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diff --git a/old/3772-h/files/ch6.html b/old/3772-h/files/ch6.html new file mode 100644 index 0000000..557bcd9 --- /dev/null +++ b/old/3772-h/files/ch6.html @@ -0,0 +1,610 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 96">[ 96 ]</a></p> + +<p> </p> + +<center><b>Chapter VI</b><br> +<br> +DENUDATION</center> + +<p class="intro">Denudation defined. — Its Amount more than +equal to the entire Mass of Stratified Deposits in the Earth’s +Crust. — Subaërial Denudation. — Action of the +Wind. — Action of Running Water. — Alluvium defined. +— Different Ages of Alluvium. — Denuding Power of +Rivers affected by Rise or Fall of Land. — Littoral +Denudation. — Inland Sea-Cliffs. — Escarpments. — +Submarine Denudation. — Dogger-bank. — Newfoundland +Bank. — Denuding Power of the Ocean during Emergence of +Land.</p> + +<p>Denudation, which has been occasionally spoken of in the +preceding chapters, is the removal of solid matter by water in +motion, whether of rivers or of the waves and currents of the sea, +and the consequent laying bare of some inferior rock. This +operation has exerted an influence on the structure of the earth’s +crust as universal and important as sedimentary deposition itself; +for denudation is the necessary antecedent of the production of all +new strata of mechanical origin. The formation of every new deposit +by the transport of sediment and pebbles necessarily implies that +there has been, somewhere else, a grinding down of rock into +rounded fragments, sand, or mud, equal in quantity to the new +strata. All deposition, therefore, except in the case of a shower +of volcanic ashes, and the outflow of lava, and the growth of +certain organic formations, is the sign of superficial waste going +on contemporaneously, and to an equal amount, elsewhere. The gain +at one point is no more than sufficient to balance the loss at some +other. Here a lake has grown shallower, there a ravine has been +deepened. Here the depth of the sea has been augmented by the +removal of a sandbank during a storm, there its bottom has been +raised and shallowed by the accumulation in its bed of the same +sand transported from the bank.</p> + +<p>When we see a stone building, we know that somewhere, far or +near, a quarry has been opened. The courses of stone in the +building may be compared to successive strata, the quarry to a +ravine or valley which has suffered denudation. As the strata, like +the courses of hewn stone, have been laid one upon another +gradually, so the excavation both of the valley and quarry have +been gradual. To pursue the comparison still farther, the +superficial heaps of mud, sand, and gravel, usually called +alluvium, may be likened to the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 97">[ 97 ]</a></p> + +<p>rubbish of a quarry which has been rejected as useless by the +workmen, or has fallen upon the road between the quarry and the +building, so as to lie scattered at random over the ground.</p> + +<p>But we occasionally find in a conglomerate large rounded pebbles +of an older conglomerate, which had previously been derived from a +variety of different rocks. In such cases we are reminded that, the +same materials having been used over and over again, it is not +enough to affirm that the entire mass of stratified deposits in the +earth’s crust affords a monument and measure of the denudation +which has taken place, for in truth the quantity of matter now +extant in the form of stratified rock represents but a fraction of +the material removed by water and redeposited in past ages.</p> + +<p><b>Subaërial +Denudation.</b>—Denudation may be divided into +subaërial, or the action of wind, rain, and rivers; and +submarine, or that effected by the waves of the sea, and its tides +and currents. With the operation of the first of these we are best +acquainted, and it may be well to give it our first attention.</p> + +<p><i>Action of the Wind.</i>—In desert regions where no rain +falls, or where, as in parts of the Sahara, the soil is so salt as +to be without any covering of vegetation, clouds of dust and sand +attest the power of the wind to cause the shifting of the +unconsolidated or disintegrated rock.</p> + +<p>In examining volcanic countries I have been much struck with the +great superficial changes brought about by this power in the course +of centuries. The highest peak of Madeira is about 6050 feet above +the sea, and consists of the skeleton of a volcanic cone now 250 +feet high, the beds of which once dipped from a centre in all +directions at an angle of more than 30°. The summit is formed +of a dike of basalt with much olivine, fifteen feet wide, +apparently the remains of a column of lava which once rose to the +crater. Nearly all the scoriæ of the upper part of the cone +have been swept away, those portions only remaining which were +hardened by the contact or proximity of the dike. While I was +myself on this peak on January 25, 1854, I saw the wind, though it +was not stormy weather, removing sand and dust derived from the +decomposing scoriæ. There had been frost in the night, and +some ice was still seen in the crevices of the rock.</p> + +<p>On the highest platform of the Grand Canary, at an elevation of +6000 feet, there is a cylindrical column of hard lava, from which +the softer matter has been carried away; and other similar remnants +of the dikes of cones of eruption</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 98">[ 98 ]</a></p> + +<p>attest the denuding power of the wind at points where running +water could never have exerted any influence. The waste effected by +wind aided by frost and snow, may not be trifling, even in a single +winter, and when multiplied by centuries may become indefinitely +great.</p> + +<center><img src="../images/fig80.jpg" width="367" height="126" alt= +"Fig. 80: Section through several eroded formations."></center> + +<p><i>Action of Running Water.</i>—There are different +classes of phenomena which attest in a most striking manner the +vast spaces left vacant by the erosive power of water. I may +allude, first, to those valleys on both sides of which the same +strata are seen following each other in the same order, and having +the same mineral composition and fossil contents. We may observe, +for example, several formations, as Nos. 1, 2, 3, 4, in the diagram +(Fig. 80): No. 1, conglomerate, No. 2, clay, No. 3, grit, and No. +4, limestone, each repeated in a series of hills separated by +valleys varying in depth. When we examine the subordinate parts of +these four formations, we find, in like manner, distinct beds in +each, corresponding, on the opposite sides of the valleys, both in +composition and order of position. No one can doubt that the strata +were originally continuous, and that some cause has swept away the +portions which once connected the whole series. A torrent on the +side of a mountain produces similar interruptions; and when we make +artificial cuts in lowering roads, we expose, in like manner, +corresponding beds on either side. But in nature, these appearances +occur in mountains several thousand feet high, and separated by +intervals of many miles or leagues in extent.</p> + +<p>In the “Memoirs of the Geological Survey of Great Britain” (vol. +i), Professor Ramsay has shown that the missing beds, removed from +the summit of the Mendips, must have been nearly a mile in +thickness; and he has pointed out considerable areas in South Wales +and some of the adjacent counties of England, where a series of +primary (or palæozoic) strata, no less than 11,000 feet in +thickness, have been stripped off. All these materials have of +course been transported to new regions, and have entered into the +composition of more modern formations. On the other hand, it is +shown by</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 99">[ 99 ]</a></p> + +<p>observations in the same “Survey,” that the Palæozoic +strata are from 20,000 to 30,000 feet thick. It is clear that such +rocks, formed of mud and sand, now for the most part consolidated, +are the monuments of denuding operations, which took place on a +grand scale at a very remote period in the earth’s history. For, +whatever has been given to one area must always have been borrowed +from another; a truth which, obvious as it may seem when thus +stated, must be repeatedly impressed on the student’s mind, because +in many geological speculations it is taken for granted that the +external crust of the earth has been always growing thicker in +consequence of the accumulation, period after period, of +sedimentary matter, as if the new strata were not always produced +at the expense of pre-existing rocks, stratified or unstratified. +By duly reflecting on the fact that all deposits of mechanical +origin imply the transportation from some other region, whether +contiguous or remote, of an equal amount of solid matter, we +perceive that the stony exterior of the planet must always have +grown thinner in one place, whenever, by accessions of new strata, +it was acquiring thickness in another.</p> + +<p>It is well known that generally at the mouths of large rivers, +deltas are forming and the land is encroaching upon the sea; these +deltas are monuments of recent denudation and deposition; and it is +obvious that if the mud, sand, and gravel were taken from them and +restored to the continents they would fill up a large part of the +gullies and valleys which are due to the excavating and +transporting power of torrents and rivers.</p> + +<p><b>Alluvium.</b>—Between the +superficial covering of vegetable mould and the subjacent rock +there usually intervenes in every district a deposit of loose +gravel, sand, and mud, to which when it occurs in valleys the name +of alluvium has been popularly applied. The term is derived from +<i>alluvio</i>, an inundation, or <i>alluo</i>, to wash, because +the pebbles and sand commonly resemble those of a river’s bed or +the mud and gravel washed over low lands by a flood.</p> + +<p>In the course of those changes in physical geography which may +take place during the gradual emergence of the bottom of the sea +and its conversion into dry land, any spot may either have been a +sunken reef, or a bay, or estuary, or sea-shore, or the bed of a +river. The drainage, moreover, may have been deranged again and +again by earthquakes, during which temporary lakes are caused by +landslips, and partial deluges occasioned by the bursting of the +barriers of such lakes. For this reason it would be unreasonable +to</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 100">[ 100 ]</a></p> + +<p>hope that we should ever be able to account for all the alluvial +phenomena of each particular country, seeing that the causes of +their origin are so various. Besides, the last operations of water +have a tendency to disturb and confound together all pre-existing +alluviums. Hence we are always in danger of regarding as the work +of a single era, and the effect of one cause, what has in reality +been the result of a variety of distinct agents, during a long +succession of geological epochs. Much useful instruction may +therefore be gained from the exploration of a country like +Auvergne, where the superficial gravel of very different eras +happens to have been preserved and kept separate by sheets of lava, +which were poured out one after the other at periods when the +denudation, and probably the upheaval, of rocks were in progress. +That region had already acquired in some degree its present +configuration before any volcanoes were in activity, and before any +igneous matter was superimposed upon the granitic and fossiliferous +formations. The pebbles therefore in the older gravels are +exclusively constituted of granite and other aboriginal rocks; and +afterwards, when volcanic vents burst forth into eruption, those +earlier alluviums were covered by streams of lava, which protected +them from intermixture with gravel of subsequent date. In the +course of ages, a new system of valleys was excavated, so that the +rivers ran at lower levels than those at which the first alluviums +and sheets of lava were formed. When, therefore, fresh eruptions +gave rise to new lava, the melted matter was poured out over lower +grounds; and the gravel of these plains differed from the first or +upland alluvium, by containing in it rounded fragments of various +volcanic rocks, and often fossil bones belonging to species of land +animals different from those which had previously flourished in the +same country and been buried in older gravels.</p> + +<center><img src="../images/fig81.jpg" width="372" height="125" alt= +"Fig. 81: Lavas of Auvergne resting on alluviums of different ages."> +</center> + +<p>The annexed drawing (Fig. 81) will explain the different heights +at which beds of lava and gravel, each distinct from the other in +composition and age, are observed, some on the flat tops of hills, +700 or 800 feet high, others on the slope of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 101">[ 101 ]</a></p> + +<p>the same hills, and the newest of all in the channel of the +existing river where there is usually gravel alone, although in +some cases a narrow strip of solid lava shares the bottom of the +valley with the river.</p> + +<p>The proportion of extinct species of quadrupeds is more numerous +in the fossil remains of the gravel No. 1 than in that indicated as +No. 2; and in No. 3 they agree more closely, sometimes entirely, +with those of the existing fauna. The usual absence or rarity of +organic remains in beds of loose gravel and sand is partly owing to +the friction which originally ground down the rocks into small +fragments, and partly to the porous nature of alluvium, which +allows the free percolation through it of rain-water, and promotes +the decomposition and removal of fossil remains.</p> + +<p>The loose transported matter on the surface of a large part of +the land now existing in the temperate and arctic regions of the +northern hemisphere, must be regarded as being in a somewhat +exceptional state, in consequence of the important part which ice +has played in comparatively modern geological times. This subject +will be more specially alluded to when we describe, in the eleventh +chapter, the deposits called “glacial.”</p> + +<p><b>Denuding Power of Rivers affected by Rise +or Fall of Land.</b>—It has long been a matter of +common observation that most rivers are now cutting their channels +through alluvial deposits of greater depth and extent than could +ever have been formed by the present streams. From this fact it has +been inferred that rivers in general have grown smaller, or become +less liable to be flooded than formerly. It may be true that in the +history of almost every country the rivers have been both larger +and smaller than they are at the present moment. For the rainfall +in particular regions varies according to climate and physical +geography, and is especially governed by the elevation of the land +above the sea, or its distance from it and other conditions equally +fluctuating in the course of time. But the phenomenon alluded to +may sometimes be accounted for by oscillations in the level of the +land, experienced since the existing valleys originated, even where +no marked diminution in the quantity of rain and in the size of the +rivers has occurred.</p> + +<p>We know that many large areas of land are rising and others +sinking, and unless it could be assumed that both the upward and +downward movements are everywhere uniform, many of the existing +hydrographical basins ought to have the appearance of having been +temporary lakes first filled with fluviatile strata and then +partially re-excavated.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 102">[ 102 ]</a></p> + +<p>Suppose, for example, part of a continent, comprising within it +a large hydrographical basin like that of the Mississippi, to +subside several inches or feet in a century, as the west coast of +Greenland, extending 600 miles north and south, has been sinking +for three or four centuries, between the latitudes 60° and +69° N.* It will rarely happen that the rate of subsidence will +be everywhere equal, and in many cases the amount of depression in +the interior will regularly exceed that of the region nearer the +sea. Whenever this happens, the fall of the waters flowing from the +upland country will be diminished, and each tributary stream will +have less power to carry its sand and sediment into the main river, +and the main river less power to convey its annual burden of +transported matter to the sea. All the rivers, therefore, will +proceed to fill up partially their ancient channels, and, during +frequent inundations, will raise their alluvial plains by new +deposits. If then the same area of land be again upheaved to its +former height, the fall, and consequently the velocity, of every +river will begin to augment. Each of them will be less given to +overflow its alluvial plain; and their power of carrying earthy +matter seaward, and of scouring out and deepening their channels, +will be sustained till, after a lapse of many thousand years, each +of them has eroded a new channel or valley through a fluviatile +formation of comparatively modern date. The surface of what was +once the river-plain at the period of greatest depression, will +then remain fringing the valley-sides in the form of a terrace +apparently flat, but in reality sloping down with the general +inclination of the river. Everywhere this terrace will present +cliffs of gravel and sand, facing the river. That such a series of +movements has actually taken place in the main valley of the +Mississippi and in its tributary valleys during oscillations of +level, I have endeavoured to show in my description of that +country;† and the fresh-water shells of existing species and +bones of land quadrupeds, partly of extinct races, preserved in the +terraces of fluviatile origin, attest the exclusion of the sea +during the whole process of filling up and partial +re-excavation.</p> + +<p><b>Littoral Denudation.</b>—Part +of the action of the waves between high and low watermark must be +included in subaërial denudation, more especially as the +undermining of cliffs by the waves is facilitated by land-springs, +and these often lead to the sliding down of great masses of land +into the sea. Along our coasts we find numerous submerged +forests,</p> + +<p class="fnote">* Principles of Geology 7th ed., p. 506; 10th ed., +vol. ii, p. 196.<br> +† Second Visit to the United States, vol. i, chap. +xxxiv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 103">[ 103 ]</a></p> + +<p>only visible at low water, having the trunks of the trees erect +and their roots attached to them and still spreading through the +ancient soil as when they were living. They occur in too many +places, and sometimes at too great a depth, to be explained by a +mere change in the level of the tides, although as the coasts waste +away and alter in shape, the height to which the tides rise and +fall is always varying, and the level of high tide at any given +point may, in the course of many ages, differ by several feet or +even fathoms. It is this fluctuation in the height of the tides, +and the erosion and destruction of the sea-coast by the waves, that +makes it exceedingly difficult for us in a few centuries, or even +perhaps in a few thousand years, to determine whether there is a +change by subterranean movement in the relative level of sea and +land.</p> + +<p>We often behold, as on the coasts of Devonshire and +Pembrokeshire, facts which appear to lead to opposite conclusions. +In one place a raised beach with marine littoral shells, and in +another immediately adjoining a submerged forest. These phenomena +indicate oscillations of level, and as the movements are very +gradual, they must give repeated opportunities to the breakers to +denude the land which is thus again and again exposed to their +fury, although it is evident that the submergence is sometimes +effected in such a manner as to allow the trees which border the +coast not to be carried away.</p> + +<p><b>Inland Sea-cliffs.</b>—In +countries where hard limestone rocks abound, inland cliffs have +often retained faithfully for ages 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 1000 feet in elevation. A consolidated beach +with marine shells is usually found at the base of each cliff, and +a line of littoral caverns. These ranges of cliff probably imply +pauses in the process of upheaval when the waves and currents had +time to undermine and clear away considerable masses of rock.</p> + +<p>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</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 104">[ 104 ]</a></p> + +<p>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.</p> + +<p><b>Escarpments.</b>—Besides the +inland cliffs above alluded to which mark the ancient limits of the +sea, there are other abrupt terminations of rocks of various kinds +which resemble sea-cliffs, but which have in reality been due to +subaërial denudation. These have been called “escarpments,” a +term which it is useful to confine to the outcrop of particular +formations having a scarped outline, as distinct from cliffs due to +marine action.</p> + +<p>I formerly supposed that the steep line of cliff-like slopes +seen along the outcrop of the chalk, when we follow the edge of the +North or South Downs, was due to marine action; but Professor +Ramsay has shown* that the present outline of the physical +geography is more in favour of the idea of the escarpments having +been due to gradual waste since the rocks were exposed in the +atmosphere to the action of rain and rivers.</p> + +<p>Mr. Whittaker has given a good summary of the grounds for +ascribing these apparent sea-cliffs to waste in the open air. 1. +There is an absence of all signs of ancient sea-beaches or littoral +deposits at the base of the escarpment. 2. Great inequality is +observed in the level of the base line. 3. The escarpments do not +intersect, like sea-cliffs, a series of distinct rocks, but are +always confined to the boundary-line of the same formation. 4. +There are sometimes different contiguous and parallel +escarpments—those, for example, of the greensand and +chalk—which are so near each other, and occasionally so +similar in altitude, that we can not imagine any existing +archipelago if converted into dry land to present a like +outline.</p> + +<p>The above theory is by no means inconsistent with the opinion +that the limits of the outcrop of the chalk and greensand which the +escarpments now follow, were originally determined by marine +denudation. When the south-east of England last emerged from +beneath the level of the sea, it was acted upon, no doubt, by the +tide, waves, and currents, and the chalk would form from the first +a mass projecting above the more destructible clay called Gault. +Still the present escarpments so much resembling sea-cliffs have no +doubt, for reasons above stated, derived their most characteristic +features subsequently to emergence from subaërial waste by +rain and rivers.</p> + +<p class="fnote">* Physical Geography and Geology of Great Britain, +p. 78, 1864.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 105">[ 105 ]</a></p> + +<p><b>Submarine +Denudation.</b>—When we attempt to estimate the amount +of submarine denudation, we become sensible of the disadvantage +under which we labour from our habitual incapacity of observing the +action of marine currents on the bed of the sea. We know that the +agitation of the waves, even during storms, diminishes at a rapid +rate, so as to become very insignificant at the depth of a few +fathoms, and is quite imperceptible at the depth of about sixteen +fathoms; but when large bodies of water are transferred by a +current from one part of the ocean to another, they are known to +maintain at great depths such a velocity as must enable them to +remove the finer, and sometimes even the coarser, materials of the +rocks over which they flow. As the Mississippi when more than 150 +feet deep can keep open its channel and even carry down gravel and +sand to its delta, the surface velocity being not more than two or +three miles an hour, so a gigantic current, like the Gulf Stream, +equal in volume to many hundred Mississippis, and having in parts a +surface velocity of more than three miles, may act as a propelling +and abrading power at still greater depths. But the efficacy of the +sea as a denuding agent, geologically considered, is not dependent +on the power of currents to preserve at great depths a velocity +sufficient to remove sand and mud, because, even where the +deposition or removal of sediment is not in progress, the depth of +water does not remain constant throughout geological time. Every +page of the geological record proves to us that the relative levels +of land and sea, and the position of the ocean and of continents +and islands, has been always varying, and we may feel sure that +some portions of the submarine area are now rising and others +sinking. The force of tidal and other currents and of the waves +during storms is sufficient to prevent the emergence of many lands, +even though they may be undergoing continual upheaval. It is not an +uncommon error to imagine that the waste of sea-cliffs affords the +measure of the amount of marine denudation of which it probably +constitutes an insignificant portion.</p> + +<p><b>Dogger-bank.</b>—That great +shoal called the Dogger-bank, about sixty miles east of the coast +of Northumberland, and occupying an area about as large as Wales, +has nowhere a depth of more than ninety feet, and in its shallower +parts is less than forty feet under water. It might contribute +towards the safety of the navigation of our seas to form an +artificial island, and to erect a light-house on this bank; but no +engineer would be rash enough to attempt it, as he would feel sure +that the ocean in the first heavy gale would</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 106">[ 106 ]</a></p> + +<p>sweep it away as readily as it does every temporary shoal that +accumulates from time to time around a sunk vessel on the same +bank.*</p> + +<p>No observed geographical changes in historical times entitle us +to assume that where upheaval may be in progress it proceeds at a +rapid rate. Three or four feet rather than as many yards in a +century may probably be as much as we can reckon upon in our +speculations; and if such be the case, the continuance of 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.</p> + +<p><i>Newfoundland Bank.</i>—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 can not 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.</p> + +<p>In the same way currents which flow over the Newfoundland bank 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, but the same rocky fragments may be made to sink by the +undermining of beds consisting of finer matter on which the blocks +and gravel repose. In this way gravel and boulders may continue to +overspread a submarine bottom after the latter has been lowered for +hundreds of feet, the</p> + +<p class="fnote">* Principles, 10th ed., vol. i, p. 569.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 107">[ 107 ]</a></p> + +<p>surface never having been able to emerge and become land. It is +by no means improbable that the annual removal of an average +thickness of half an inch of rock might counteract the ordinary +upheaval which large submarine areas are undergoing; and the real +enigma which the geologist has to solve is not the extensive +denudation of the white chalk or of our tertiary sands and clays, +but the fact that such incoherent materials have ever succeeded in +lifting up their heads above water in an open sea. Why were they +not swept away during storms into some adjoining abysses, the +highest parts of each shoal being always planed off down to the +depth of a few fathoms? The hardness and toughness of some rocks +already exposed to windward and acting as breakwaters may perhaps +have assisted; nor must we forget the protection afforded by a +dense and unbroken covering of barnacles, limpets, and other +creatures which flourish most between high and low water and +shelter some newly risen coasts from the waves.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch5.html"> +Chapter V</a> / <a href="ch7.html">Chapter VII</a></small> +</body> +</html> + |