dated beach with marine shells is usually found at the base of each cliff, and a line of old shore caverns. But the beginner should be warned not to expect to find evidence of the former sojourn of the sea on all those lands which we are nevertheless sure have been submerged at periods comparatively modern; for notwithstanding the enduring nature of the marks left by littoral action on some rocks, especially limestones, we can by no means detect sea-beaches and inland cliffs everywhere. On the contrary, they are, upon the whole, extremely partial, and are often entirely wanting in districts composed of argillaceous and sandy formations, which must, nevertheless, have been upheaved at the same time, and by the same intermittent movements, as the adjoining harder rocks. CHAPTER VII. JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S CRUST. How we obtain an insight at the surface of the arrangement of rocks at great depths-Why the height above sea-level of the successive strata in a given region is so disproportionate to their thickness-Computation of the average annual amount of subaerial denudation-Antagonism of the internal energies of the globe to those acting on its surface-How far the transfer of sediment from the land to a neighbouring sea-bottom may affect subterranean movements-Permanence and mutability of continental and oceanic areas. How we obtain an insight at the surface of the arrangement of rocks at great depths.-The reader has been already informed that in the structure of the earth's crust we often find proofs of the direct superposition of marine to freshwater strata, and also evidence of the alternation of deep-sea and shallowwater formations. Sedimentary deposits cannot become thick if exposed to concurrent denudation. Darwin suggested years ago that all deep sediments accumulated during subsidence of their area. In order to explain how such a series of rocks could be made to form our present continents and islands, we have not only to assume that there have been alternate upward and downward movements of great vertical extent, but that the upheaval in the areas which we at present inhabit has, in later geological times, sufficiently predominated over subsidence so as to cause these portions of the earth's crust to be land instead of sea. The sinking down of a delta beneath the sea-level may cause strata the upward movement might easily be counteracted by the denuding force of such currents aided by such waves as during a gale are known to prevail in the German Ocean. What parts of the bed of the ocean are stationary at present, and what areas may be rising or sinking, is a matter of which we are very ignorant, as the taking of accurate soundings is but of recent date. Newfoundland-bank.—The great bank of Newfoundland may be compared in size to the whole of England. This part of the bottom of the Atlantic is surrounded on three sides by a rapidly deepening ocean, the bank itself being from twenty to fifty fathoms (or from 120 to 300 feet) under water. We are unable to determine by the comparison of different charts, made at distant periods, whether it is undergoing any change of level, but if it be gradually rising we cannot anticipate on that account that it will become land, because the breakers in an open sea would exercise a prodigious force even on solid rock brought up to within a few yards of the surface. We know, for example, that when a new volcanic island rose in the Mediterranean in 1831, the waves were capable in a few years of reducing it to a sunken rock. In the same way currents which flow over the Newfoundlandbank a great part of the year at the rate of two miles an hour, and are known to retain a considerable velocity to near the bottom, may carry away all loose sand and mud and make the emergence of the shoal impossible, in spite of the accessions of mud, sand, and boulders derived occasionally from melting icebergs which, coming from the northern glaciers, are frequently stranded on various parts of the bank. They must often leave at the bottom large erratic blocks which the marine currents may be incapable of moving. 'Needles' and 'No Man's Lands' are portions of cliffs unworn down by the sea; they indicate the former extension of the land up to and beyond them seawards. They are, as it were, outlines of the strata which have been worn away, and which are recognised in the main cliffs of the land. Earth pillars with stones on their tops, are relics of the country worn away all around them. Inland sea-cliffs. In countries where hard limestone rocks abound, inland cliffs have often retained the characters which they acquired when they constituted the boundary of land and sea. Thus, in the Morea, no less than three or even four ranges of cliffs are well preserved, rising one above the other at different distances from the actual shore, the summit of the highest and oldest occasionally attaining 1,000 feet in elevation. A consoli dated beach with marine shells is usually found at the base of each cliff, and a line of old shore caverns. But the beginner should be warned not to expect to find evidence of the former sojourn of the sea on all those lands which we are nevertheless sure have been submerged at periods comparatively modern; for notwithstanding the enduring nature of the marks left by littoral action on some rocks, especially limestones, we can by no means detect sea-beaches and inland cliffs everywhere. On the contrary, they are, upon the whole, extremely partial, and are often entirely wanting in districts composed of argillaceous and sandy formations, which must, nevertheless, have been upheaved at the same time, and by the same intermittent movements, as the adjoining harder rocks. CHAPTER VII. JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH'S CRUST. How we obtain an insight at the surface of the arrangement of rocks at great depths-Why the height above sea-level of the successive strata in a given region is so disproportionate to their thickness-Computation of the average annual amount of subaërial denudation-Antagonism of the internal energies of the globe to those acting on its surface-How far the transfer of sediment from the land to a neighbouring sea-bottom may affect subterranean movements-Permanence and mutability of continental and oceanic areas. How we obtain an insight at the surface of the arrangement of rocks at great depths.-The reader has been already informed that in the structure of the earth's crust we often find proofs of the direct superposition of marine to freshwater strata, and also evidence of the alternation of deep-sea and shallowwater formations. Sedimentary deposits cannot become thick if exposed to concurrent denudation. Darwin suggested years ago that all deep sediments accumulated during subsidence of their area. In order to explain how such a series of rocks could be made to form our present continents and islands, we have not only to assume that there have been alternate upward and downward movements of great vertical extent, but that the upheaval in the areas which we at present inhabit has, in later geological times, sufficiently predominated over subsidence so as to cause these portions of the earth's crust to be land instead of sea. The sinking down of a delta beneath the sea-level may cause strata of fluviatile or even terrestrial origin, such as peat with trees proper to marshes, to be covered by deposits of deep-sea origin. There is also no limit to the thickness of mud and sand which may accumulate in shallow water, provided that fresh sediment is brought down from the wasting land at a rate corresponding to that of the sinking of the bed of the sea. The succession of strata here alluded to would be consistent with the occurrence of gradual downward and upward movements of the land and bed of the sea without any disturbance of the horizontality of the several formations. But the arrangement of rocks composing the earth's crust differs materially from that which would result from a mere series of radial vertical movements. Had the internal energies of the globe only produced such movements, and had the stratified rocks been first formed beneath the sea and then raised above it, without any lateral compression, the geologist would never have obtained an insight into the monuments of various ages, some of extremely remote antiquity. What we have said in Chapter V. of dip and strike, of the folding and inversion of strata, of anticlinal and synclinal flexures, and in Chapter VI. of denudation at different periods, whether subaërial or submarine, must be understood before the student can comprehend what may at first seem to him an anomaly, but which it is his business particularly to understand. I allude to the small height above the level of the sea attained by strata, often many miles in thickness, and about the chronological succession of which, in one and the same region, there is no doubt whatever. Had stratified rocks in general remained horizontal, the waves of the sea would have been enabled during oscillations of level to plane off entirely the uppermost beds as they rose or sank during the emergence or submergence of the land. But the occurrence of a series of formations of widely different ages, all remaining horizontal and in conformable stratification, is exceptional, and for this reason the total annihilation of the uppermost strata has rarely taken place. We owe, indeed, to the side to side movement produced by tangential thrust those anticlinal and synclinal curves of the beds already described (fig. 55, p. 51), which, together with denudation, subaërial and submarine, enable us to investigate the structure of the earth's crust many miles below those points which the miner can reach under other circumstances. I have already shown in fig. 78, p. 68, how, at St. Abb's Head, a series of strata of indefinite thickness may become vertical, and then denuded, so that the edges of the beds alone shall be exposed to view, the altitude of the upheaved ridges being reduced to a moderate height above the sea-level. The breadth of an exposed edge of a stratum is equivalent to its height were the horizontal position maintained. It may be observed that, although the incumbent strata of Old Red Sandstone (d d') are nearly horizontal, yet they will in other places be found so folded as to present vertical strata, the edges of which are abruptly cut off, as in 2, 3, 4 on the right-hand side of the diagram, fig. 55, p. 51. Why the height above sea-level of the successive strata in a given region is so disproportionate to their thickness. -We cannot too distinctly bear in mind how dependent we are, for our power of consulting the different pages of those stony records of which the crust of the globe is composed, on the joint action of the internal energies and agents of denudation, the one in disturbing the original position of rocks, and the other in destroying large portions of them. Why, it may be asked, if the ancient bed of the sea has been in many regions uplifted to the height of two or three miles, and sometimes twice that altitude, and if it can be proved that some single formations are of themselves two or three miles thick, do we so often find several important groups resting one upon the other, yet attaining only the height of a few hundred feet above the level of the sea? The American geologists, after carefully studying the Alleghany or Appalachian mountains, have ascertained that the older fossiliferous rocks of that chain (from the Silurian to the Carboniferous inclusive) are not less than 42,000 feet thick, and if they were now superimposed on each other in the order in which they were deposited, they ought to equal in height the Himalayas with the Alps piled upon them. Yet they rarely reach an altitude of 5,000 feet, and their loftiest peaks are no more than 7,000 feet high. The Carboniferous strata forming the highest member of the series, and containing beds of coal, can be shown to be of shallow-water origin, or even sometimes to have originated in swamps in the open air. But what is more surprising, the lowest part of this great Palæozoic series, instead of having been deposited at the bottom of an abyss more than 40,000 feet deep, consists of sediment (the Potsdam sandstone), evidently spread out on the bottom of a shallow sea on which ripple-marked sands were occasionally formed. This vast thickness of 40,000 feet is estimated by measuring the denuded edges of the vertical strata forming the parallel folds into which the originally horizontal Silurian and Carboniferous rocks had been forced, and which 'crop out' at the surface. A like phenomenon is exhibited in every mountainous country, |