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DIVISION II
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See also:DIVISION II .—EPIGENE OR SUPERFICIAL See also:ACTION It is on the See also:surface of the globe, and by the operation of agents working there, that at See also:present the See also:chief amount of visible geo- logical See also:change is effected . In considering this See also:branch of inquiry, we are not involved in a preliminary difficulty regarding the very nature of the agencies as is the See also:case in the investigation of plutonic action . On the contrary, the surface agents are carrying on their See also:work under our very eyes . We can See also:watch it in all its stages, measure its progress, and See also:mark in many ways how accurately it represents similar changes which, for See also:long ages previously, must have been effected by the same means . But in the systematic treatment of this subject we encounter a difficulty of another See also:kind; We discover thatwhile the operations to be discussed are numerous and readily observable, they are so interwoven into one See also:great network that any separation of them under different subdivisions is sure to be more or less artificial and to convey an erroneous impression . While, therefore, under the unavoidable See also:necessity of making use of such a See also:classification of subjects, we must always See also:bear in mind that it is employed merely for convenience, and that in nature superficial See also:geological action must be continually viewed as a whole, since the work of each See also:agent has See also:constant reference to that of the others,, and is not properly intelligible unless that connexion be kept in view . The movements of The See also:air; the evaporation from See also:land and See also:sea; the fall of See also:rain, See also:hail and See also:snow; the flow of See also:rivers and glaciers; the tides, currents and waves of the ocean; the growth and decay of organized existence, alike on land and in the depths of the sea;-in See also:short, the whole circle of See also:movement, which is continually in progress upon the surface of our See also:planet, are the subjects now to be examined . It is desirable to adopt some See also:general See also:term to embrace the whole of this range of inquiry . For this end the word epigene (Gr. See also:iris upon) has been suggested as a convenient term, and antithetical to hypogene (Gr. fnro, under), or subterranean action . A See also:simple arrangement of this See also:part of Geological See also:Dynamics is in three sections: A . Air.—The See also:influence of the See also:atmosphere in destroying and forming rocks . B . See also:Water.—The geological functions of the circulation of water through the air and between sea and land, and the action of the sea . C . See also:Life.—The part taken by See also:plants and animals in preserving, destroying or reproducing geological formations . The words destructive, reproductive and conservative, employed in describing the operations of the epigene agents, do not necessarily imply that anything useful to See also:man is destroyed, reproduced or preserved . On the contrary, the destructive action of the atmosphere may turn barren See also:rock into See also:rich See also:soil, while its reproductive effects sometimes turn rich land into barren See also:desert . Again, the conservative influence of vegetation has sometimes for centuries retained as barren morass what might otherwise have become rich meadow or luxuriant See also:wood-land . The terms, therefore, are used in a strictly geological sense, to denote the removal and re-deposition of material, and its agency in preserving what lies beneath it . (A) The Air . As a geological agent, the air brings about changes partly by its component gases and partly by its movements . Its destructive action is both chemical and See also:mechanical . The chemical changes are probably mainly, if not entirely, due to the moisture of the air, and particularly to the gases, vapours and organic See also:matter which the moisture contains . Dry air seems to have little or no appreciable influence in promoting these reactions .
As the changes in question are similar to those much more abundantly brought about by rain they are described in the following See also:section under the See also:division on rain
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Among the more recognizable mechanical changes effected in the atmosphere, one of considerable importance is to be seen in the result of great and rapid changes of temperature
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See also:Heat expands rocks, while See also:cold contracts them
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In countries with a great See also:annual range of temperature, considerable difficulty is sometimes experienced in selecting See also:building materials liable to be little affected by the alternate expansion and contraction, which prevents the See also:joints of See also:masonry from remaining See also:close and tight
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In dry tropical climates, where the days are intensely hot and the nights extremely cold, the rapid nocturnal contraction produces a See also:strain so great as to See also:rival See also:frost in its influence upon the surface of exposed rocks, disintegrating them into See also:sand, or causing them to crack or See also:peel off in skins or irregular pieces
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Dr See also:Livingstone found in See also:Africa (12° S. See also:lat., 34° E. long.) that surfaces of rock which during the See also:day were heated up to 137° Fahr., cooled so rapidly by. See also:radiation at See also:night that, unable to sustain the strain of contraction, they split and threw off See also:sharp angular fragments from a few ounces to See also:loo or 200 lb in See also:weight
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In temperate regions this action, though much less pronounced, still makes itself See also:felt
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In these climates, however, and still more in high latitudes, somewhat similar results are brought about by frost
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By its See also:motion in See also:wind the air drives loose sand over rocks, and in course of See also:time abrades and smoothes them
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" Desert See also:polish " is the name given to the characteristic lustrous surface thus imparted
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Holes are said to be drilled in window See also:glass at Cape See also:Cod by the same agency
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Cavities are now and then hollowed out of rocks by the gyration in them of little fragments of See also: Hurricanes See also:form important geological agents upon land in uprooting trees, and thus sometimes impeding the drainage of a See also:country and giving rise to the formation of See also:peat mosses . The reproductive action of the air arises partly from the effect of the chemical and mechanical disintegration involved in the See also:process of " weathering," and partly from the transporting See also:power of wind and of aerial currents . The layer of soil, which covers so much of the surface of the land, is the result of the decay of the underlying rocks, mingled with See also:mineral matter blown over the ground by wind, or washed thither by rain, and with the mouldering remains of plants and animals . The extent to which See also:fine dust may be transported over the surface of the land can hardly be realized in countries clothed with a covering of vegetation, though even there, in dry See also:weather during See also:spring, clouds of dust may often be seen blown away by wind from See also:bare ploughed See also:fields . Intercepted by the leaves of plants and washed down to their roots by rain, this dust goes to increase the soil below . In arid climates, where dust clouds are dense and frequent, enormous quantities of fine mineral particles are thus See also:borne along and accumulated . The remarkable See also:deposit of " See also:Loess," which is sometimes more than 15oo ft. thick and covers extensive areas in See also:China and other countries, is regarded as due to the drifting of dust by wind . Again the See also:dunes of sand so abundant along the inner See also:side of sandy sea-beaches in many different parts of the See also:world are attributable to the same action . (B) Water . In treating of the epigene action of water in geological processes it will be convenient to See also:deal first with its operations in traversing the land, and then with those which it performs in the sea . The circulation of water from land to sea and again from sea -to land constitutes the fundamental cause of most of the daily changes by which the surface of the land is affected . 1 .. Rain.—Rain effects two kinds of changes upon the surface of the land . It acts chemically upon soils and stones, and sinking underground continues a great See also:series of similar reactions there . It acts mechanically, by washing away loose materials, and thus powerfully affecting the contours of the land . Its chemical action depends mainly upon the nature and proportion of the substances which, in descending to the See also:earth, it abstracts from the atmosphere . Rain always absorbs a little air, which, in addition to its See also:nitrogen and See also:oxygen, contains carbonic See also:acid, and in See also:minute proportions, See also:sodium chloride, sulphuric acid and other ingredients, especially inorganic dust, organic particles and living germs . Probably the most generally efficient of these constituents are oxygen, carbonic acid and organic matter . Armed with these reagents, rain effects a chemical decomposition of the rocks on which it falls, and through which it sinks underground . The See also:principal changes thus produced are as follows: (a) Oxidation.—Owing to the prominence of oxygen in rain-water, and its readiness to unite with any substance which can contain more of it, a thin oxidized pellicle is formed on the surface of many rocks on which rain falls, and this oxidized layer if not at once washed off, sinks deeper until a crust is formed over the stone . A See also:familiar See also:illustration of this action is afforded by the See also:rust, or See also:oxide, which forms on See also:iron when exposed to moisture, though this iron may be kept long See also:bright if allowed to remain screened from moist air and rain . (b) Deoxidation.—Organic matter having an See also:affinity for more oxygen decomposes peroxides by depriving them of some part of their See also:share of that See also:element and reducing them to protoxides . These changes are especially noticeable among the iron oxides so abundantly diffused among rocks . Hence rain-water, in sinking through soil and obtaining such organic matter, becomes thereby a reducing agent . (c) See also:Solution.—This may take See also:place either by the simple action of the water, as in the solution of rock-See also:salt, or by the influence of the carbonic acid present in the rain . (d) Formation of See also:Carbonates.—A familiar example of the action of carbonic acid in rain is to be seen in the corrosion of exposed See also:marble slabs . The carbonic acid dissolves some of the See also:lime, which, as a bicarbonate, is held in solution in the carbonated water, but is deposited again when the water loses its carbonic acid or evaporates . It is not merely carbonates, however, which are liable to this kind of destruction . Even silicates of lime, potash and soda, combinations existing abundantly as constituents of rocks, are attacked; their See also:silica is liberated, and their alkalis or alkaline earths, becoming carbonates, are removed in solution . (e) Hydration.—Some minerals, containing little or no water, and therefore called anhydrous, when exposed to the action of the atmosphere, absorb water, or become hydrous, and are then usually more prone to further change . Hence the rocks of which they form part become disintegrated . Besides the reactions here enumerated, a considerable amount of decay may be observed as the result of the presence of sulphuric and nitric acid in the air, especially in that of large towns and manufacturing districts, where much See also:coal is consumed . Metallic surfaces, as well as various kinds of stone, are there corroded, while the See also:mortar of walls may often be observed to be slowly swelling out and dropping off, owing to the See also:conversion of the lime into sulphate . Great injury is likewise done from a similar cause to marble monuments in exposed graveyards . The general result of the disintegrating action of the air and of rain, including also that of plants and animals, to be noticed in the sequel, is denoted by the term " weathering." The amount of decay depends partly on conditions of See also:climate, especially the range of temperature, the abundance of moisture, height above the sea and exposure to prevalent winds . Many rocks liable to be saturated with rain and rapidly dried under a warm See also:sun are See also:apt to disintegrate at the surface with See also:comparative rapidity . The nature and progress of the weathering are mainly governed by the See also:composition and texture of the rocks exposed to it . Rocks composed of particles liable to little chemical change from the influence of moisture are best fitted to resist weathering, provided they possess sufficient cohesion to withstand the mechanical processes of disintegration . Siliceous sandstones are excellent examples of this permanence . Consisting wholly or mainly of the durable mineral See also:quartz, they are sometimes able so to withstand decay that buildings made of them still retain, after the See also:lapse of centuries, the See also:chisel-marks of the builders . Some rocks, which yield with comparative rapidity to the chemical attacks of moisture, may show little or no mark of disintegration on their surface . This is particularly the case with certain calcareous rocks . See also:Limestone when pure is wholly soluble in acidulated water . Rain falling on such a rock removes some of it in solution, and will continue to do so until the whole is dissolved away . But where a limestone is full of impurities, a weathered crust of more or less insoluble particles remains after the solution of the calcareous part of the stone . Hence the relative purity of limestones may be roughly determined by examining their weathered surfaces, where, if they contain much sand, the grains will be seen projecting from the calcareous See also:matrix, and where, should the rock be very ferruginous, the yellow hydrous peroxide, or ochre, will be found as a powdery crust . In limestones containing abundant encrinites, shells, or other organic remains, the weathered surface commonly presents the fossils See also:standing out in See also:relief . The crystalline arrangement of the lime in the organic structures enables them to resist disintegration better than the general mechanically aggregated matrix of the rock .
An experienced fossil See also:collector will always See also:search well such weathered surfaces, for he often finds there, delicately
picked out by the weather, minute and frail fossils which are wholly invisible on a freshly broken surface of the stone
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Many rocks weather with a thick crust, or even decay inwards for many feet or yards
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See also:Basalt, for example, often shows a yellowish-See also: But further See also:attention will show such an impression to be singularly erroneous . It loses sight of the fact that a change which may be hardly appreciable within a human lifetime, or even within the comparatively brief span of geological time embraced in the See also:compass of human See also:history, may nevertheless become gigantic in its results in the course of immensely protracted periods . An instructive See also:lesson in the erosive action of rain may be found in the pitted and channelled surface of ground lying under the drip of the See also:eaves of a cottage . The fragments of stone and pebbles of See also:gravel that form part of the soil can there be seen sticking out of the ground, because being hard they resist the impetus of the falling drops, protecting for a time the earth beneath them, while that which surrounded and covered them is washed away . From this familiar illustration the observer may advance through every See also:stage in the disappearance of material which once covered the surface, until he comes to examples where once continuous and thick sheets of solid rock have been reduced to a few fragments or have been entirely removed . Since the whole land surface over which rain falls is exposed to this See also:waste, the superficial covering of decayed rock or soil, as See also:Hutton insisted, is constantly, though imperceptibly, travel-See also:ling outward and downward to the sea . In this process of transport rain is an important carrying agent, while at the same time it serves to connect the work of the other disintegrating forces, and to make it conducive to the general degradation of the land . Though this decay is general and constant, it is obviously not See also:uniform . In some places where, from the nature of the rock, from the flatness of the ground, or from other causes, rain See also:works 'under great difficulties, the See also:rate of waste may be extremely slow . In other places it may be rapid enough to be appreciable from See also:year to year . A survey of this See also:department of geological activity shows how unequal wasting by rain, combined with the operations of brooks and rivers, has produced the details of the present relief of the land, those tracts where the destruction has been greatest forming hollows and valleys, others, where it has been less, rising into ridges and hills (Part VIII.) . Rain-action is not merely destructive, but is accompanied with reproductive effects, chief of which is the formation of soil . In favourable situations it has gathered together accumulations of See also:loam and earth from neighbouring higher ground, such as the " See also:brick-earth," " See also:head," and " rain-See also:wash " of the See also:south of See also:England—earthy deposits, sometimes full of angular stones, derived from the subaerial waste of the rocks of the neighbourhood . 2 . Underground Water.—Of the rain which falls upon the land one portion flows off into brooks and rivers by which the water is conducted back to the ocean; the larger part, however, sinks into the ground and disappears . It is this latter part which has now to be considered . Over and above the proportion of the rainfall, which is absorbed by living vegetation and by the soil, there is a` continual filtering down of the water from the surface into the rocks that See also:lie below, where it partly lodges in pores and interstices, and partly finds its way into subterranean joints and fissures, in which it performs an underground circulation, and ultimately issues once more at the surface in the form of springs (q.v.) . In the course of this circulation the water performs an important geological task . Not. only carrying down with it the substances which the rain has abstracted from the air, but obtaining more acids and organic matter from the soil, it is enabled to effect chemical changes in therocks underneath, and especially to dissolve limestone and other calcareous formations . So considerable is the extent of this solution in some places that the springs which come to the surface, and begin there to evaporate and lose some of their carbonic acid, contain more dissolved lime than they can hold . They consequently deposit it in the form of calcareous See also:tuff or See also:sinter (q.v.) . Other subterranean See also:waters issue with a large proportion of iron-salts in solution which form deposits of ochre . The various mineral springs so largely made use of for the mitigation or cure of diseases owe their properties to the various salts which they have dissolved out of rocks underground . As the result of prolonged subterranean solution in limestone districts, passages and caves (q.v.), sometimes of great width and length, are formed . When these lie near the surface their See also:roofs sometimes fall in and engulf brooks and rivers, which then flow for some way underground until the tunnels conduct them back again to daylight on some See also:lower ground . Besides its chemical activity water exerts among subterranean rocks a mechanical influence which leads to important changes in the See also:topography of the surface . In removing the mineral matter, either in solution or as fine sediment, it sometimes loosens the support of overlying masses of rock which may ultimately give way on sloping ground, and See also:rush down the declivities in the form of landslips . These destructive effects are specially frequent on the sides of valleys in mountainous countries and on lines of sea-cliff . 3 . Brooks and Rivers.—As geological agents the See also:running waters on the See also:face of the land See also:play an important part in epigene changes . Like rain and springs they have both a chemical and a mechanical action . The latter receives most attention, as it undoubtedly is the more important; but the former ought not to be omitted in any survey of the general waste of the earth's surface . The water of rivers must possess the See also:powers of a chemical solvent like rain and springs, though its actual work in this respect can be less easily measured, seeing that See also:river water is directly derived from rain and springs, and necessarily contains in solution mineral sub-stances supplied to it by them and not by its own operation . Nevertheless, it is sometimes easy to prove that streams dissolve chemically the rocks of their channels . Thus, in limestone districts the See also:base of the cliffs of river ravines may be found eaten away into tunnels, See also:arches, and overhanging projections, presenting in their smooth surfaces a great contrast to the angular jointed faces of the same rock, where now exposed to the influence only of the weather on the higher parts of the cliff . The mechanical action of rivers consists (a) in transporting mud, sand, gravel and blocks of stone from higher to lower levels; (b) in using these loose materials to widen and deepen their channels by erosion; (c) in depositing their load of detritus wherever possible and thus to make new geological formations .
(a) Transporting Power.—River-water is distinguished from that of springs by being less transparent, because it contains more or less mineral matter in suspension, derived mainly from what is washed down by rain, or carried in by brooks, but partly also from the See also:abrasion of the water-channels by the erosive action of the rivers themselves
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The progress of this See also:burden of detritus may be instructively followed from the See also:mountain-tributaries of a river down to the mouth of the See also:main stream
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In the high grounds the water-courses may be observed to be choked with large fragments of rock disengaged from the cliffs and crags on either side
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Traced downwards the blocks are seen to become gradually smaller and more rounded
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They are ground against each other, and upon the rocky sides and bottom of the channel, getting more and more reduced as they descend, and at the same time abrading the rocks over or against which they are driven
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Hence a great deal of debris is produced, and is swept along by the onward and downward movement of the water
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The finer portions, such as mud and fine sand, are carried in suspension, and impart the characteristic turbidity to river-water; the coarser sand and gravel are driven along the river-bottom
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The proportion of suspended mineral matter has been ascertained with more or less precision for a number of rivers
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As an illustrative example of a river draining a vast See also:area with different climates, forms of surface and geological structure the See also:Mississippi may be cited
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The See also:average proportion of sediment in its water was ascertained by See also:Humphreys and See also: 
Not only is the coarse detritus reduced in See also:size by the See also:friction of the stones against each other, but, at the same time, these materials abrade the rocks against which they are driven by the current . Where, owing to the shape of the bottom of the channel, the stones are caught in eddies, and are kept whirling See also:round there, they become more and more worn down themselves, and at the same time scour out See also:basin-shaped cavities, or " pot-holes," in the solid rock below . The uneven bed of a swiftly flowing stream may in this way be honeycombed with such eroded basins which coalesce and thus appreciably lower the surface of the bed . The steeper the channel, other conditions being equal, the more rapid will be the erosion . Geological structure also affects the See also:character and rate of the excavation . Where the rocks are so arranged as to favour the formation and persistence of a See also:waterfall, a long chasm may be hollowed out like that of the See also:Niagara below the falls, where a hard thick bed of nearly See also:flat limestone lies on softer and more easily eroded shales . The latter are scooped out from underneath the limestone, which from time to time breaks off in large masses and the waterfall gradually retreats up stream, while the See also:ravine is proportionately lengthened . To the excavating power of rivers the origin of the valley systems of the dry land must be mainly assigned (see Part VIII . ) . (c) Reproductive Power.—So. long as a stream flows over a steep declivity its velocity suffices to keep the sediment in suspension, but when from any cause, such as a diminution of slope, the velocity is checked, the transporting power is lessened and the sediment begins to fall to the bottom and to remain there . Hence various river-formed or " alluvial " deposits are laid down . These some-times See also:cover considerable spaces at the See also:foot of mountains . The floors of valleys are strewn with detritus, and their level may thereby be sensibly raised . In floods the ground inundated on either side of a stream intercepts some part of the detritus, which is then spread over the See also:flood-See also:plain and gradually heightens it . At the same time the stream continues to See also:erode the channel, and ultimately is unable to reach the old flood-plain . It consequently forms a new plain at a lower level, and thus, by degrees, it comes to be flanked on either side by a series of successive terraces or platforms, each of which marks one of its former levels . Where a river enters a large See also:body of water its current is checked . Some of its sediment is consequently dropped, and by slow See also:accumulation forms a See also:delta (q.v.) . On land, every See also:lake in mountain districts furnishes instances of this kind of See also:alluvium . But the most important deltas are those formed in the sea at the mouths of the larger rivers of the globe . Off many See also:coast-lines the detritus washed from the land gathers into bars, which enclose long strips of water more or less completely separated from the sea outside and. known as lagoons . A See also:chain of such See also:lagoon-barriers stretches for hundreds of See also:miles round the Gulf of Mexico and the eastern shores of the See also:United States . 4 . Lakes.—These sheets of water, considered as a whole, do not belong to the normal See also:system of drainage on the land whereby valleys are excavated . On the contrary they are exceptional to it; for the constant tendency of running water is to fill them up, or to drain them by wearing down the barriers that contain them at their outflow . Some of them are referable to movements of the terrestrial crust whereby depressions arise on the surface of the land, as has been noted after earthquakes . Others have arisen from solution such as that of rock-salt or of limestone, the removal of which by underground water causes a subsidence-of the ground above . A third type of lake-basin occurs in regions that are now or have once been subject to the erosive action of glaciers (see under next sub-division, Terrestrial See also:Ice) . Many small lakes or tarns have been caused by the deposit of debris across a valley as by landslips or moraines . Considered from a geological point of view, lakes perform an important See also:function in regulating the drainage of the ground below their outfall and diminishing the destructive effects of floods, in filtering the water received from their affluent streams, and in providing undisturbed areas of deposit in which thick and extensive lacustrine formations may be accumulated . In the inland basins of some dry climates the lakes are salt, owing to excess of evaporation, and their bottoms become the sites of chemical deposits, particularly of chlorides of sodium and See also:magnesium, and calcium sulphate and carbonate . 5 . Terrestrial Ice.—Each of the forms assumed by frozen water has its own characteristic action in geological processes . Frost has a powerful influence in breaking up See also:damp soils and surfaces of stone in the pores or cracks of which moisture has lodged . The water in freezing expands, and in so doing pushes asunder the component particles of soil or stone, or widens the space between the walls of joints or crevices . When the ice melts the loosened grains remain apart ready to be washed away by rain or blown off by wind, while by the widening of joints large blocks of rock are detached from the faces of cliffs . Where rivers or lakes are frozen over the ice exerts a marked pressure on their See also:banks; and when it breaks up large sheets of it are driven ashore, pushing up quantities of gravel and stones above the level of the water . The piling up of the disrupted ice against obstructions in rivers ponds back the water, and often leads to destructive floods when the ice barriers break . Where the ice has formed round boulders in shallow water, or at the bottom (" See also:anchor-ice "), it may lift these up when the frost gives way,' and may transport them for some distance . Ice formed in the atmosphere, and descending to the ground in the form of hail, often causes great destruction to vegetation and not infrequently to See also:animal life . Where the frozen moisture reaches the earth as snow, it serves to protect rock, soil and vegetation from the effects of frost; but on sloping ground it is apt to give rise to destructive avalanches or landslips, while indirectly, by its rapid melting, it may cause serious floods in rivers . But the most striking geological work performed by terrestrial ice is that achieved by glaciers (q.v.) 'and ice-sheets . These vast masses of moving ice, when they descend from mountains where the steeper rocks are clear of snow, receive on their surface the debris detached by frost from the declivities above, and bear these materials to lower levels or to the sea . Enormous quantities of rock-rubbish are thus transported in the See also:Alps and other high mountain ranges . When the ice retreats the boulders carried by it are dropped where it melts, and See also:left there as memorials of the former See also:extension of the glaciers . See also:Evidence of this nature proves the much wider extent of the Alpine ice at a comparatively See also:recent geological date . It can also be shown that detritus from Scandinavia has been ice-borne to the south-See also:east of England and far into the See also:heart of See also:Europe . The ice, by means of grains of sand and pieces of stone which it drags along, scores, scratches and polishes the surfaces of rock underneath it, and, in this way, produces the abundant fine sediment that gives the characteristic milky See also:appearance to the rivers that issue from the lower ends of glaciers . By such long-continued See also:attrition the rocks are worn down, portions of them of softer nature, or where the ice acts with especial vigour, are hollowed out into cavities which, on 'the disappearance of the ice, may be filled with water and become tarns or lakes . Rocks over which land-ice has passed are marked' by a See also:peculiar smooth, flowing outline, which forms a contrast to the more rugged surface produced by See also:ordinary weathering . They are covered with groovings, which range from the finest striae left by sharp grains of sand to deep ruts ground out by blocks of stone, The trend of these markings shows the direction in which the ice flowed . By their evidence the position and movement of former glaciers in countries from which the ice has entirely vanished may be clearly determined (see GLACIAL See also:PERIOD) . 6 . The Sea.—The See also:physical features of the sea are discussed in See also:separate articles (see OCEAN AND OCEANOGRAPHY) . The sea must be regarded as the great regulator of temperature and climate over the globe, and as thus exerting a profound influence on the See also:distribution of plant and animal life . Its distinctly geological work is partly erosive and partly reproductive . As an eroding agent it must to some extent effect chemical decompositions in the rocks and sediments over which it spreads; but these changes have not yet been satisfactorily studied . Undoubtedly, its chief destructive power is of a mechanical kind, and arises from the action of its waves in beating upon See also:shore-cliffs . By the alternate . See also:compression and expansion of the air in crevices of the rocks on which heavy breakers fall, and by the See also:hydraulic pressure which these masses of sea-water exert on the walls of the fissures into which they rush, large masses of rock are loosened and detached, and caves and tunnels are drilled along the base of sea-cliffs . Probably still more efficacious are the blows of the loose See also:shingle, which, caught up and hurled forward by the waves, falls with great force upon the shore rocks, battering them as with a kind of See also:artillery until they are worn away . The smooth surfaces of the rocks within reach of the waves contrasted with their angular forms above that limit bear See also:witness to the amount of waste, while the rounded forms of the boulders and shingle show that they too are being continually reduced in size . Thus the sea, by its action on the coasts, produces much sediment, which is swept away by its waves and currents and strewn over its See also:floor . Besides this material, it is constantly receiving the fine silt and sand carried down by rivers . As the floor of the ocean is thus the final receptacle for the waste of the land, it becomes the chief era on the surface of the globe for the accumulation of new stratified formations . And such has been one of its great functions since the beginning of geological time, as is proved by the rocks that form the visible part of the earth's crust, and consist in great part of marine deposits . Chemical precipitates take place more especially in enclosed parts of the sea, where concentration of the water by evaporation can take place, and where layers of sodium chloride, calcium sulphate and carbonate, and other salts are laid down . But the chief marine accumulations are of detrital origin . Near the land and for a variable distance extending sometimes to 200 or 300 m. from shore the deposits consist chiefly of sediments derived from the waste of the land, the finer silts being transported farthest from their source . At greater depths and distances the ocean floor receives a slow deposit of exceedingly fine See also:clay, which is believed to be derived from the decomposition of See also:pumice and volcanic dust from insular or sub-marine volcanoes . Wide tracts of the bottom are covered with various forms of See also:ooze derived from the accumulation of the remains of minute organisms . (C) Life . Among the agents by which geological changes are carried on upon the surface of the globe living organisms must be enumerated . Both plants and animals co-operate with the inorganic agents in promoting the degradation of the land . In some cases, on the other See also:hand, they protect rocks from decay, while, by the accumulation of their remains, they give rise to extensive formations both upon the land and in the sea . Their operations may hence be described as alike destructive, conservative and reproductive . Under this heading also the influence of Mali as a geological agent deserves See also:notice . (a) Plants.—Vegetation promotes the disintegration of rocks and soil in the following ways: (t) By keeping the surfaces of stone moist, and thus promoting both mechanical and chemical See also:dissolution, as is especially shown by liverworts, mosses and other moisture-loving plants . (2) By producing through their decay carbonic and other acids, which, together with decaying organic matter taken up by passing moisture, become potent in effecting the chemical decomposition of rocks and in promoting the disintegration of soils . (3) By inserting their roots or branches between joints of rock, which are thereby loosened, so that large slices may be eventually wedged off . (4) By attracting rain, as thick See also:woods, forests and peat-mosses do, and thus accelerating the general waste of a country by running water . (5) By promoting the decay of diseased and dead plants and animals, as when See also:fungi overspread a damp rotting See also:tree or the carcase of a dead animal . That plants also exert a conservative influence on the surface of the land is shown in various ways . (I) The formation of a stratum of See also:turf protects the soil and rocks underneath from being rapidly disintegrated and washed away by atmospheric action . (2) Many plants, even without forming a layer of turf, serve by their roots or branches to protect the loose sand or soil on which they grow from being removed by wind . The See also:common sand-carex and other arenaceous plants bind the loose sand-dunes of our coasts, and give them a permanence, which would at once be destroyed were the sand laid bare again to storms . The growth of shrubs and brushwood along the course of a stream not only keeps the alluvial banks from being so easily undermined and removed as would otherwise be the case, but serves to See also:arrest the sediment in floods, filtering the water and thereby adding to the height of the flood plain . (3) Some marine plants, like the calcareous nullipores, afford See also:protection to shore rocks by covering them with a hard incrustation . The tangles and smaller Fuci which grow abundantly on the littoral See also:zone break the force of the waves or diminish the effects of ground swell . (4) Forests and brushwood protect the soil, especially on slopes, |