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 passage 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 interstitial

pores, some binding matter by which the partially or wholly incoherent

grains are agglutinated into a solid mass. 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

interstitial 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. _Stalactites_ 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 (_stalactites_) 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 analysis 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 mass 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 quantities 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 mass 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 passages 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 masses 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 mass 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 accumulations of

clay, loam, and brick-earth, which often reach many feet in thickness,

and which are undoubtedly the results of rain washing 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 analyses 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-shells of the usual size--a quantity 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 passes 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

coarser 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 mass 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 mass 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 passes, 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 accumulations. 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 coarse 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

passing 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

accumulate in water which is comparatively still, the latter are often

brought under the influence of waves and currents, and become shifted

and sifted to such a degree that fine and coarse detritus are frequently

commingled; and there is, therefore, not the same orderly succession of

coarse 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

cumbered 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 passage 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, passes into

the solid state, and in doing so pushes the grains of the rocks asunder,

or wedges out large masses. 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 masses 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 accumulates 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 constitutes what are called _glaciers_ or ice-rivers.

These great masses 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 mass 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 _crevasses_.

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

crevasses 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 coarse 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 masses 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 accumulating 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 quantities in sea-water, are yet of the very utmost

importance to marine creatures. This is the case with _carbonate of

lime_, vast quantities of which are carried down by many rivers to the

sea. But it must be nearly all used up in the formation of hard shells

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 masses fall down and get broken up and pounded into

gravel and sand. The new wall of rock thus exposed becomes in turn

assaulted, 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 masses 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 shore-lines.

Close inshore, the bulkier debris derived from the waste of the land

often accumulates, forming beds and banks of shingle 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

coarser-grained the deposit, the smaller will be the area it covers;

while conversely, the finer the accumulation, 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 coarse 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 coarse sand, and even

small stones, have been brought up from depths of a hundred fathoms, so

that currents may occasionally carry coarser materials for great

distances from the shore. The shifting 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 shore, and approached areas which

were once deeply submerged, we should find that the change of material

was generally from coarse to fine.