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Geological effects of Water seeking its Level.

312. This earth, with the circulation of water upon its surface, has been likened to the animal body, with its circulation of renovating blood. In the animal machine the moving agent is the heart, which acts as a forcing pump, sending the blood charged with fresh nourishment along the arterial channels to every part of the system, and thence back again by the veins to the heart to be re-charged. In the machine of nature, the great motive agent is the sun, whose heat raises aloft from the extended surface of the oceans and lakes, perfectly pure water to be diffused over the earth by the winds-these also, as we shall see, are effects of the solar heat-and to be deposited again in the form of rain, dew, or snow, as a life-giving drink to the animal and vegetable creations, and ultimately to return to the bosom of the ocean from which at first it sprung.

313. In order to understand the effect of this water, as it seeks the level of the ocean, in shaping the present features of our globe, we shall consider the following miniature

case:

A mill-pond, suddenly emptied by a sluice at its lowest part, would exhibit a variety of pits and pools left among the inequalities of the bottom. The subsequent fall of rain would cause each pool to overflow, and send out a streamlet either into another lower pool, or into a channel leading directly to the sluice. Thus there would be a constant wearing down of the side of the pool over which the water is running, and a lowering of the surface of water in the pool, while, at the same time, the bottom would be rising, owing to the deposit of matter washed down by the rain from the elevations around. These two operations continuing, the pool would at last disappear, and so the whole bottom of the emptied mill-pond would at last become only a wrinkled surface of dry land, with a beautiful ramification of water channels, all sloping with inimitable precision towards the general mouth or estuary. In every case, a watercourse soon becomes singularly uniform, both as to dimension and descent, because any hollows are gradually filled up by the sand and mud carried along in the stream, and deposited where the current is slack; while any elevations are gradually worn down by the action of the more rapid current which accompanies shallowness.

This is but a picture of what has been going on over the face of this earth ever since the local convulsions or more tranquil upheavals and depressions which led to the present unequal distri

Formation of Deltas and Plains by Rivers.

181

bution of land and water. In many places the effect of the gradual draining is already complete ; in others it is only in progress. Geologists have proved that much of the present dry land had, in remote ages, been sea-bottom, and that great part of it is but the hardened deposit from moving water of mud and sand, having embedded within it the innumerable shells, bones, and other remains now found, of the living beings which inhabited the earth during the change. It is thence concluded that our present continents and islands must have been upheaved from the bottom of an ocean, or an ocean must have subsided away from them; and that in either case the land must have emerged as chequered and unsightly as the bottom of our emptied mill-pond. But the gradual operation of "water seeking its level" has converted these primary inequalities and corrugations into the lovely plains and regular alternating water-courses which we now enjoy.

Nor is this a mere fanciful picture. Slight observation of the face of our globe shows that the extensive plains along the course of many rivers are evidently formed of the clay or sand which the stream has borne down, and may still be carrying down from the higher lands; while the remains found embedded in the soil are the shells and bones of such animals as have lived in the river or on its banks. These plains had been in remote time deep hollows or lakes surrounded by barriers of elevated land, through which the chief passages were those by which the river now enters and leaves them, which passages had evidently been gradually cut deep by the action of running water. The great delta of the Nile is now proved to be made up of the surface soil of the lofty mountains of Abyssinia, brought down annually by that river and deposited in Lower Egypt during the floods.

314. In former ages the Rhine, for instance, has been the drain of a chain of such basins or lakes, which have in this way been gradually filled up. This operation is seen to be still going on in all the lakes of the earth. It is ascertained that since the time of Julius Cæsar an extent of about three miles at the upper part of the Lake of Geneva has been converted into dry land by the wearings of the Alpine mountains, brought down by the winter torrents. Several villages that were close upon the lake some centuries ago, have now fields and gardens spreading between them and the shore; and if the town of Geneva last long enough, its inhabitants will have to speak of the river threading the neighbouring valley, instead of the picturesque lake which now fills it. In illustration of this subject,

182

The Hydrostatics of Geology.

it is interesting to observe the contrast between the proverbially pure blue water of the Rhone as it issues from the Lake of Geneva, and the turbid streams which enter it from above and around. These having deposited all their load of mud and sand in the still bosom of the lake, become the clear water of the river below. The streams which, below the lake, join the Rhone directly from the Alps, are long distinguishable by their muddy waters.

When, in the course of a river, there is no lake to intercept the solid matters which it carries down, these ultimately reach the sea, and form the deltas or regions of flat country seen at the mouths of rivers. There is an extensive formation of this kind at the mouth of the Rhone. The greater part of Holland is a similar deposit from the Rhine. The whole of Lower Egypt, and much of the flat fertile land higher up, has thus been formed by the Nile; much of Bengal has been formed by the Ganges, and so forth.

Where the soil or bed of a country through which a water-track passes is not of a soft consistence, so as to allow readily the wearing down of higher parts, and the filling up of hollows by deposited sand, lakes, rapids, and great irregularities of current remain. We have, for instance, the line of lakes in North America, the rapids of the St. Lawrence, and the stupendous Niagara, where at one leap the river falls one hundred and sixty feet. A softer barrier than the rock over which the river pours would soon be cut through, and the line of lakes might be emptied.

The consideration of the fact that water is constantly wearing where it flows, and carrying the abraded portions down to lower levels, and ultimately to the ocean bed, forces upon us the idea that this earth can have but a limited existence in its present state. Every shower sends portions of mountain and plain into the depths of the ocean, and thus causes a corresponding encroachment on the shores by the rising water. It is ascertained that the River Ganges alone carries down every year, and deposits in the Bay of Bengal, more solid matter than would cover the whole surface of England to a considerable thickness. With revolving ages, therefore, unless the causes which have operated in past time to upheave portions of the earth's crust from beneath the sea continue to operate, the whole of the present dry land must disappear. Human art may for a time succeed, as in Holland and elsewhere, in shutting out the ocean from some low tracks by means of sea-dykes or embankments, but its power is utterly insignificant when set against the great forces of nature.

Tides.-Inundations.

183

There is, perhaps, no fact that illustrates in a more striking manner the exact accordance of all nature's phenomena with the few general expressions called laws which describe them, than the steady maintenance of the mean height and level of the ocean as a liquid surface. The sea, although having in most parts a depth of thousands of feet, which fluctuates several feet twice in every day with the flood and ebb-tides, never rises or falls in any place, even one inch, but in obedience to fixed laws, which men can study. Were it not for this perfect exactness, in what a precarious state would the inhabitants exist on the sea-shores, and on the banks of low rivers! Few of the inhabitants of London, perhaps, reflect, when standing by the side of their noble river, and gazing on the rapid flood-tide pouring inland through the bridges, that, at the moment, the level of the wide ocean around the mouth of their river is several feet higher than that of the water near them.

The destruction that would follow a slight alteration in the level of the ocean, may be judged of by the effects of occasional floods, produced by rains and melting snow in the interior of countries, or by these combined with winds and high tides on the coasts. The accounts which have been published, under the title of Inundations, are truly appalling. In Holland, which is a low flat, formed chiefly by the mud and sand brought down by the Rhine and neighbouring rivers, much of the country is really below the level of the common spring-tides, and is protected from daily inundations only by artificial dykes or ramparts, intended to be strong enough to resist the ocean. Partial failures of these have been frequent; and, in the year 1580, a more extensive failure caused an inundation which drowned four hundred thousand people.

Where moderate inundation is regularly periodical, as in the Nile and many other rivers, the hurtful effects can be guarded against, and the occurrence may even be rendered highly useful in fertilizing the soil. Tracts of land in contact with rivers, where the surface lies between the levels of ebb and flood-tide, if surrounded with dykes, may be kept constantly covered with water, by sluices made to open only at high water, or may be kept constantly drained, by sluices which open only at low water. A vast extent of rice-fields, near the mouths of rivers in India and China are managed in this way, the admission or exclusion of water being regulated by the age of the rice plant. A great part also of the rich sugar plantations of Demerara, Essequibo, &c., on the coast of South America, are supplied with water under similar circumstances.

184 Liquid Pressure illustrated by Water Springs.

315. The subject of fluid level leads to the consideration of springs or wells, and of the operation of boring for water.

The rain which falls over the land, and which must ultimately return to the sea, may find its way to the river channels, either by running directly along the surface of soils which refuse it admittance, or by first sinking into porous earth, and then oozing out at lower situations in the form of springs. If a spring be as low as the bottom of the porous earth from which it issues, that is to say, as low as the surface of the impermeable clay or rock on which at some depth all such earth rests, it may drain the whole; but if not, the water will stand at a certain level among the earth as water stands among bullets in a tub. If a hole be then dug in such earth to below the level of the water lying there, it will soon be filled with water up to the level around, and will be called a well. In many places this water-level is very far below the surface of the ground; and in some places, by reason of the water having an easy drainage from the earth towards the sea, or of the superficial soil being altogether impervious to water, no well is to be found at all.

A remarkable illustration of this subject occurred some years ago in Kent, on the occasion of cutting between Rochester and Gravesend the canal then called the Thames and Medway Canal, now transformed into a railway. This canal consisted of but one cut or level, about seven miles long, two of which were in a tunnel through the hill. The level was that of high water in the connected rivers ; the intention having been to let the canal be filled always from the rivers at high tide: but as the permanent level of the subterranean water in the surrounding land, and therefore of all the wells of the inhabitants was, as should have been anticipated, half-way between the sea levels of high and low tides, the salt water from the rivers was no sooner admitted to the canal, than it spread into the land on both sides, where the resisting internal water-level was lower, and spoiled all the wells. If the canal had been dug a few feet lower, the evil would not have occurred, and the company would have escaped paying the heavy damages which rendered their undertaking a very profitless speculation.

The case has occurred, and it illustrates the general principle of water seeking its level, where, on wells being sunk to a lower level than cess-pools in the immediate vicinity, the water has been soon contaminated with the noxious products of sewage. Typhoid fever and other fatal diseases have been traced to accidents of this kind.