Page images

feet may completely bury valleys and water-courses, and thus, tending that the supply so furnished, cven if it could reach and be greatly affect the drainage. The coarsest materials accumulate dissolved in the magma, would yet be insufficient to furnish the nearest to the vent that emits them. The finer dust is not in re- prodigious quantity

of aqueous vapour discharged during an eruption, fuently hurled forth with such an impetus as to be carried for maintain that the water belongs to the magma itsell. They point housands of feet into the tracks of upper air-currents, whereby it to the admitted fact that many substances, particularly metals in may be borne for hundreds of miles away from the vent so as últi a state of fusion, can absorb large quantities of vapours and gases mately to fall to the ground in countries far removed from any active without chemical combination, and on cooling discharge them with vokano. Outflows of lava, from their greater solidity and durability, eruptive phenomena somewhat like those of volcanoes. This produce still more serious and lasting changes in the external fcatures question must be regarded as one of the still unsolved problems of of the ground over which they flow. As they naturally seek the geology. lowest levels, they find their way into the channels of streams.

(B) Movements of the Earth's Crust. If they keep along the channels, they seal them up under a mass of compact stone which the running water, is not wholly diverted Among the hypogene forces in geological dynamics an important elsewhere, will take many long centuries to cut through. '!, on the place must be assigned to movements of the terrestrial crust. Though other hand, the lava crosses a stream, it forms a massive dam, the expression " the solid earth has become proverbial, it appears above which the water is ponded back so as to form a lake.

singularly inappropriate in the light of the results obtained in recent As the result of prolonged activity a volcanic cone is gradually years by the use of delicate instruments of observation. With the built up by successive outflows of lava and showers of dust and facilities supplied by these instruments (sce SEISMOMETER), it has stones. These materials are arranged in beds, or sheets, inclined becn ascertained that the ground bencath our feet is subject to outwards from the central vent. On surrounding level ground the continual slight tremors, and sceble pulsations of longer duration, alternating beds are flat. In course of time, decp gullies are cut on some of which may be due to daily or scasonal variations of temthe outer slopes of the cone by rain, and by the heavy showers that perature, atmospheric pressure or other meteorological causes. arise from the condensation of the copious discharges of steam The establishment of sell-recording seismometers all over the world during eruptions. Along the sides of these ravines instructive has led to the detection of many otherwise imperceptible shocks. sections may be studied of the volcanic strata. The larger rivers of over and above the appreciable earth-waves propagated from earthsome volcanic regions have likewise eroded vast gorges in the more quake centres of disturbance. Morcover, it has been ascertained horizontal lavas and ashes of the Aatter country, and have thus laid that some parts of the surface of the land are slowly risingwhile bare stupendous cliffs, along which the successive volcanic shects others are falling with reference to the sea level. From time to can be seen piled above each other for many hundred feet. Ona time the surface suffers calamitous devastation from earthquakes, small scale, some of these features are well displayed among the when portions of the crust under great strain suddenly give way. rivers that drain the volcanic tracts of central France; on a great Lastly, at intervals, probably separated from each other by vast scale, they are presented in the course of the Snake river, and other periods of time, the terrestrial crust undergoes intense plication streams that traverse the great volcanic country of western North and fracture, and is consequently ridged up into mountain-chains. America. Similar volcanic scenery has been produced in western No event of this kind has been witnessed since man began to record Europe by the action of denudation in dissecting the flat Tertiary his experiences. But from the structure of mountains. as laid open lavas of Scotland, the Faeroe Isles and Iceland.

by prolonged denudation, it is possible to form a vivid conception Of special interest to the geologist are those volcanoes which have of the nature and effects of these most stupendous of all geological taken their rise on the sea-bottom; for the volcanic intercalations revolutions. among the stratified formations of the earth's crust are almost In considering this department of geological inquiry it will be entirely of submarine origin. Many active volcanoes situated on convenient to treat it under the following heads: (i) slow depresislands have begun their eruptions below sea-level. Both Vesuvius sion and upheaval; (2) Earthquakes; (3) Mountain-making: (4) and Etna sprang up on the floor of the Mediterranean sea, and have Metamorphism of rocks. gradually built up their cones into conspicuous parts of the dry land. 1. Slow Depression and Uphcaval.-On the west side of Japan Examples of a similar history are to be found among the volcanic the land is believed to be sinking below the sea, for fields are replaced islands of the Pacific Ocean. In some of these cases a movement by beaches of sand or shingle, while the depth of the sca off shore of clevation has carried the submarine lavas, tuffs and agglomerates has perceptibly increased. A subsidence of the south of Sweden has above sea level, and has furnished opportunities of comparing these taken place in comparatively recent times, for streets and loundations materials with those of recent subaerial origin, and also with the of houses at successive levels are found below high-water mark. ancient records submarine eruptions which have been preserved | The west coast of Greenland over an extent of more than 600 m. among the stratified formations. From the evidence thus supplied, is sinking, and old settlements are now submerged. Proofs of it can be shown that the materials ejected from modern submarine submergence of land are furnished by ." submerged forests," and volcanic vents closely resemble those accumulated by subaerial beds of terrestrial peat now lying at various depths below the level volcanoes; that the dust, ashes and stones become intermingled or of the sea, of which many examples have been collected along the interstratified with coral-mud, or other non-volcanic deposit of the shores of the British Isles, Holland and France. Interesting evidence sea-bottom, that vesicular lavas may be intercalated among them that the west of Europe now stands at a lower level than it did at a as on land, and that between the successive sheets of volcanic late geological period is supplied in the charts of the North Sea and origin, layers of limestone may be laid down which are composed Atlantic, which show that the valleys of the land are prolonged chiefly, or wholly, of the remains of calcareous marine organisms. under the sea. These valleys have been eroded out of the rocks by

Though active volcanoes are widely distributed over the globe, the streams which flow in them, and the depth of their submerged and are especially abundant around the vast basin

of the Pacific portions below the sea level affords an indication of the extent of the Ocean, they afford an incomplete picture of the extent to which subsidence. volcanic action has displayed itsell on the surface of our planet.

The uprise of land has been detected in various parts of the world. When the

rocks of the land are attentively studied they disclose one of the most celebrated instances is that of the shores of the Gulf proofs of that action in many districts where there is now no outward of Bothnia, where, at Stockholm, the elevation, between the years sign of it. Not only so, but they reveal that volcanoes have been in 1774 and 1875, appears to have been 48 centimetres (18ļ in.) in eruption in some of these districts during many different periods of a century. But on the west side of Sweden, fronting the Skager Rak, the past, back to the beginnings of geological history. The British the coast, between the years 1820 and 1870, rose 30 centimetres, Islands furnish a remarkable example of such a series of ancient which is at the rate of 60 centimetres, or ncarly 2 ii. in a century. eruptions from the Cambrian period all through Palaeozoic times in the region of the Great Lakes in the interior of Canada and the there rose at intervals in that country a succession of volcanic centres United States it has been ascertained that the land is undergoing a from some of which thousands of feet of lavas and tuffs were dis- slow tilt towards the south-west, of which the mean rate appears to charged. Again in older Tertiary times the same region witnessed be rather less than 6 in. in a century. If this rate of change should a stupendous outpouring of basalt, the surviving relics of which continue the waters of Lake Michigan, owing to the progress of the are more than 3000 ft. thick, and cover many hundreds of square tilt, will, in some 500 or 600 years, submerge the city of Chicago. miles. Similar evidence is supplied in other countries both in the and eventually the drainage of the lakes will be diverted into the Old and the New world. Hence it is proved that, in the geological basin of the Mississippi Proof of recent emergence of land is supplied past, volcanic action has been vigorous at long intervals on the same by what are called "raised beaches or "strand-lines," that is, sites during a vast series of ages, though no active vents are to be lines of former shores marked by sheets of littoral deposits, or seen there now. The volcanoes now active form but a small pro- platforms cut by shore-waves in rock and flanked by old sea-cliffs portion of the total number which has appeared on the surface of and lines of sea-worn caves. Admirable examples of these features the earth. With regard to the cause of volcanic action much has been England to the north of Norway. These lines of old shores become

are to be seen along the west coast of Europe from the south of speculated, but little can be confidențy affirmed. That water in fainter in proportion to their antiquity. In Britain they occur at the form of occluded gas plays the chief part in forcing the lava various heights, the platforms at 25. 50 and 100 ft. being well column up a volcanic chimney, and in the violent explosions that marked. accompany the rise of the molten material, is generally admitted. The cause of these slow upward and downward movements of the But opinions differ as to the source of this water. According to crust of the earth is still imperfectly understood. Upheaval might some investigators, it should be regarded as in large measure of conceivably be produced by an ascent of the internal magma, and the meteoric origin, derived from the descent of rain into the carth, and consequent expansion of the overlying crust by heat; while depresits absorption by the molten magma in the interior. Others, con sion might follow any subsidence of the magma, or its displacement

to another district. If, as is generally believed, the globe is still which are probably not yet all recognized, but among them we should contracting, the shrinkage of the surface may cause both these doubtless include a high temperature, intense pressure, mechanical movements. Subsidence will be in excess, but between subsiding movement resulting in crushing, shearing and foliation, and the tracts lateral thrust may suffice to push upward intervening more presence of water in their pores. It is among igneous rocks that the solid and stable ground; but no solution of the problem yet proposed progressive stages of metamorphism can be most easily traced. is wholly satisfactory.

Their definite original structure and mineral composition afford a 2. Earthquakes.-As this subject is discussed in a separate article starting point from which the investigation may be begun and it will

be sufficient here to take note of its more important geological pursued. Where an igneous rock has been invaded by metamorphic bearings. It was for many centuries taken for granted that earth changes, it may be observed to have been first broken down into quakes and volcanoes are due to a common cause. We have seen separate lenticles, the cores of which may still retain, with little or that in classical antiquity, they were looked on as the results of the no alteration, the original characteristic minerals and crystalline movements of wind imprisoned within the earth. Long after this structure of the rock. Between these lenticle the intervening notion was discarded, and a more scientific appreciation of volcanic portions have been crushed down into a powder or paste, which action was reached, it was still thought that earthquakes should be seems to have been squeezed round and past them, and shows a regarded as manifestations of the same source of energy as that laminated arrangement that resembles the flow-structure in lavas. which displays itself in volcanic eruptions. It is true that earth As the degree of metamorphism increases, the lenticles diminish in quakes are frequent in districts of active volcanoes, and they may size, and the intervening crushed and foliated matrix increases in undoubtedly be often due there to the explosions of the magma, amount, until at last it may form the entire mass of the rock. While or to the rupture of rocks caused by its ascent towards the surlace. the original minerals are thus broken down, new varieties make But such shocks are comparatively local in their range and feeble their appearance. Of these, among the earliest to present themselves in their effects. There

is now a general agreement that between the are usually the micas, that impart their characteristic silvery sheen great world-shaking earthquakes and volcanic phenomena, no to the surfaces of the folia along which they sprcad. Younger immediate and intimate relationship can be traced, though they may felspars, as well as mica, are developed, and there arise also sillibe connected in ways which are not yet perceived. Some of the manite, garnet, andalusite and many others. The texture becomes more recent great earthquakes on land have proved that the waves more coarsely crystalline, and the segregation of the constituent of shock are produced by the sudden rupture or collapse of rocks minerals more definite along the lines of foliation. From the finest under great strain, either along lines of previous fracture or of new silky phyllites a graduation may be traced through successively rents in the terrestrial crust; and that such ruptures may occur at coarser mica-schists, until we reach the almost granitic texture of a remote distance from any volcano. Thus the recent disastrous the coarsest gneisses. San Francisco earthquake has been recognized to have resulted from Regional metamorphism has arisen in the heart of mountain. a slipping of ground along the line of an old fault, which has been chains, and in any other district where the deformation of the crust traced for a long distance in California generally parallel to the has been sufficiently intense. There is another type of alteration coast. The position of this fault at the surface has long been clearly termed contact-metamorphism," which is developed around followed by its characteristic topography. After the earthquake masses of igneous rock, especially where these have been intruded in these superficial features were found to have been removed by the large bosses among stratified formations. It is particularly displayed same cause that had originated them. For some 300 m. on the track around masses of granite, where sandstones are found altered into of this old fault-line a renewed slipping was seen to have taken place quartzite, shales and grits into schistose compounds, and where some. along one or both sides, and the ground at the surface was ruptured times fossils are still recognizable among the metamorphic minerals. as well as displaced horizontally. Obviously, the jar occasioned by the sudden and simultaneous subsidence of a portion of the earth's

DIVISION II.-EPIGENE OR SUPERFICIAL ACTION, crust several hundred miles long, must be far more serious than could be produced by an carthquake radiating from a single local working there, that at present the chief amount of visible geo

It is on the surface of the globe, and by the operation of agents volcanic focus.

From their disastrous effects on buildings and human lives, an logical change is effected. In considering this branch of inquiry, exaggerated importance has been imputed to earthquakes as agents we are not involved in a preliminary difficulty regarding the very of geological change. Experience shows that even after a severe nature of the agencies as is the case in the investigation of shock which may have destroyed numerous towns and villages, together with thousands of their inhabitants, the face of the country plutonic action. On the contrary, the surface agents are carrying has suffered scarcely any perceptible change, and that, in the course

on their work under our very eyes. We can watch it in all its of a year or two, when the ruined houses and prostrate trees have stages, measure its progress, and mark in many ways how been cleared away, little or no obvious trace of the catastrophe may accurately it represents similar changes which, for long ages

But may be enumerated (a) landslips, which lay bare hillsides, and some previously, must have been effected by the same means. times pond back the drainage of valleys so as to give rise to lakes: in the systematic treatment of this subject we encounter a (6) alterations of the topography, as in fissuring of the ground, or in difficulty of another kind. We discover that while the operations the production of inequalities whereby the drainage is affected; to be discussed are numerous and readily observable, they are so new valleys and new lakes may thus be formed, while previously interwoven into one great network that any separation of them existing lakes may be emptied; C) permanent changes of level, under different subdivisions is sure to be more or less artificial either in an upward or downward direction.

3. Mountain-making.–This subject may be referred to here for and to convey an erroneous impression. While, therefore, under the striking evidence which it supplies of the importance of move the unavoidable necessity of making use of such a classification nents of the earth's crust among geological processes. The structure of subjects, we must always bear in mind that it is employed of the earth has been intensely plicated, crumpled and fractured merely for convenience, and that in nature superficial geological Vast piles of sedimentary strata have been folded to such an extent action must be continually viewed as a whole, since the work of as to occupy now only half of their original horizontal extent. This each agent has constant reference to that of the others, and is to as much as 120,000 metres of 74 English miles, so that two points not properly intelligible unless that connexion be kept in view. on the opposite sides of that chain have been brought by so much

The movements of the air; the evaporation from land and nearer to each other than they were originally before the movements. sea; the fall of rain, hail and snow; the flow of rivers and Besides such intense plication, extensive rupturing of the crust has glaciers; the tides, currents and waves of the ocean; the growth taken place in the same range of mountains. Not only have the and decay of organized existence, alike on land and in the depths chain, but huge slices of them have been torn away from the main of the sea;-in short, the whole circle of movement, which is body, and thrust forward for many miles, so as now actually to continually in progress upon the surface of our planet, are the lorin the summits of mountains, which are almost entirely composed subjects now to be examined. It is desirable to adopt some of much younger formations. 1f these colossal disturbances occurred

For rapidly, they would give rise to cataclysms of inconceivable general term to embrace the whole of this range of inquiry. magnitude over the surface of the globe No record has been dis. this end the word epigene (Gr. éti, upon) has been suggested as covered of such accompanying devastation. But whether sudden a convenient term, and antithetical to hypogene (Gr. Útó, under), and violent, or prolonged and gradual, such stupendous upturnings or subterranean action. of the crust did undoubtedly take place, as is clearly revealed in innumerable natural sections, which have been laid open by the

A simple arrangement of this part of Geological Dynamics is denudation of the crests and sides of the mountains.

in three sections: 4. Melamorphism of Rocks (see METAMORPHISM).-During the A. Air.-The influence of the atmosphere in destroying and movements to which the crust of the earth has been subject, not forming rocks. only have the rocks been folded and fractured, but they have likewise, in many regions, acquired new internal structures, and have

B. Waler.— The geological functions of the circulation of thus undergone a process of " regional metamorphism." This

water through the air and between sea and land, and the rearrangement of their substance has been governed by conditions action of the sea.


C. Life.-The part taken by plants and animals in preserving, ground continues a great series of similar reactions there. It acts destroying or reproducing geological formations.

mechanically, by washing away loose materials, and thus powerfully

affecting the contours of the land. Its chemical action depends The words destructive, reproductive and conservative, mainly upon the nature and proportion of the substances which, in tmployed in describing the operations of the epigene agents, do descending to the earth, it abstracts from the atmosphere. Rain not necessarily imply that anything useful to man is destroyed, always absorbs a little air, which, in addition to its nitrogen and reproduced or preserved. On the contrary, the destructive oxygen, contains carbonic acid, and in minute proportions, sodium

chloride, sulphuric acid and other ingredients, especially inorganic action of the atmosphere may turn barren rock into rich soil, dust, organic particles and living germs. Probably the most generally while its reproductive effects sometimes turn rich land into efficient of these constituents are oxygen, carbonic acid and organic barren desert. Again, the conservative influence of vegetation matter.

Armed with these reagents, rain effects a chemical decomhas sometimes for centuries retained as barren morass what position of the rocks on which

it fails, and through which it sinks

underground. The principal changes thus produced are as follows: might otherwise have become rich meadow or luxuriant wood: (@) Oxidation.-Owing to the prominence of oxygen in rain-water, land. The terms, therefore, are used in a strictly geological and its readiness to unite with any substance which can contain sense, to denote the removal and re-deposition of material, and more of it, a thin oxidized pellicle is formed on the surface of many

rocks on which rain falls, and this oxidized layer is not at once its agency in preserving what lies beneath it.

washed off, sinks deeper until a crust is formed over the stone. A (A) The Air.

familiar illustration of this action is afforded by the rust, or oxide, As a geological agent, the air brings about changes partly by its which forms on iron when exposed to moisture, though this iron component gases and partly by its movements. Its destructive may be kept long bright if allowed to remain screened from moist action is both chemical and mechanical. The chemical changes are

air and rain. (6) Deoxidation.- Organic matter having an affinity probably mainly, if not entirely, due to the moisture of the air,

for more oxygen decomposes peroxides by depriving them of some and particularly to the gases, vapours and organic matter which part of their share of that element and reducing them to protoxides. the moisture

contains. Dry air seems to have little or no appreciable These changes are especially noticeable among the iron oxides so influence in promoting these reactions. As the changes in question abundantly diffused among rocks. Hence rain-water, in sinking they are described in the following section under the division on rain. simple action

of the water, as in the solution

of rock salt, or by the are similar to those much more abundantly brought about by rain through soil and obtaining such organic matter, becomes thereby the atmosphere, one of considerable importance is to be seen in the influence of the carbonic acid present in the rain. (d) Formation of result of great and rapid changes of temperature. Heat expands in rain is to be seen in the corrosion of exposed marble slabs.

Carbonates.-A familiar example of the action of carbonic acid rocks, while cold contracts them. In countries with a great annual range of temperature, considerable difficulty is sometimes experienced is held in solution in the carbonated water, but is deposited again

cartonic acid dissolves some of the lime, which, as a bicarbonate, in selecting building materials liable to be little affected by the

when the water loses its carbonic acid or evaporates. It is not alternate expansion and contraction, which prevents the joints of masonry from remaining close and tight. In dry tropical climates, merely carbonates, however, which are liable to this kind of destruc: where the days are intensely hot and the nights extremely cold,

the abundantly as constituents of rocks, are atracked; their silica is

tion. Even silicates of lime, potash and soda, combinations existing rapid nocturnal contraction produces a strain so great as to rival liberated, and their alkalis or alkaline earths, becoming carbonates, frost in its influence upon the surface of exposed rocks,

disintegrating are removed in solution. (e) Hydration.-Some minerals, captaining them into sand, or causing them to crack or peel off in ski.s or

little or no water, and therefore called anhydrous, when exposed to irregular pieces. Dr Livingstone found in Africa (12° S. lat., 34° E.

the action of the atmosphere, absorb water, or become hydrous, long.) that surfaces of rock which during the day were heated up to

and are then usually more prone to further change. Hence the rocks 137° Fahr., cooled so rapidly by radiation at night that, unable to sustain the strain of contraction, they split and threw off sharp

of which they form part become disintegrated.

Besides the reactions here enumerated, a considerable amount of angular fragments from sew ounces to 100 or 200 18 in weight decay may be observed as the result of the presence of

sulphuric In temperate regions this action, though much less pronounced, and nitric acid in the air, especially in that of large towns and still makes itself felt. In these climates, however, and still more in manufacturing districts, where much coal is consumed. Metallic high latitudes, somewhat similar results are brought about by frost.

By its motion in wind the air drives loose sand over rocks, and in surfaces, as well as various kinds of stone, are there corroded, while course of time abrades and smoothes them. the name given to the characteristic lustrous surface thus imparted and dropping off,

owing to the conversion of the lime into sulphate. Holes are said to be drilled in window glass at Cape Cod by the same

Great injury is likewise done from a similar cause to marble monuagency. Cavities are now and then hollowed out of rocks by the

ments in exposed graveyards. gyration in them of little fragments of stone of grains of sand kept rain, including also that of plants and animals,

to be noticed in the

The general result of the disintegrating action of the air and of in motion by the wind. Hurricanes form important geological sequel, is denoted by the term " weathering." The amount of decay agents upon land in uprooting trees, and thus sometimes impeding depends partly on conditions of climate, especially the range of the drainage of a country and giving rise to the formation of peat temperature, the abundance of moisture, height above the sea and The reproductive action of the air arises partly from the effect

exposure to prevalent winds. Many rocks liable to be saturated of the chemical and mechanical disintegration involved in the

with rain and rapidly dried under a warm sun are apt to disintegrate process of " weathering," and partly from the transporting power of the weathering are mainly governed by the composition and

at the surface with comparative rapidity. The nature and progress of wind and of aerial currents. The layer of soil, which covers so

texture of the rocks exposed to it, much of the surface of the land, is the result of the decay of the liable to little chemical change from the influence of moisture are

Rocks composed of particles underlying rocks, mingled with mineral matter blown over the ground best fitted to resist weathering, provided they possess sufficient by wind, or washed thither by rain, and with the mouldering remains cohesion to withstand the mechanical processes of disintegration. of plants and animals. The extent to which fine dust may, be Siliceous sandstones are excellent examples of this permanence. transported over the surface of the land can hardly be realized in Consisting wholly or mainly of the durable mineral quartz, they are countries clothed with a covering of vegetation, though even there, sometimes ablc so to withstand decay that buildings made of them in dry weather during spring, clouds of dust may often be seen still retain, after the lapse of centuries, the chisel-marks of the blown away by wind from bare ploughed fields. Intercepted by the builders. Some rocks, which yield with comparative rapidity to 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 disintegration on their surface. This is particularly the case with

the chemical attacks of moisture, may show little or no mark of arc dense and frequent, enormous quantities of fine mineral particles certain calcareous rocks. Limestone when pure is wholly soluble are thus borne along and accumulated. The remarkable deposit in acidulated water. Rain falling on such a rock removes some of it of " Loess," which is sometimes more than 1500 ft. thick and covers in solution, and will continue to do so until the whole is dissolved extensive areas in China and other countries, is regarded as due to

away. But where a limestone is full of impurities, a weathered crust the drifting of dust by wind. Again the dunes of sand so abundant of more or less insoluble particles remains after the solution of the along the inner side of sandy sea-beaches in many different parts calcareous part of the stone. Hence the relative purity of limestones of the world are attributable to the same action.

may be roughly determined by examining their weathered surfaces, (B) Water.

where, if they contain much sand, the grains will be seen projecting In treating of the epigene action of water in gcological processes ferruginous, the yellow hydrous peroxide, or ochre, will be found as

from the calcareous matrix, and where, should the rock be very it will be convenient to deal first with its operations in traversing a powdery crust. In limestones containing abundant encrinites, the land, and then with those which it performs in the sea. circulation of water from land to sea and again from sca to land presents the fossils standing out in relicf. The crystalline arrange.

The shells, or other organic remains, the weathered surface commonly constitutes the fundamental cause of most of the daily changes by inent of the lime in the organic structures enables them to resist which the surface of the land is affected.

disintegration better than the general mechanically aggregated 1. Rain. ---Rain effects two kinds of changes upon the surface of marrix of the rock. the land. It acts chemically upon soils and stones, and sinking under scarch well such weathered surfaces, for he often finds there, delicately

An experienced fossil collector will always



picked out by the weather, minute and frail fossils which are wholly | rocks underneath, and especially to dissolve limestone and other invisible on a freshly broken surface of the stone. Many rocks calcareous formations. So considerable is the extent of this solution weather with a thick crust, or even decay inwards for many feet or in some places that the springs which come to the surface, and begin yards. Basalt, for example, often shows a yellowish-brown fer. there to evaporate and lose some of their carbonic acid, contain more ruginous layer on its surface, formed by the conversion of its felspar dissolved lime than they can hold. They consequently deposit it into kaolin, and the removal of its calcium silicate as carbonate, in the form of calcareous tuff or sinter (9.v.). Other subterranean by the hydration of its olivine and augite and their conversion into waters issue with a large proportion of iron-salts in solution which serpentine, or some other hydrous magnesian silicate, and by the form deposits of ochre. The various mineral springs so largely conversion of its magnetite into limonite. Granite sometimes shows made use of for the mitigation or cure of diseases owe their proin a most remarkable way the distance to which weathering can perties to the various salts which they have dissolved out of rocks rcach. It may occasionally be dug into for a depth of 20 or 30 ft., underground. As the result of prolonged subterranean solution in the quartz crystals and veins retaining their original positions, while limestone districts, passages and caves (q.v.), sometimes of great the felspar is completely kaolinized. It is to the endlessly varied width and length, are formed. When these lic near the surface their effects of weathering that the abundant fantastic shapes assumed rooss sometimes fall in and engulf brooks and rivers, which then by crags and other rocky masses are due. Most varieties of rock now for some way underground until the tunnels conduct them back have their own characteristic modes of weathering, whereby they again to daylight on some lower ground. may be recognized even from a distance. To some of these features Besides its chemical activity water exerts among subterrancan reference will be made in Part VIII.

rocks a mechanical influence which leads to important changes in The mechanical action of rain, which is intimately bound up with the topography of the surface. In removing the mineral matter, its chemical action, consists in washing off the fine superficial cither in solution or as fine sediment, it sometimes loosens the support particles of rocks which have been corroded and loosened by the of overlying masses of rock which may ultimately give way on sloping process of weathering, and in thus laying open fresh portions to the ground, and rush down the declivities in the form of landslips. same influences of decay. The detritus so removed is partly carried These destructive effects are specially frequent on the sides of valleys down into the soil which is thereby enriched, partly held in suspension in mountainous countries and on lines of sca-cliff. in the little runnels into which the rain-drops gather as they begin 3. Brooks and Rivers.--As geological agents the running waters to flow over the land, partly pushed downwards along the surface on the face of the land play an important part in epigene of sloping ground. A good deal of it finds its way into the nearest changes. Like rain and springs they have both a chemical and a brooks and rivers, which are consequently made muddy by heavy mechanical action. The latter receives most attention, as it un. rain.

doubtedly is the more important; but the former ought not to be It is natural that a casual consideration of the subject should lead omitted in any survey of the general waste of the earth's surface. to an impression that, though the general result of the fall of rain The water of rivers must possess the powers of a chemical solvent upon a land-sursace must lead to some amount of disintegration and like rain and springs, though its actual work in this respect can be lowering of that surface, the process must be so slow and slight as less easily measured, seeing that river water is directly derived from hardly to be considered of much importance among geological rain and springs, and necessarily contains in solution mineral suboperations. But further attention will show such an impression to stances supplied to it by them and not by its own operation. Neverbe singularly erroneous. It loses sight of the fact that a change theless, it is sometimes easy to prove that streams dissolve chemically which may be hardly appreciable within a human lifetime, or even the rocks of their channels. Thus, in limestone districts the base within the comparatively brief span of geological time embraced in of the cliffs of river ravines may be found eaten away into tunnels, the compass of human history, may nevertheless

become gigantic arches, and overhanging projections, presenting in their smooth in its results in the course of immensely protracted periods. An surfaces a great contrast to the angular jointed faces of the same instructive lesson in the erosive action of rain may be found in the rock, where now exposed to the influence only of the weather on the pitted and channelled surface of ground lying under the drip of the higher parts of the cliff. eaves of a cottage. The fragments of stone and pebbles of gravel The mechanical action of rivers consists (a) in transporting mud, that form part of the soil can there be seen sticking out of the ground, sand, gravel and blocks of stone from higher to lower levels; (6) because being hard they resist the impetus of the falling drops in using these loose materials to widen and deepen their channels protecting for a time the earth beneath them, while that which by erosion; (c) in depositing their load of detritus wherever possible surrounded and covered them is washed away. From this familiar and thus to make new geological formations. illustration the observer may advance through every stage in the (a) Transporting Power.-River-water is distinguished from that disappearance of material which once covered the surface, until he of springs by being less transparent, because it contains more or less comes to examples where once continuous and thick sheets of solid mineral matter in suspension, derived mainly from what is washed rock have been reduced to a few fragments or have been entirely down by rain, or carried in by brooks, but partly also from the removed. Since the whole land surface over which rain falls is abrasion of the water-channels by the erosive action of the rivers exposed to this waste, the superficial covering of decayed rock or themselves. The progress of this burden of detritus may be instrucsoil, as Hutton insisted, is constantly, though imperceptibly, travel- tively followed from the mountain-tributaries of a river down to ling outward and downward to the sea. In this process of transport the mouth of the main stream. In the high grounds the waterrain is an important carrying agent, while at the same time it serves courses may be observed to be choked with large fragments of rock to connect the work of the other disintegrating forces, and to make disengaged from the cliffs and crags on either side. Traced downwards it conducive to the general degradation of the land. Though this the blocks are seen to become gradually smaller and more rounded. decay is general and constant, it is obviously not uniform. In some They are ground against each other, and upon the rocky sides and places where, from the nature of the rock, from the flatness of the bottom of the channel, getting more and more reduced as they ground, or from other causes, rain works under great difficulties, descend, and at the same time abrading the rocks over or against the rate of waste may be extremely slow. In other places it may which they are driven. Hence a great deal of debris is produced, be rapid enough to be appreciable from year to year. A survey of and is swept along by the onward and downward movement of the this department of geological activity shows how unequal wasting water. The finer portions, such as mud and fine sand, are carried by rain, combined with the operations of ks and rivers, has in suspension, an impart the characteristic turbidity to riverproduced the details of the present relief of the land, those tracts water; the coarser sand and gravel are driven along the river. where the destruction has been greatest forming hollows and valleys, bottom. The proportion of suspended mineral matter has been others, where it has been less, rising into ridges and hills (Part VIII.)! ascertained with more or less precision for a number of rivers. As

Rain-action is not merely destructive, but is accompanied with an illustrative example of a river draining a vast area with different reproductive effects, chief of which is the formation of soil. In climates, forms of surface and geological structure the Mississippi favourable situations it has gathered together accumulations of loam may be cited. The average proportion of sediment in its water was and earth from neighbouring higher ground, such as the “ brick- ascertained by Humphreys and Abbot to be iibo by weight or earth, head," and "rain-wash" of the south of England-earthy too by volume. These engineers found that, in addition to this deposits, sometimes full of angular stones, derived from the subaerial suspended material, coarse detritus is constantly being pushed waste of the rocks of the neighbourhood.

forward along the bed of the river into the Gulf of Mexico, to an 2. Underground Water. --Of the rain which falls upon the land amount which they estimated at about 750,000,000 cubic ft. of one portion flows off into brooks and rivers by which the water is sand, earth and gravel; they concluded that the Mississippi carries conducted back to the ocean; the larger part, however, sinks into into the gulf every year an amount of mechanically transported the ground and disappears. 'It is this latter part which has now sediment sufficient to make a prism one square mile in area and to be considered. Over and above the proportion of the rainfall 268 ft. in height. which is absorbed by living vegetation and by the soil, there is a (6) Excavating Power.---It is by means of the sand, gravel and continual filtering down of the water from the surface into the rocks stones which hey drive against the sides and bottoms of their that lie below, where it partly lodges in pores and interstices, and channels that streams have hollowed out the beds in which they partly finds its way into subterranean joints and fissures, in which flow. Not only is the coarse detritus reduced in size by the friction it performs an underground circulation, and ultimately issues once of the stones against each other, but, at the same time, these materials more at the surface in the form of springs (9.9.). In the course of abrade the rocks against which they are driven by the current. this circulation the water performs an important geological task. Where, owing to the shape of the bottom of the channel, the stones Not only carrying down with the substances which the rain has are caught in eddies, and are kept whirling round there, they become abstracted from the air, but obtaining more acids and organic more and more worn down themselves, and at the same time scour matter from the soil, it is enabled to eficct chemical changes in the out basin-shaped cavities, or "pot-holes," in the solid rock lek it,

The uneven bed of a swiftly, flowing, stream may in this way be ice is that achieved by glaciers (9.v.) and ice-sheets. These vast honeycombed with such eroded basins which coalesce and thus masses of moving ice, when they descend from mountaints where the appreciably lower the surface of the bed. The steeper the channel, steeper rocks are clear of snow, receive on their surface the débris other conditions being equal, the more rapid will be the erosion. detached by frost from the declivities above, and bear these materials Geological structure also affects the character and rate of the excava to lower levels or to the sea. Enormous quantities of rock-rubbish tion. Where the rocks are so arranged as to favour the formation are thus transported in the Alps and other high mountain ranges. and persistence of a waterfall, a long chasm may be hollowed out When the ice retreats the boulders carried by it are dropped where like that of the Niagara below the falls, where a hard thick bed of it melts, and left there as memorials of the former extension of the ncarly flat limestone lies on softer and more easily eroded shales. glaciers. Evidence of this nature proves the much wider extent of The latter are scooped out from underneath the limestone, which the Alpine ice at a comparatively recent geological date. It can from time to time breaks off in large masses and the waterfall also be shown that detritus from Scandinavia has been ice-borne to gradually retreats up stream, while the ravine is proportionately the south-east of England and far into the heart of Europe. lengthened. To the excavating power of rivers the origin of the The ice, by means of grains of sand and pieces of stone which it valley systems of the dry land must be mainly assigned (see Part VIII.). drags along, scores, scratches and polishes the surfaces of rock

(c) Reproductive Power.-So long as a stream flows over a steep underneath it, and, in this way, produces the abundant fine sediment declivity' its velocity suffices to keep the sediment in suspension, that gives the characteristic milky appearance to the rivers that but when from any cause, such as a diminution of slope, the velocity issue from the lower ends of glaciers. By such long-continued is checked, the transporting power is lessened and the sediment attrition the rocks are worn down, portions of them of softer nature, begins to fall to the bottom and to remain there. Hence various or where the ice acts with especial vigour, are hollowed out into river-formed or "alluvial " deposits are laid down. These some cavities which, on the disappearance of the ice, may be filled with times cover considerable spaces at the foot of mountains. The water and become tarns or lakes. Rocks over which land-ice has floors of valleys are strewn with detritus, and their level may thereby passed are marked by a peculiar smooth, flowing outline, which be sensibly raised. lo floods the ground inundated on either side forms a contrast to the more rugged surface produced by ordinary of a stream intercepts some part of the detritus, which is then spread weathering. . They are covered with groovings, which range from over the flood-plain and gradually heightens it. At the same time the finest striae left by sharp grains of sand to deep ruts ground out the stream continues to erode the channel, and ultimately is unable by blocks of stone. The trend of these markings shows the direction to reach the old flood-plain. It consequently forms a new plain at in which the ice flowed. By their evidence the position and movea lower level, and thus, by degrees, it comes to be flanked on either ment of former

glaciers in countries from which the ice has entirely side by a series of successive terraces or platforms,.each of which vanished may be clearly determined (see GLACIAL PERIOD). marks one of its former levels. Where a river enters a large body of 6. The Sea.The physical features of the sea are discussed in water its current is checked. Some of its sediment is consequently separate articles (see OCEAN AND OCEANOGRAPHY). The sea must dropped, and by slow accumulation forms a delta (9.0.). On land, be regarded as the great regulator of temperature and climate over every lake in mountain districts furnishes instances of this kind of the globe, and as thus exerting a profound influence on the distribu. alluvium. But the most important deltas are those formed in the tion of plant and animal life. Its distinctly geological work is partly sea at the mouths of the larger rivers of the globe. Off many coast erosive and partly reproductive. As an eroding agent it must to lines the detritus washed from the land gathers into bars, which some extent effect chemical decompositions in the rocks and sedienclose long strips of water more or less completely separated from ments over which it spreads; but these changes have not yet been the sea outside and known as lagoons. A chain of such lagoon- satisfactorily studied. Undoubtedly, its chief destructive power barriers stretches for hundreds of miles round the Gulf of Mexico is of a mechanical kind, and arises from the action of its waves in and the eastern shores of the United States.

beating upon shore-cliffs. By the alternate compression and 4. Lakes. These sheets of water, considered as a whole, do not expansion of the air in crevices of the rocks

on which heavy breakers belong to the normal system of drainage on the land whereby valleys fall, and by the hydraulic pressure which these masses of sea-water are excavated. On the contrary they are exceptional to it;, for exert on the walls of the fissures into which they rush, large masses the constant tendency of running water is to fill them up, or to drain of rock are loosened and detached, and caves and tunnels are drilled them by wearing down the barriers that contain them at their along the base of sea-cliffs. Probably still more efficacious are the outflow. Some of them are referable to movements of the terrestrial blows of the loose shingle, which, caught up and hurled forward by crust whereby depressions arise on the surface of the land, as has the waves, falls with great force upon the shore rocks, battering been noted after earthquakes. Others have arisen from solution them as with a of artillery until they are worn away. The such as that of rock-sali or of limestone, the removal of which by smooth surfaces of the rocks within reach of the waves contrasted underground water causes a subsidence of the ground above. A with their angular forms above that limit bear witness to the amount third type of lake-basin occurs in regions that are now or have once of waste, while the rounded forms of the boulders and shingle show been subject to the erosive action of glaciers (see under next sub- that they too are being continually reduced in size. Thus the sea, division, Terrestrial Ice). Many small lakes or tarns have been by its action on the coasts, produces much sediment, which is swept caused by the deposit of débris across a valley as by landslips or away by its waves and currents and strewn over its floor. Besides moraincs. Considered from a geological point of view, lakes perform this material, it is constantly receiving the fine silt and sand carried an important function in regulating the drainage of the ground below down by rivers. As the floor of the ocean is thus the final receptacle their outfall and diminishing the destructive effects of floods, in for the waste of the land, it becomes the chief era on the surface of filtering the water received from their affluent streams, and in the globe for the accumulation of new stratified formations. And providing undisturbed areas of deposit in which thick and extensive such has been one of its great

functions since the beginning

of lacustrine formations may be accumulated. In the inland basins geological time, as is proved by the rocks that form the visible part of some dry climates the lakes are salt, owing to excess of evaporation, of the carth's crust, and consist in great part of marine deposits. and their bottoms become the sites of chemical deposits, particularly Chemical precipitates take place more especially in enclosed parts of chlorides of sodium and magnesium, and calcium sulphate and of the sea, where concentration of the water by evaporation can take carbonate.

place, and where layers of sodium chloride, calcium sulphate and 5. Terrestrial Ice. Each of the forms assumed by frozen water carbonate, and other salts are laid down. But the chief marine has its own characteristic action in geological processes. Frost has accumulations are of detrital origin. Near the land and for a variable a powerful influence in breaking up damp soils and surfaces of stone distance extending sometimes to 200 or 300 m. from shore the in the pores or cracks of which moisture has lodged. The water in deposits consist chiefly of sediments derived from the waste of the freezing expands, and in so doing pushes asunder the component land, the finer silts being transported farthest from their source. particles of soil or stone, or widens the space between the walls of At greater depths and distances the ocean floor receives a slow deposit joints or crevices. When the ice melts the loosened grains remain. of exceedingly fine clay, which is believed to be derived from the apart ready to be washed away by rain or blown off by wind, while decomposition of pumice and volcanic dust from insular or subby the widening of joints large blocks of rock are detached from marine volcanoes." Wide tracts of the bottom are covered with the faces of cliffs. Where rivers or lakes are frozen over the ice various forms of ooze derived from the accumulation of the remains exerts a marked pressure on their banks; and when it breaks up of minute organisms. large sheets of it are driven ashore, pushing up quantities of gravel

(C) Life. and stones above the level of the water. The piling up of the dis, Among the agents by, which geological changes are carried on rupted ice against obstructions in rivers ponds back the water, and upon the surface of the globe living organisms must be enumerated. often leads to destructive floods when the ice barriers break. Where Both plants and animals co-operate with the inorganic agents in the ice has formed round boulders in shallow water, or at the bottom promoting the degradation of the land. In some cases, on the other ("anchor-ice"), it may lift these up when the frost gives way, hand, they protect rocks from decay, while, by the accumulation of and may transport them for some distance. Ice formed in the their remains, they give rise to extensive formations both upon the atmosphere, and descending to the ground in the form of hail, often land and in the sea. Their operations may hence be described as causes great destruction to vegetation and not infrequently to alike destructive, conservative and reproductive. Under this headanimal life. Where the frozen moisture reaches the earth as snow, ing also the influence of Man as a geological agent deserves notice. it serves to protect rock, soil and vegetation from the effects of a) Plants.-Vegetation promotes the disintegration of rocks and frost; but on sloping ground it is apt to give rise to destructive soil in the following ways (1) By keeping the surfaces of stone avalanches or landslips, while indirectly, by its rapid melting, it moist, and thus promoting both mechanical and chemical dissolution, may cause serious floods in rivers.

as is especially shown by liverworts, mosses and other moisture But the most striking geological work performed by terrestrial ' loving plants. (a) By producing through their decay carbonic and

« ՆախորդըՇարունակել »