If the relative length be multiplied by 180, it will give the length in miles; and if the relative discharge be multiplied by 1800, it will give the feet discharged per second.

SECTION III.

CHANGES OF the earth's SURFACE.

The alternations of heat and cold, the sweeping of the wind, and the dashing of the rain, have a constant tendency to wear away the lofty and prominent parts of the earth; and the waters which descend from the mountains carry along with them the finer parts of the fragments, and very often masses of considerable magnitude. But there is no known power by which matter may again be made to return to the higher regions, unless it can be evaporated like water, or changed into air. These are not changes to which the stony and earthy matters, carried down by the rivers, can, in general, be subjected; and, therefore, the general tendency of the everyday operations of nature, is to equalize the surface of the earth; to level down the hills, and fill up the valleys. When the velocity of the water becomes so much diminished as that it cannot carry forward the substances

with which it is charged, they are deposited, and, in the event of the river's running into a still lake, form beds of deposit along its bottom; or, if the river empty itself into the sea, the tide rolls back the matter discharged along with the water; and a delta, or piece of flat alluvial land is formed. If the river be subject to sudden floods, from the fall of rain or the melting of snow, and if it have no lake to regulate the discharge of the water, this process will go on with considerable rapidity. Thus the Nile, which runs rapidly among the central mountains of Africa, and then slowly through Egypt, without any lake, has, in the course of ages, formed a very considerable portion of land. The same thing has happened with the Ganges. the Rhine, the Mississippi, and a number of other rivers. If the whole course of the river be slow, and it pass through a lake, a delta will not be formed with so much rapidity or to such an extent. Thus, generally speaking, the St. Lawrence runs with not very great rapidity, and it passes through lakes of very great size, so that, instead of projecting a delta out into the sea, the St. Lawrence has left its channel clear. The Rhone is a very rapid river, but, in passing through the lake of Geneva, it is prevented from forming a delta so rapidly as the Po, which, descending from the same mountains, sweeps along the plain of Lombardy. If a river be rapid in the early part of its course, and become slower afterwards, as by descending from a mountainous country into a valley, it will, if the valley is level, deposit the greater part of the matters which it brings down in the upper part of the valley; but if the valley slope, they will be deposited along the whole channel, with the fragments and substances finer and lighter as the current advances. In this way, while the river is cutting in the mountainous country a deep channel for itself, it may form in the valley a new channel, elevated above the common surface. Thus the Po, in the valley of Lombardy, and the Mississippi, in the great central valley of America, are confined within banks which they have formed for themselves; and sometimes, when they are swollen to a height much greater than usual, they overflow those artificial banks, and deluge the country. In many places the water, which in this way escapes from the Mississippi, renders the adjoining country a marsh.

When a river descends rapidly from one plain to another, it invariably cuts into the face or breast-work of the elevation. If that be soft, it of course cuts more rapidly than if it be hard; but no degree of hardness that is known to exist, can, altogether, prevent the river from cutting. When large pieces are removed, that is done by the mere force of the water, but where the rock is cut away, the effect is chiefly to be attributed to the particles or sand which the water carries along with it.

When the river has filled the valley through which it flows, with a mass of alluvial matter, it is always, if the current be rapid, changing its channel among that matter; and the general tendency is to cut away from the high bank, and throw the spoil upon the low, and this, although the high

bank may
be composed of more hard and compact materials than the other.
In like manner, the sea is often found cutting in upon a shore of rock, and
throwing the debris upon a flat shore of sand or mud. Nor is it difficult to
account for this: It has already been stated that the water is always
deepest at the steep bank, and that upon a low bank, it is always shallow.
Thus, the force of the water, when put in motion, either by a tide of the
sea, or a current of a river, is much greater upon a stcep shore or bank,
than upon a low one, and, in the case of the steep bank, the gravitation of
the matter of which it is composed, adds to the power of the water.

Hence it is easy to see how solid matters, even rocks, may be reduced to
sand and dust, by the waters of seas and rivers, and how, in proportion to
the force, length, and continuance of the currents, either of the one or the
other, the spoils of one district may be transported to another, and how,
where the water which, when in motion, held the sand and mud in suspen-
sion, may deposit it in new strata, when it becomes stagnant. In this
way the upper part of a lake, into which a rapid river discharges itself, will
be, in time, filled up by the larger fragments, while the whole bed or bottom
will be covered with a deposit of the more attenuated matters.

But the bed of such a lake may, in time, become a fertile valley, by the river which it discharges wearing away the barrier by which the water of the lake is confined; and there is hardly a mountainous country, in which there are not abundant proofs of this; for we find, in the courses of all mountain rivers, narrow passes which have been cut through the rocks, and by this means, the waters which filled the more extended valleys above, has been discharged, and the surface of that valley has been gradually converted into land. The valley of Athol, in Perthshire, is a striking instance of this. The rocks which cross that valley, immediately above Dunkeld, have evidently once confined the waters of the Tay, at a height of several hundred feet above their present level; the rocks of the pass of Killicrankie must, in like manner, have confined the waters of the Tummel and Garry; and, indeed, there is hardly a river in the whole extent of the Scottish mountains, which does not offer an instance of what has been here stated; and more extended instances are found in the Rhine, which has evidently cut through the rocky barrier about Coblentz; in the Elbe, which, by opening for itself a passage through the mountains of Saxony, has emptied water which covered the whole of Bohemia; and in many other places both of the eastern and western continent. At the great fall of Niagara, too, the St. Lawrence is cutting through the lime-stone rock, and must in time empty in succession, all the great lakes which lie between the territory of the United States and Upper Canada.

By means of this operation, the surface of the earth must be gradually getting more warm; for when a great portion of the surface is covered by lakes, the evaporation destroys a great portion of the solar heat, and the quantity

of humidity with which the atmosphere is thus continually loaded, produces frequent rain or snow, according to the season of the year.

One would, at first, be very apt to suppose that the discharge of so many lakes as has taken place in the lapse of ages, must have added to the water in the sea, and raised its surface so as to flood the land. But in the very process of emptying the lake, the river provides that the land shall not be permanently flooded; for, before the water is let out, the whole matters cut away by the river are deposited along its course; and we know, from the fact that, in those rivers which do not deposit rubbish enough for forming a delta there is always a bar, that the sea throws back upon the shore immediately joining the greater part of the matters brought down by the river. Hence we find, near the mouths of the larger rivers, beds of peat and wood, and even boats and anchors buried at a considerable depth below the present surface.

The sudden discharge of a great lake in the upper country, explains another circumstance, which we often meet with in the lower part of the valley, and for which, without this it would not be very easy to account. We often find two or three channels for the river at different elevations, and on opposite sides of the present one; and we also find hills of gravel that could hardly be accounted for without such an agency. But when we reflect upon the mass of, perhaps, many cubic miles of water, passing rapidly over the soft alluvial soil, we have an explanation of their appearances equally simple and satisfactory; and if the action of this water be very violent, and if it be immediately met by the tide of the sea, we can conceive the possibility of such an accumulation of matter, close by the shore, as to form a new lake in the lower part of the valley.

Water-spouts bursting upon mountains, composed of loose materials, produce nearly the same effect as the discharge of lakes; and the writer of these pages has seen a village, with all its fields, buried to the very tops of the cottages, in a single day, almost in an hour, by the effect of a waterspout, which tore away a gravelly mountain immediately above.

But, besides the slow changes which are produced by the action of the water, there are others of a more rapid and extensive nature, produced by a more active agent. Earthquakes may alter the surface in a moment, and volcanoes may either bury under ashes and lava, the surface which was formerly fair and fertile, or they may force up new land from the bottom of the ocean; and as, when the volcano casts its molten matters upon the surface, the place whence they come must be an internal cavity, the roof of that may, in time, give way, and the external surface may be much below its present depth, and even buried under the sea.

It is not improbable that earthquakes and volcanoes are produced by the same agent differently modified; and that, in both, this agent is gas of a very expansive and inflammable nature, produced in the interior of the earth. This gas may, in the earthquake, find its escape, without entering a

new combination by being burnt, or meeting with substances that can be put or kept in a state of ignition; and which during that escape tears the earth in pieces. In volcanoes, again, it may meet with a supply of combustible matters; and as we know that of all combustibles, the component parts of water are among the most violent, it is probable that these feed the volcanic fire. This conjecture receives a colour of truth from the fact, that volcanoes are never at any very great distance from the sea. Etna, Vesuvius, and Hecla, rise almost immediately from the shore; the Lipari volcanoes are in the sea; and those of the Andes and the eastern parts of Asia are at no great distance from it. In the mountains of Cevennes, in France, there are 100 volcanic mountains which have been extinct since the earliest records of history; the land which lies between them and the Mediterranean is chiefly alluvial, and has evidently been in great part formed by the water of the Rhone when it cut its way to the present level of the lake of Geneva; and this, at whatever time it may have happened, may have removed the Mediterranean to as great a distance from the ridge of the Cevennes, as to cut off the supply of water and extinguish the volcanoes. The volcanic products do not, however, enter deeply into the earth, they rather extend along the surface, and therefore, if the great strata were produced and elevated by the action of fire, that fire must have been more deeply seated than that which produces the present volcanoes, and the mode of its elevation must also have been different.

The following extract, from a very able article in the Supplement to the Encyclopedia Britannica, contains an estimate of the alluvial deposits made by some of the principal rivers, both of the eastern and the western continent, and also of the floodings to which they are subjected. "Where rivers which pass through low and level tracts are subject to annual inundation, the earth, sand, and gravel, they bring down, deposited most abundantly on their banks, raise them gradually above the surrounding country, while a part of the matter carried to the sea enlarges the coast, or forms sand or mud banks, which rise, by degrees, above the water. It is thus that the Ganges, Po, Nile, Mississippi, and many other rivers, flow on the top of ridges. During floods, the elevated sides are sometimes burst through, and the waters which escape either stagnate in lakes, or return into the main stream, lower down, forming islands, or travel to the sea by a separate mouth and form a delta. Hence the Amazons, Orinoco, Mississippi, Nile, Danube, Wolga, Ganges, abound in islands, and have deltas from 50 to 200 miles in breadth at their mouths. In the tracts of new ground which are thus gained from the sea, the soil may be seen of every degree of solidity, from soft mud to firm land. In the broad plain at the mouth of the Ganges, between the Tipperah hills and Bardwan, wherever the ground is penetrated, no virgin soil is found, but an alluvial deposit of sand and mud arranged in strata. The finest soil transported to the sea renders the water muddy to the distance of twenty miles from the coast. The Mississippi is