter of the globe.

We are all acquainted with fossils (Plate X). By this term we now mean the remains of any plant or animal that are found included in a rock.1 We must use our discretion as to how far we should apply the word "fossil to modern cases of entombment. A goat might become involved in a recent mud-flow on the flanks of a mountain; the rootlets of living plants, again, penetrate a long way down into the crevices of rocks; but neither of these cases gives rise to fossils in the generally accepted sense. Some fossil shells have preserved even their lustre, while the calcium carbonate of others has actually become changed, molecule by molecule, into some other chemical substance. Silica, iron carbonate, iron sulphide, and gypsum, are thus found actually replacing the original shells or hard parts of many organisms; and silicified tree-stems are well known from many parts of the world. Very often, all that is left us is a cast, formed by the fine particles of the rock itself, which have penetrated the hollows of the shell or skeleton and have taken an accurate mould of them. Clay forms excellent internal casts, and the fine limestone-mud associated with shell-banks has a similar effect. Often the substance of the fossil itself has become dissolved away, after its entombment, and a hollow space has arisen; but the enveloping and consolidated rock has at the same time taken an external cast of the fossil, from which many of its characters can be ascertained. Moreover, an internal cast may have been formed as well, which is generally fixed to the surrounding rock at the points where the material penetrated into the hollow of the shell. In some cases, however, the solution of the shell leaves the internal cast to rattle loosely

1 Up to the earlier years of the present century, a "fossil" meant anything dug up (fodio, fossum) out of the earth, and the objects to which the term is now restricted were called "organised fossils."