Geology Part 3

52. True _igneous rocks_ occur either in beds or as irregular amorphous ma.s.ses. When they occur as beds interstratified with aqueous strata, they are said to be _contemporaneous_, because they have evidently been erupted at the time the series of strata among which they appear was being ama.s.sed. When, on the other hand, they cut across the bedding, they are said to be _subsequent_ or _intrusive_, because in this case they have been formed at a period _subsequent_ to the strata among which they have been _intruded_. The bed upon which a contemporaneous igneous rock reclines, often affords marks of having been subjected to the action of heat; sandstones being hardened, and frequently much jointed and cracked, owing to the shrinking induced by the heat of the once molten rock above, and clay-rocks often a.s.suming a baked appearance.

There is generally, also, some discoloration both in the pavement of rock upon which the igneous ma.s.s lies, and in the under portions of the latter itself. The beds overlying a contemporaneous igneous rock, however, do not exhibit any marks of the action of heat; the old lava-stream having cooled before the sediment, now forming the overlying strata, was acc.u.mulated over its surface. One may often notice how the sand and mud have quietly settled down into the irregular hollows and crevices of the old lava, as in the following section, where _i_ represents the igneous rock; _a_ being the baked pavement of sandstone, &c.; and _b_ the overlying sedimentary deposits. When the igneous rock itself is examined, its upper portions are often observed to be scoriaceous or cinder-like, and the under portions likewise frequently exhibit a similar appearance. It is generally most solid towards the centre of the bed. The vesicles, or pores, in the upper and lower portions are often flattened, and are frequently filled with mineral matter. Sometimes these cavities may have been filled at the time the rock was being erupted, but in most cases the mineral matter would appear to have been introduced subsequently by the action of water percolating through the rock. Occasionally we meet with igneous rocks which are more or less vesicular and amygdaloidal throughout their entire ma.s.s. Others, again, often shew no vesicular structure, but are h.o.m.ogeneous from top to bottom. The texture is also very variable, and this even in the same rock-ma.s.s; some portions being compact or fine-grained, and others coa.r.s.ely crystalline. As a rule the rock is most crystalline towards the centre, and gets finer-grained as the top and bottom of the bed are approached. Not unfrequently, however, an igneous rock will preserve the same texture throughout. The jointing is also highly irregular as a rule. But in many cases, especially when the rock is fine-grained, the jointing is very regular. The basaltic columns of the Giants' Causeway and the Isle of Staffa are well-known examples of such regularly jointed ma.s.ses. Igneous rocks frequently decompose into a loose earthy ma.s.s (_wacke_), and this is most markedly the case with those belonging to the basic group.

[Ill.u.s.tration: Fig. 20.--Contemporaneous Igneous Rock.]

53. Contemporaneous igneous rocks are frequently a.s.sociated with more or less regular beds of _breccia_, _conglomerate_, _ash_, _tuff_, &c. These are evidently the loose volcanic ejectamenta which accompanied former eruptions of lava, and have been arranged by the action of water. Beds of such materials, however, frequently occur without any accompanying lava-form rocks. Nor are they always arranged in bedded ma.s.ses. They sometimes appear filling vertical pipes which seem to have been the funnels of old volcanoes. The following section exhibits the general appearance of one of these volcanic _necks_. They are very common in some parts of Scotland, as in Ayrs.h.i.+re, and are frequently ranged along the line of a fault in the strata. Fig. 21 shews such a neck of ejectamenta, made up of fragments of various kinds of rock, such as sandstone, shale, limestone, coal, &c., sometimes without any admixture of igneous rocks. The strata through which the pipe has been pierced usually dip in towards the latter, and at their junction with the coa.r.s.e agglomerate often shew marks of the action of heat, coal-seams having sometimes been 'burned' useless for a number of yards away from the 'neck.'

[Ill.u.s.tration: Fig. 21.--Neck filled with Volcanic Agglomerate.]

54. Intrusive igneous rocks occur as _sheets_, _d.y.k.es_, and _necks_. The sheets frequently conform for long distances to the bedding of the strata among which they occur, and are thus liable to be mistaken for contemporaneous rocks. But when they are closely examined, it will be seen that they not only bake or alter the beds above and below them, but seldom keep precisely to one horizon or level--occasionally rising to a higher, or sinking to a lower position in the strata, as shewn in the following diagram-section. d.y.k.es are wall-like ma.s.ses of igneous strata which cut across the strata, generally at a high angle (see _d, d_, fig.

22). In the neighbourhood of a recent volcanic orifice, numerous d.y.k.es are seen ramifying in all directions. In the British Islands some d.y.k.es have been followed in a linear direction for very long distances.

Sometimes these occupy the sites of large dislocations, at other times they have cut through the strata without displacing them. Occasionally they appear to have been the feeders of the great sheets of igneous rock which here and there occur in their vicinity. The phenomena presented by the _necks_ of intrusive rock do not differ from those characteristic of _agglomerate_ or _tuff necks_. The strata are bent down towards the central plug of igneous rock, and are generally more or less altered at the line of junction.

[Ill.u.s.tration: Fig. 22.--Intrusive Sheet and d.y.k.es: _i_, igneous intrusive sheet; _d_, _d_, d.y.k.es; _s_, _s_, sedimentary strata.]

55. Intrusive rocks offer, as a rule, some contrasts in texture to contemporaneous ma.s.ses. They are seldom amygdaloidal, but when they are so it is generally towards the centre of the ma.s.s. The kernels are usually minute and more or less spherical.

[Ill.u.s.tration: Fig. 23.--Contemporaneous and Intrusive Igneous Rocks: _c_, _c_, contemporaneous trap-rocks[1]; _t_, _t_, contemporaneous fragmental igneous rocks; _i_, _p_, _n_, _d_, intrusive igneous rocks.]

The diagram (fig. 23) shews the general mode of occurrence of igneous rocks on the large scale. The stratified aqueous deposits are indicated at _a_, _a_. These are overlaid by a series of alternating beds of crystalline (_c_) and fragmental (_t_) igneous rocks. An irregular intrusive sheet at _i_ cuts across the beds _a_, _a_. At _p_, another intrusive ma.s.s is seen rising in a pipe, as it were, and overflowing the beds _a_, _a_, so as to form a cap. A volcanic neck filled with angular stones intersects the strata at _n_, and two d.y.k.es, approaching the vertical, traverse the bedded rocks at _d_, _d_. It will be noticed that the contemporaneous igneous rocks form a series of escarpments rising one above the other.

The alteration effected by igneous rocks is generally greatest in the case of intrusive ma.s.ses. This is well seen in some of our coal-fields, where the coal has frequently been destroyed over large areas by the proximity of ma.s.ses of what was once melted rock. It is curious to notice how the intrusive sheets in a great series of strata have forced their way along the lines of least resistance. Thus, in the Scottish coal-fields, we find again and again that intrusive sheets have been squirted along the planes occupied by coal-seams, these having been more easily attacked than beds of sandstone or shale. The coal in such cases is either entirely 'eaten up,' as it were, or converted into a black soot. At other times, however, it is changed into a kind of c.o.ke, while other seams at a greater distance from the intrusive ma.s.s have been altered into a kind of 'blind coal' or _anthracite_.

These remarks on the mode of occurrence of igneous rocks are meant to refer chiefly to those ma.s.ses which occur in regions where volcanic action has long been extinct, as, for instance, in the British Islands.

In the sequel, some account will be given of the appearances presented by modern volcanoes and volcanic rocks.

[1] It has been usual to apply the term _trap_ or _trappean_ rock to all the old igneous rocks which could neither be cla.s.sed with the granites and syenites, nor yet with the recent lavas, &c., which are connected with a more or less well-marked volcanic vent. The term _trap_ (Swedish _trappa_, a flight of steps) was suggested by the terraced or step-like appearance presented by hills which are built up of successive beds of igneous rock. But the pa.s.sage from the granitic into the so-called trap rocks, and from these into the distinctly volcanic, is so very gradual, that it is impossible to say where the one cla.s.s ends and the other begins. The term _trap_, therefore, has no scientific precision, although it is sometimes very convenient as a kind of broad generic term to include a large number of rocks.

MINERAL VEINS.

56. The cracks and crevices and joint planes which intersect all rocks in a greater or less degree, are not unfrequently filled with subsequently introduced mineral matter, forming what are termed _veins_.

This introduced matter may either be harder or less durable than the rock itself; in the former case, the veins will project from the surface of the stone, where that has been subjected to the weathering action of the atmosphere; in the latter case, the veins, under like circ.u.mstances, are often partially emptied of their mineral matter. Not unfrequently, however, the more or less irregularly ramifying, non-metalliferous veins appear as if they had segregated from the body of the rock in which they occur, as in the case of the quartz veins in granite. Besides these irregular veins, the rocks of certain districts are traversed in one or more determinate directions by fissures, extending from the surface down to unknown depths. These great fissures are often in like manner filled with mineral matter. The minerals are usually arranged in bands or layers which run parallel to the walls of the vein. Quartz, fluor-spar, barytes, calcite, &c. are among the commonest vein-minerals, and with these are frequently a.s.sociated ores of various metals. A vein may vary in width from less than an inch up to many yards, and the arrangement of its contents is also subject to much variation. Instead of parallel layers of spars and ores, frequently a confused ma.s.s of clay and broken rocks, which are often cemented together with sparry matter, chokes up the vein. The ore in a vein may occur in one or more ribs, which often vary in thickness from a mere line up to ma.s.ses several yards in width.

Sometimes the rocks are dislocated along the line of fissure occupied by a great vein; at other times no dislocation can be observed. Mineral veins, however, do not necessarily occupy dislocation fissures. They often occur in cavities which have been formed by the erosive action of acidulated water, in the way described in pars. 59, 60, and 61. This is frequently the case in calcareous strata. Such veins usually coincide more or less with the bedding of the rocks, but in the case of thick limestones they not unfrequently cut across the bedding in a vertical or nearly vertical direction, forming what are termed _pipe-veins_.

DYNAMICAL GEOLOGY.

57. Having considered the composition, structure, and arrangement of the rock-ma.s.ses which form the solid crust of our globe, we have next to inquire into the nature of those physical agencies by the action of which the rocks, as we now see them, have been produced. The work performed by the various forces employed in modifying the earth's crust is at one and the same time destructive and reconstructive. Rocks are being continually demolished, and out of their ruins new rocks are being built. In other words, matter is constantly entering into new relations--now existing as solid rock, or in solution in water, or carried as the lightest dust on the wings of the wind; now being swept down by rivers into the sea, or brought under the influence of subterranean heat--but always changing, sooner or later, slowly or rapidly, from one form to another. The great geological agents of change are these: 1. THE ATMOSPHERE; 2. WATER; 3. PLANTS AND ANIMALS; 4.

SUBTERRANEAN FORCES. We shall consider these in succession.

THE ATMOSPHERE.

58. All rocks have a tendency to waste away under the influence of the atmosphere. This is termed _weathering_. Under the influence of the sun's heat, the external portions of a rock expand, and again contract when they cool at night. The effect of this alternate expansion and contraction is often strikingly manifest in tropical countries: some rocks being gradually disintegrated, and crumbling into grit and sand; others becoming cracked, and either exfoliating or breaking up all over their surface into small angular fragments. Again, in countries subject to alternations of extreme heat and cold, similar weathering action takes place. The chemical action of the atmosphere is most observable in the case of calcareous rocks. The carbonic acid almost invariably present acts as a solvent, so that dew and rain, which otherwise would in many cases have but feeble disintegrating power, are enabled to eat into such rocks as chalk and limestone, calcareous sandstones, &c. The oxygen of the atmosphere also unites with certain minerals, such as the proto-salts of iron, and converts them into peroxides. It is this action which produces the red and yellow ferruginous discolorations in sandstone. Chemical changes also take place in the case of many igneous rocks, the result being that a weathered 'crust' forms wherever such rocks are exposed to the action of the atmosphere. Of course, the rate at which a rock weathers depends upon its mineralogical and chemical composition. Limestones weather much more rapidly than clay-rocks; and augitic igneous rocks, as a rule, disintegrate more readily than the more highly silicated species. The weathering action of the atmosphere is also greatly aided by frost, as we shall see presently. The result of all this weathering is the formation of _soil_--soil being only the fine-grained debris of the weathered rocks. The angular debris found at the base of all cliffs in temperate and arctic regions, and on every hill and mountain which is subjected to alternations of extreme heat and cold, is also the effect of weathering. But these and other effects of frost will be treated of under the head of _Frozen Water_. The hillocks and ridges of loose sand (_sand dunes_) found in many places along the sea-margin, and even in the interior of some continents, as in Africa and Asia, are due to the action of the wind, which drives the loose grains before it, and piles them up. Sometimes also the wind carries in suspension the finest dust, which may be transported for vast distances before it falls to the ground. Thus, fine dust shot into the air by the volcanoes of Iceland has been blown as far as the Shetland Islands; and in tropical countries the dust of the dried-up and parched beds of lakes and rivers is often swept away during hurricanes, and carried in thick clouds for leagues. Rain falling through this dust soaks it up, and comes down highly discoloured, brown and red. This is the so-called _blood-rain_. Minute microscopic animal and vegetable organisms are often commingled with this dust, and falling into streams, lakes, or the sea, may thus become eventually buried in sediments very far removed from the place that gave them birth.

WATER.

59. The geological action of water in modifying the crust of the earth is twofold--namely, _chemical_ and _mechanical_.

_Underground Water._--All the moisture which we see falling as rain or snow does not flow immediately away by brooks and rivers to the sea.

Some portion of it soaks into the ground, and finds a pa.s.sage for itself by cracks and fissures in the rocks below, from which it emerges at last as springs, either at the surface of the earth, or at the bottom of the sea. Such are the more obvious courses pursued by the water--it flows off either by sub-aerial or subterranean channels. But a not inconsiderable portion soaks into the solid rocks themselves, which are all more or less porous and pervious. Water thus slowly soaking often effects very considerable chemical changes. Sometimes the binding matter which held the separate particles of the rock together is dissolved out, and the rock is thus rendered soft and crumbling; at other times, the reverse takes place, and the water deposits, in the minute interst.i.tial pores, some binding matter by which the partially or wholly incoherent grains are agglutinated into a solid ma.s.s. Thus what were originally hard and tough rocks become disintegrated to such a degree, that they crumble to powder soon after they are exposed to the air; while some again are converted into a clay, and may be dug readily with a spade.

And, on the other hand, loose sand is glued into a hard building-stone.

There are many other changes effected upon rocks by water, in virtue of the chemical agents which it holds in solution. Indeed, it may be said that there are very few, if any, rocks in which the chemical action of interst.i.tial water has not formerly been, or is not at present being, carried on. Besides that which soaks through the rocks themselves, there is always a large proportion of underground water, which, as we have said above, finds a circuitous route for itself by joints, cracks, and crevices. After coursing for, it may be, miles underground, such water eventually emerges as springs, which contain in solution the various ingredients which the water has chemically extracted from the rocks.

These ingredients are then deposited in proportion as the mineral water suffers from evaporation. Water impregnated with carbonate of lime, for example, deposits that compound as soon as evaporation has carried off a certain percentage of the water itself, and the carbonic acid gas which it held. This is the origin of the mineral called _travertine_ or _calcareous tufa_, which is so commonly met with on the margins of springs, rivers, and waterfalls.

60. _Stalact.i.tes_ and _stalagmites_ have been formed in a similar way.

Water slowly oozing from the roof of a limestone cavern partially evaporates there, and a thin pellicle of carbonate of lime is formed; while that portion of the water which falls to the ground, and is there evaporated, likewise gives rise to the formation of carbonate of lime.

By such constant dropping and evaporating, long tongue-and icicle-like pendants (_stalact.i.tes_) grow downwards from the roof; while at the same time domes and bosses (_stalagmites_) grow upwards from the floor, so as sometimes to meet the former and give rise to continuous pillars and columns. The great solvent power of carbonated water is shewn first by the chemical a.n.a.lysis of springs, and, secondly, by the great wasting effects which the long-continued action of these has brought about.

Thus, it has been estimated that the fifty springs near Carlsbad, which yield eight hundred thousand cubic feet of water in twenty-four hours, contain in solution as much lime as would go to form a ma.s.s of stone weighing two hundred thousand pounds. Warm, or, as they are termed, _thermal_ springs, frequently carry away with them, out of the bowels of the earth, vast quant.i.ties of mineral matter in solution. The waters at Bath, for instance, are estimated to bring to the surface an annual amount of various salts, the ma.s.s of which is not less than 554 cubic yards. One of the springs of Loueche, France, however, carries out with it no less than 8,822,400 pounds of gypsum annually, which is equal to about 2122 cubic yards.

61. It is easy to conceive, therefore, that in the course of ages great alterations must be caused by springs. Caves and winding galleries, and irregular channels, will be worn out of the rocks which are thus being dissolved. Especially will this be the case in countries where calcareous rocks abound. It is in such regions, accordingly, where we meet with the most striking examples of caves and underground river-channels. The largest cave at present known is the Mammoth Cave, in Kentucky. This remarkable hollow consists of numerous winding galleries and pa.s.sages that cross and recross, and the united length of which is said to be 217 miles. In calcareous countries, rivers, after flowing for, it may be, miles at the surface, suddenly disappear into the ground, and flow often for long distances before they reappear in the light of day. In some regions, indeed, nearly all the drainage is subterranean. The surface of the ground, in calcareous countries, frequently shews circular depressions, caused by the falling in of the roofs of caverns. Sometimes, also, great ma.s.ses of rock, often miles in extent, get loosened by the dissolving action of subterranean water, and crash downwards into the valleys. Such _landslips_, as they are called, are not, however, confined to calcareous regions. In 1806, a large section of the Rossberg, a mountain lying to the north of the Righi, consisting of conglomerate overlying beds of clay, rushed down into the plains of Goldau, overwhelming four villages and nearly a thousand inhabitants. The cause of this catastrophe was undoubtedly the softening into mud of the clay-beds on which the conglomerate rested, for the season which had just terminated when the slip took place had been very wet. The ma.s.s of material that slid down was estimated to contain upwards of fifty-four millions of cubic yards; it reached not less than two and a half miles in length, by some three hundred and fifty yards wide, and thirty-five yards thick.

62. _Surface-water--Rain._--Having now learned something as to the modifications produced by underground water, we turn next to consider the action of surface-water, and the results arising from that action.

Rain, when it falls to the ground, carries with it some carbonic acid gas which it has absorbed from the atmosphere. Armed with this solvent, it attacks certain rocks, more especially limestones and chalk, a certain proportion of which it licks up and delivers over to brooks and streams. Under its influence, also, the finer particles of the soil are ever slowly making their way from higher to lower levels. Rocks which are being gradually disintegrated by weathering have their finer grains and particles, thus loosened, carried away by rain. Nor is this rain-action so inconsiderable as might be supposed. In the gentler hollows of an undulating country, we frequently find acc.u.mulations of clay, loam, and brick-earth, which often reach many feet in thickness, and which are undoubtedly the results of rain was.h.i.+ng down the particles of soil, &c. from the adjacent slopes.

63. _River-action._--The water of streams and rivers almost invariably contains in solution one or more chemical compounds, and in this respect does not differ from the water of springs. Of course, this mineral matter is derived in considerable measure from springs, but is also no doubt to a large extent taken up by the rivers themselves, as they wash the rocks and soils on their journey to the sea. The amount of mineral matter thus transported must be something enormous, as is shewn by the chemical a.n.a.lyses of river-water. Bischof calculated that the Rhine carries in solution as much carbonate of lime as would suffice for the yearly formation of three hundred and thirty-two thousand millions of oyster-sh.e.l.ls of the usual size--a quant.i.ty equal to a cube five hundred and sixty feet in the side, or a square bed a foot thick, and upwards of two miles in the side. But the mechanical erosion effected by running water is what impresses us most with the importance of rivers as geological agencies. This erosive action is due to the gravel, sand, and mud carried along by the water. These ingredients act as files in the hand of a workman, and grind, polish, and reduce the rocks against which they are borne. The beds of some streams that flow over solid rock are often pitted with circular holes, at the bottom of which one invariably finds a few rounded stones. These stones, kept in constant motion by the water, are the means by which the _pot-holes_, as they are called, have been excavated. When pot-holes are numerous, they often unite so as to form curious smooth-sided trenches and gullies. The same filing action goes on all over the bed of the stream wherever the solid rock is exposed. And while the latter is being gradually reduced, the stones and grit which act as the files are themselves worn and reduced; so that stones diminish in size, and grit pa.s.ses into fine sand and mud, as they move from higher to lower levels. No doubt the erosive action of running water appears to have but small effect in a short time, and we are apt, therefore, to underestimate its power. But when our observations extend, we see it is quite otherwise, and that, so far from being unimportant, running water is really one of the most powerful of all the geological agencies that are employed in modifying the earth's crust. Even within a comparatively short time, it is able to effect very considerable changes. Thus, the river Simeto, in Sicily, having become dammed by a stream of lava flowing from Etna, succeeded, in two hundred and fifty years, in cutting through hard solid basalt a new channel for itself, which measured from twenty to fifty metres in depth, and from twelve to eighteen in breadth. When, also, we remember the fact, that no river is absolutely free from mineral matter held in suspension, but that, on the contrary, all running water is more or less discoloured with sediment, which is merely the material derived from the disintegration of rocks, it will appear to us difficult to overestimate the power of watery erosion. To the mineral matter held in suspension, we have to add the coa.r.s.er detritus, gravel and sand, which is being gradually pushed along the beds of rivers, and which, in the case of the Mississippi, has been estimated to equal a ma.s.s of seven hundred and fifty million cubic feet, discharged annually into the Gulf of Mexico. By careful measurements, it has also been ascertained that the same river carries down annually into the sea a weight of mud held in suspension which reaches the vast sum of 812,500,000,000 pounds. The total annual amount of mineral matter, whether held in suspension or pushed along the bottom of this great river, has been estimated to equal a ma.s.s 268 feet in height, with an area of one square mile.

64. _Alluvium._--The sediment carried along and deposited by a river is called _alluvium_. Sometimes this alluvium covers wide areas, forming broad flats on one or both sides of a river, and in such cases it is due to the action of the floodwaters of the stream. Every time the river overflows the low grounds through which it pa.s.ses, a layer of sediment is laid down, which has the effect of gradually raising the level of the alluvial tract. By and by a time comes when the river, which has all the while been slowly deepening its channel, is unable to flood the flats, and thereupon it begins to cut into these, and to form new flats at a somewhat lower level. In this way we often observe a series of alluvial terraces, consisting of gravel, sand, and silt, rising one above another along a river valley. Such are the terraces of the Thames and other rivers in England, and of the Tweed, Clyde, Tay, &c. in Scotland. The great plains through which the Rhine flows between Basel and Bingen, are also well-marked examples of alluvial acc.u.mulations. There are very few streams, indeed, which have not formed such deposits along some portion of their course.

65. When a river enters a lake, the motion of the water is of course checked, and hence the heavier detritus, such as gravel and coa.r.s.e sand, moves more slowly forward, and at last comes to rest on the bed of the lake, at no great distance from the mouth of the river. Finer sand and mud are carried out for some distance further, but eventually they also cease to move, and sink to the bottom. When the lake is sufficiently large, it catches all or nearly all the matter brought down by the river, which, as it issues from the lower end of the lake, is bright and clear. A well-known example of this phenomenon is that of the Rhone, which enters the Lake of Geneva turbid and muddy, but rushes out quite clear at the lower end of the lake. Lakes, therefore, are all being slowly or more rapidly silted up, and this, of course, is most conspicuous at the points where they are entered by rivers. Thus, at the head of the Lake of Geneva, it is manifest that the wide flat through which the river flows before it pours into the lake, has been conquered by the Rhone from the latter. In the times of the Romans, the lake, as we know, extended for more than one mile and a half further up the valley.

66. _Deltas._--When there are no lakes to intercept fluviatile sediment, this latter is borne down to the sea, where it is deposited in precisely the same way as in a lake: the heavier detritus comes to rest first, the finer sediment being swept out for some distance further. So that, in pa.s.sing from the river-mouth outwards, we have at first gravel, which gradually gets finer and finer until it is replaced by sand, while this in turn is succeeded by mud and silt. There is this difference, however, between lacustrine and fluvio-marine deposits, that while the former acc.u.mulate in water which is comparatively still, the latter are often brought under the influence of waves and currents, and become s.h.i.+fted and sifted to such a degree that fine and coa.r.s.e detritus are frequently commingled; and there is, therefore, not the same orderly succession of coa.r.s.e and fine materials which characterises lacustrine deposits.

Often, indeed, the currents opposite the mouth of a river are so strong, that little or no sediment is permitted to gather there. Usually, however, we find that rivers have succeeded in reclaiming more or less wide tracts from the dominion of the waves, or at all events have c.u.mbered the bed of the sea with banks and bars of detritus. The broad plains formed at the mouth of a river are called _deltas_, from their resemblance to the Greek letter [Delta]. The deltas of the Nile, Ganges, and Mississippi are among the most noted. The term _delta_, however, is not exclusively applied to fluvio-marine deposits; rivers also form deltas in fresh-water lakes. It is usual, however, to restrict the term to extensive alluvial plains which are intersected by many winding channels, due to the rapid bifurcation of the river, which begins to take place at the very head of the great flat--that is to say, at the point where the river originally entered the sea (or lake).

67. _Frozen Water._--We have now seen what can be done by the mechanical action of running water. We have next to consider what modifications are effected by freezing and frozen water. Water, as every one knows, expands in the act of freezing, and in doing so exerts great force. Let the reader bear in mind what has been said as to the pa.s.sage of water through the minute and often invisible pores of rocks, and to its presence in cracks and crevices after every shower of rain, and he will readily see how excessive must be the waste caused by the action of frost. The water, to as great a depth as the frost extends, pa.s.ses into the solid state, and in doing so pushes the grains of the rocks asunder, or wedges out large ma.s.ses. No sooner does thaw ensue than the water, becoming melted, allows the grains of the rock to fall asunder; the outer skin of the rock, as it were, is disintegrated, and crumbles away, while fragments and ma.s.ses lose their balance in many cases, and topple down. Hence it is, that in all regions where frost acts, the hill-tops and slopes are covered with angular fragments and debris, and a soil is readily formed by the disintegration of the rocks.

River-ice is often a potent agent of geological change. Stones get frozen in along the margins of a river, and often debris falls down from cliff and scaur upon the surface of the ice; when thaw sets in, and the ice breaks up, stones and rubbish are frequently floated for long distances, and may even be carried out to sea before their support fails them, and they sink to the bottom. In some cases, when the ice is very thick, it may run aground in a river, and confuse and tumble up the deposits gathering at the bottom. Ice sometimes forms upon stones at the bottom of a river, and floats these off; and this curious action may take place even although no ice be forming at the time on the surface of the water.

68. _Glaciers, Icebergs, and Ice-foot._--In certain mountainous districts, and in arctic and antarctic regions, snow acc.u.mulates to such an extent that its own weight suffices to press the lower portions into ice. Alternate thawing and freezing also aid in the formation of the ice, which soon begins to creep down the mountain-slopes into the valleys, where it const.i.tutes what are called _glaciers_ or ice-rivers.

These great ma.s.ses of ice attain often a great thickness, and frequently extend for many miles along the course of a valley. In the Alps they occasionally reach as much as five hundred or six hundred feet in depth.

In Greenland, however, there are glaciers probably not less than five thousand feet thick; and the glacier ice of the antarctic continent has been estimated even to reach twelve miles in thickness. Glaciers flow slowly down their valleys, at a rate which varies with the slope of their beds and the ma.s.s of the ice. Some move only a few inches, others two or three feet, in a day. Their forward motion is arrested at a point where the ice is melted just as fast as it comes on. A glacier is always more or less seamed with yawning cracks, which are called _creva.s.ses_.

These owe their origin to the unequal rate at which the different parts of the ice flow; this differential motion causing strains, to which the ice yields by snapping asunder. The flanks of a glacier are usually fringed with heaps of angular blocks and debris which fall from the adjacent rocky slopes, and some of this rubbish tumbling into the gaping creva.s.ses must occasionally reach to the bottom of the ice. The rubbish heaps (_superficial moraines_) travel slowly down the valley on the surface of the ice, and are eventually toppled over the end of the glacier, where they form great banks and mounds. These are called _terminal moraines_. The rocky bed of a glacier is invariably smoothed and polished, and streaked with coa.r.s.e and fine _striae_, or scratches, which run parallel to the direction of the ice-flow. These are due to the presence, at the bottom of the ice, of such angular fragments as become detached from the underlying rocks, or of boulders and rubbish which have been introduced from above. The stones are ground by the ice along the surface of its bed, causing ruts and scratches, while the finer material resulting from the grinding action forms a kind of polisher. The stones acting as gravers are themselves covered with striae, and their sharp edges get smoothed away. In alpine districts there is always a good deal of water circulating underneath a glacier, and this washes out the sand and fine clay. Thus it is that rivers issuing from glaciers are always more or less discoloured brown, yellow, green, gray, or blue, according to the nature of the rocks which the ice has pounded down into mud. In Greenland many of the large glaciers go right out to sea, and owing to their great thickness are able to dispossess the sea sometimes for miles. But erelong the greater specific gravity of the sea-water forces off large segments from the terminal front of the ice, which float away as _icebergs_. Large ma.s.ses are also always falling down from the ice-front. Occasionally, big blocks and debris are floated away on the icebergs, but this does not appear to be common. In Greenland there is very little rock-surface exposed, from which blocks can be showered down upon the glaciers, and the surface of the latter is therefore generally free from superficial moraines. A kind of submarine terminal moraine, however, gathers in front of some glaciers, made up chiefly of the stones and rubbish that are dragged along underneath the ice, and exposed by the breaking-off of icebergs, but partly composed also of the sand and mud washed out by sub-glacial waters. A narrow belt of ice forms along the sea-coast in arctic regions, which often attains a thickness of thirty or forty feet. This is called the _ice-foot_. It becomes loaded with debris and blocks, which fall upon it from the cliffs above; and, as large portions are frequently detached from the cliffs in summer-time, they sail off with their cargoes of debris, and drop these over the sea-bottom as they gradually melt away. The ice-foot is the great distributor of _erratics_ or wandered blocks, the part taken in this action by the huge icebergs which are discharged by the glaciers being, comparatively speaking, insignificant. But when these latter run aground, they must often cause great confusion among the beds of fine material acc.u.mulating upon the floor of the sea.

69. _The Sea._--Sea-water owes its saltness to the presence of various more or less soluble substances, such as _common salt_, _gypsum_, _Epsom salts_, _chloride of magnesium_, &c. Besides these, there are other ingredients held in solution, which, although they can be detected in only minute quant.i.ties in sea-water, are yet of the very utmost importance to marine creatures. This is the case with _carbonate of lime_, vast quant.i.ties of which are carried down by many rivers to the sea. But it must be nearly all used up in the formation of hard sh.e.l.ls and skeletons by molluscs, crustaceans, corals, &c., for very little can be traced in the water itself. _Silica_ is also met with sparingly, and is abstracted by some creatures to form their hard coverings.

70. _Breaker-action--Currents._--The most conspicuous action of the sea, as a geological agent, takes place along its margin, where the breakers are hurled against the land. Stones and gravel are borne with more or less intense force against the rocks, and by their constant battering succeed eventually in undermining the cliffs, which by and by become top-heavy, and large ma.s.ses fall down and get broken up and pounded into gravel and sand. The new wall of rock thus exposed becomes in turn a.s.saulted, and in course of time is undermined in like manner. The waste of the cliffs is greatly aided by the action of frost, which loosens the jointed rocks, and renders them an easier prey to the force of the waves. Of course, the rapidity with which a coast-line is eaten into depends very much upon the nature of the rocks. Where these are formed of loose materials like sand, gravel, or clay, considerable inroads are effected by the sea in a comparatively short time. Thus, along some parts of the English coast, as between Flamborough Head and the mouth of the Humber, and between the Wash and the Thames, it is estimated that the land is wasted away at the rate of a yard per annum. Where hard rocks form the coast-line the rate of waste is often exceedingly slow, and centuries may elapse without any apparent change being effected.

When the rocks are of unequal hardness the coast-line becomes very irregular, the sea carving out bays and gullies in the softer portions, while the more durable ma.s.ses stand out as capes and bold headlands. Not unfrequently, such headlands are converted into sea-stacks and rocky islets, as one may observe along the rockier parts of our sh.o.r.e-lines.

Close insh.o.r.e, the bulkier debris derived from the waste of the land often acc.u.mulates, forming beds and banks of s.h.i.+ngle and gravel. The finer materials are carried farther out to sea, and distributed over the sea-floor by the action of the tide and currents. Tidal and other currents may also have some denuding effect upon the sea-bottom, but this can only be in comparatively shallow water. The great bulk of the material derived from the waste of the coasts by the mechanical action of the breakers, travels for no great distance. But the fine mud brought down by rivers is frequently transported for vast distances before it settles. So fine, indeed, is some of this sedimentary material, that it may be carried in suspension by sea-currents for thousands of miles before it sinks to the bottom.

71. From this short outline it becomes evident, therefore, that the coa.r.s.er-grained the deposit, the smaller will be the area it covers; while conversely, the finer the acc.u.mulation, the more widely will it be distributed. A partial exception to this rule is that of the debris scattered over the bottom of the ocean by icebergs and detached portions of ice-foot. These are often floated for vast distances by currents before they finally melt away, and hence the coa.r.s.e debris transported by them must be very widely distributed over that part of the sea-bottom which is traversed by currents flowing out of the Arctic and Antarctic Oceans. Although the deeper recesses of the ocean appear to be covered only with ooze and fine mud, yet in some instances coa.r.s.e sand, and even small stones, have been brought up from depths of a hundred fathoms, so that currents may occasionally carry coa.r.s.er materials for great distances from the sh.o.r.e. The s.h.i.+fting action of tidal currents succeeds in giving rise to very irregular deposits in shallow seas. The soundings often shew sudden changes from gravel to sand and mud, nor can there be any doubt that, could we lay bare the sea-bottom, we should often observe gravel shading off into sand, and sand into mud, and _vice versa_. But as we receded from the sh.o.r.e, and approached areas which were once deeply submerged, we should find that the change of material was generally from coa.r.s.e to fine.

GEOLOGICAL ACTION OF PLANTS AND ANIMALS.