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deemed sufficient for the purpose of irrigation, its height must exceed by 34 feet that which was requisite ten centuries ago. According to this statement, the ground must have been raised at the rate of nearly 44 inches in a century. The ancient monuments of the land all have their bases, more or less, covered by the mud which has been, for ages, accumulating around them.

The delta of the Rhone undergoes a similar augmentation, and it would appear that the arms of that river have, in the course of 1800 years, become longer by three leagues; and that many places which were once situated on the brink of the sea, or of large pools, are now several miles distant from the water. In Holland and Italy, the Rhine and the Po, since they have been banked up by dikes, raise their beds and push forward their mouths into the sea with great rapidity. Many cities which, at periods within the range of history, were flourishing sea-ports, have, by the encroachments on the water, been deprived of their importance. It is with extreme difficulty that the Venetians are able to preserve the lagunes by which their city is separated from the main land; and in all probability Venice is destined to experience the fate of Ravenna, which, according to Strabo, stood among lagunes in the time of the Roman emperor Augustus, but is now a league from the shore. M. Cuvier records some curious information which he obtained from M. de Prony, inspector-general of bridges and roads, who was appointed to investigate the remedies that might be applied to the devastations committed by the floods of the Po. This clearly displays some of the surprising changes which the coast of the Adriatic has undergone. At the beginning of the twelfth century, the whole waters of the Po flowed to the south of Ferrara, in the two channels called Po di Volano, and Po di Primaro; an irruption of the river to the north of that city happened not long after, and owing to this new direction of the stream, the two old channels in question had, in less than 100 years, been reduced to the comparative insignificance in which they still remain. Since the construction of the grand embankments of the Po, the formation of New Land has proceeded very rapidly, especially within

These are very extensive sheets of water, but so shallow that they, in no part, exceed six or seven feet in depth.

the last two centuries. Such indeed has been the increase, that the city of Adria, which there is no doubt was, at a very remote date, seated on the coast of the Adriatic, is now more than fifteen miles distant from the nearest part of it. The distance from the same city to the extreme point of the promontory of the alluvial land, deposited round what is now the principal mouth of the Po, is upwards of twenty miles. At the same time that river has so much raised the level of its bottom, that the surface of its waters is now higher than the roofs of the houses in Ferrara; and the Adige and the Po are higher than the whole tract of country lying between them. The high level above the surrounding plain, attained by the Rhine and the Meuse in Holland, since they have been banked up; the additions of land that have been made along the shores of the North Sea, in Holstein, Friesland, Groningen, &c.; and the diminution of the sea of Azof, by the entrance of alluvial matter from the Don, are further instances of the changes which nature is able to produce by the most simple means. The Yellow Sea (so named from its waters being coloured by an intermixture of particles of yellow mud) affords a similar example. This sea, which lies between the peninsula of Corea and the eastern coast of China, is exceedingly shallow, as may be seen from the account of Capt. Hall, who navigated it in the year 1816(Voyage to Loo-Choo, &c.) That officer states, that no land could be perceived from the mast-head at the time when his ship was in less than five fathoms water; and, before a sight of land was obtained, even this depth was considerably reduced. The bottom consisted of mud, formed of an impalpable powder, without the least sand or gravel. The fine particles, from which this mud is deposited, are brought down by innumerable streams from China and Tartary.

The alterations perceived to be taking place in many of those lakes which are traversed by rivers, proceed from the same cause as the extensions of alluvial land into the sea which we have just been considering. The matter brought down by rivers easily settles in the still water of lakes, and the necessary result is, that the basins of the latter are gradually undergoing a diminution. This process, carried on for a sufficient length of time, would end in the filling up of the lake, and in its place there would

be a valley intersected, of course, by the same rivers which formerly flowed into the lake. Owing to the very long time required for the purpose, there is no instance known of a lake, of any size, ever having been filled up in this manner; but there are well authenticated cases of their being very sensibly diminished. Lake Erie, one of the vast bodies of water in North America, is rapidly decreasing; in the late survey of the boundaries between the United States and Canada, it was ascertained that Long Point, opposite Big Creek river, on the north side of the lake, had, in the space of three years, increased. more than that number of miles in length by the accession of alluvial matter; and this immense basin, the average depth of which is estimated at between thirteen and seventeen fathoms, is every year becoming shallower from the influx of pebbles and earth, and the constant accumulation of reeds and shells. The diminution of the beautiful lake of Geneva is also said to have been considerable within the memory of man. There are several instances in mountainous and marshy countries of small lakes having been dried up from different causes-such as the crystallization, or deposit of substances which the waters had previously held in solution; the gradual union of floating islands, and the collection of matter arising from the lake being the seat of animal and vegetable life; but it is evident, from their very slow progress, that the effects produced in these ways cannot be upon a very large scale.

The changes which we have hitherto traced to the action of waters have been of a beneficial kind; but others of a destructive nature are brought about by the same agency. These are the breaking down of steep coasts by the waves, and the throwing up of sandhills, which the winds afterwards assist in pushing forward and dispersing over the adjacent land. The first is a very common occurrence; the sea detaches fragments from the foot of the cliffs, or else wastes it away, and then the upper parts, deprived of support, fall down. The broken portions that collect at the base, in consequence of these fallings down, serve, more or less, and for a shorter or longer period, according to their position and hardness of material, to protect the cliff from further ravages. The circumstances also which cause the slipping down or breaking away of

masses of rock and earth, among mountains, operate in a similar manner where there are shores of a steep character. Springs filter through and displace the soft strata, and thus the more solid formations are left without support; the consequence is that, at times, large spaces of land slide or fall down from above. It is by such means as these that the land-slips,' on the southern shore of the Isle of Wight, have been produced. The same thing happens, but on a far grander scale, upon the coast of the Crimea; whole tracts are there carried down, sometimes bearing upon them the houses of the natives, which have, notwithstanding, been known to escape without injury.

The action of the sea, when the coast is low, and the bottom sandy, leads to very different results. The waves then push the sand forward upon the shore, where, at every ebb of the tide, it be comes partially dried; and the wind, frequently blowing from the sea, drifts it upon the beach. By little and little, hillocks or downs of sand are created, the higher parts of which are continually carried inland; so that unless the inhabitants of the country succeed in fixing them by causing suitable plants to take root in their soil, they move slowly on and overwhelm fields and dwellings with inevitable ruin. It sometimes happens that the sand cast up by the water becomes mixed with marine and other substances, which, being enveloped therein, make what have been denominated indurated, that is, hardened downs, such as are seen upon the coast of New Holland. Perhaps the most remarkable instance of the mischief occasioned by the moving downs is to be found on the French coast of the Bay of Biscay, south of the river Gironde, where they have already overwhelmed a great number of villages mentioned in the records of the middle age, and not long ago, in the single department of the Landes, were threatening ten with unavoidable destruction. One of these villages, named Mimisan, had been struggling against them for twenty years, with the prospect of a sand-hill of more than sixty feet in height visibly approaching it. In 1802, the pools formed by the collection of waters which these downs prevent from flowing into the sea, covered five farming establishments at the village of St. Julien. They have long been over an ancient Roman road leading from Ba

yonne to Bourdeaux, and which could still be seen forty years back, when the waters were low. The river Adour is now turned to the distance of more than 2000 yards out of its former course. The progress of these downs has been estimated at 60 feet yearly, and in some places, at 72 feet; at this rate, it is calculated that it will require 2000 years to enable them to reach Bourdeaux.

The coast of Elgin or Morayshire, in Scotland, also affords a striking example of the sand-flood; an account of which is given among the notes affixed to Professor Jameson's edition of Cuvier's Theory of the Earth. West of the mouth of the river Findhorn, a district of more than ten square miles in area, (chiefly included in the barony of Coubine,) which, on account of its extreme fertility, was once termed the granary of Moray, has been depopulated and rendered unproductive by the shifting of the sand-hills. It appears that the irruption of the sand commenced about the year 1677; that in 1697, not a vestige was to be seen of the manor place, orchards, and offices of Coubine, and that two thirds of the barony were already ruined. This irruption came from Mavieston, situated on the shore, about seven miles west from the mouth of the Findhorn, where, from time immemorial, there had been large heaps of sand. These sands, which had formerly been covered with vegetation, were set at liberty by the inhabitants inconsiderately pulling up the bent and juniper for various uses, and they then drifted towards the north-east. When the wind is high, the fine particles are carried even across the bay of Findhorn. In the winter of 1816, a large portion of the only remaining farm, on the west side of the Findhorn, situated in the line of the sand's progress, was overwhelmed. The effects produced by the sand upon the river have been, and still are, of a very obvious kind. Many years ago, its mouth having become blocked up, the water cut out its present more direct channel. By this change, the old town of Findhorn, which originally stood on the east side of the river, was left upon its western bank; and the inhabitants, in consequence, removed the stones of their houses across the new channel, and erected the present village on the eastern side. The site of the old town is now covered by the sea. Even now, when the tide retires, the river almost disappears, being absorbed by

the sand; and owing to the bar formed across its entrance, it is unable, at spring tides, to force its way into the sea, so that it is made to flow back and inundate a considerable extent of land at the head of the bay. Of late, however, the great accumulations of sand have disappeared from Coubine, and the ancient rich soil has, in some places, been left bare, from which it is hoped that the barony will again become serviceable land. Such a result would be rendered much more certain, if, by putting in proper kinds of plants, they were to fix the surface of the Mavieston hills, and so prevent fresh inroads of the sand from that quarter; yet, notwithstanding the destruction that has already happened, the inhabitants are still in the habit of gathering what little bent yet remains.

The same drifting of sand occurs upon several parts of the west coast of the Outer Hebrides, and the prevention of it has been attempted in two ways. Mr. A. Macleod, surgeon of North Uist, has invented the most efficacious plan, which is that of cutting thin square turfs from the neighbouring pasture grounds, and laying them down at intervals of some inches. In the course of a few years the turfs grow together, while the ground, from which they are taken, is little injured, for as the roots remain in it, a new vegetation soon rises up. The other method was introduced by Mr. Macleod, of Harris, and tried extensively upon his estate it is to plant small bundles of arundo arenaria, (sand-reed) about a foot and a half apart; these take root and prevent the drifting, in a certain degree.

Another process, similar in its effects to those which have been already described, but much more extensively destructive, and depending solely upon the action of the wind, is the encroachment of the sands of the Libyan desert upon the cultivated lands of Egypt. These sands, driven by the west winds, have left no soil capable of tillage on any parts of the western banks of the Nile, which are destitute of the shelter of mountains. It would appear that, but for the ridge called the Libyan chain, which borders the left bank of that river, forming to the parts along which it runs a barrier against the sands, the western shores of the Nile would, long ago, have been made uninhabitable. Travellers have given a melancholy picture of the traces which bear witness to the ravages

committed by the sand-the ruins of numerous cities and villages destroyed, and the summits of the minarets of mosques, being still visible above the surface. It is partly to these resistless invasions of the desert that the decline of Egypt from her ancient splendour is to be attributed.

The formation of new islands (to which allusion has been made in a preceding part of this treatise) constitutes a distinct and interesting class among the changes to which the surface of the globe is subject. Those which have been raised up by volcanic agency are, comparatively, few; but those of coral, which owe their origin to marine insects, (of the class of zoophytes or plant-animals) are innumerable. Of the different coral tribes, the most abundant is that named the madrepore. It is most common in the tropical seas, and decreases in number and variety towards the poles; it surrounds, in vast rocks and reefs, many of the rocky islands of the South Sea and Indian Ocean, and increases their size by its daily growth. The coasts of the islands in the West Indies, of those to the east of Africa, and the shores and shoals of the Red Sea, are encircled with rocks of coral. Several navigators have furnished us with accounts of the curious manner in which these formations take place: the following is extracted from Captain Basil Hall's narrative of his voyage to the Loo Choo islands:

"The examination of a coral reef, during the different stages of one tide, is particularly interesting. When the tide has left it for some time, it becomes dry, and appears to be a compact rock exceed ingly hard and rugged; but as the tide rises, and the waves begin to wash over it, the coral worms protrude themselves from holes which were before invisible. These animals are of a great variety of shapes and sizes, and in such prodigious numbers, that, in a short time, the whole surface of the rock appears to be alive and in motion. The most common of the worms at Loo-Choo is in the form of a star, with arms from four to six inches long, which are moved about with a rapid motion, in all directions, probably to catch food. Others are so sluggish, that they may be mistaken for pieces of the rock, and are generally of a dark colour, and from four to five inches long, and two to three round. When the coral is broken, about high water mark, it is a solid

hard stone; but if any part of it be detached at a spot which the tide reaches every day, it is found to be full of worms of different lengths and colours: some being as fine as a thread, and several feet long, of a bright yellow, and sometimes of a blue colour; others resemble snails, and some are not unlike lobsters in shape, but soft, and not above two inches long.

"The growth of coral appears to cease when the worm is no longer exposed to the washing of the sea. Thus, a reef rises in the form of a cauliflower, till its top has gained the level of the highest tides, above which the worm has no power to advance, and the reef, of course, no longer extends itself upwards. The other parts, in succession, reach the surface, and there stop, forming, in time, a level field with steep sides all round. The reef, however, continually increases, and being prevented from going higher, extends itself laterally in all directions. But this growth being as rapid at the upper edge as it is lower down, the steepness of the face of the reef is still preserved. These are the circumstances which render coral reefs so dangerous in navigation; for, in the first place, they are seldom seen above the water; and in the next, their sides are so steep, that a ship's bows may strike against the rock, before any change of soundings has given warning of the danger."

Captain Flinders, who, in 1801, made a survey of the coasts of New Holland, has some observations upon the formation of coral islands, particularly of Half-Way island, on the north coast of that region, which show how, after being raised up, they gradually acquire a soil and vegetation:

--

"This little island, or rather the surrounding reef, which is three or four miles long, affords shelter from the south-east winds; and being at a moderate day's run from Murray's Isles, it forms a convenient anchorage for the night to a ship passing through Torres' Strait-I named it Half-way Island. It is scarcely more than a mile in circumference, but appears to be increasing both in elevation and extent. At no very distant period of time, it was one of those banks produced by the washing up of sand and broken coral, of which most reefs afford instances, and those of Torres' Strait a great many. These banks are in different stages of progress; some, like this, are become

PHYSICAL GEOGRAPHY.

islands, but not yet habitable; some are above high-water mark, but destitute of vegetation; whilst others are overflowed with every returning tide. "It seems to me, that when the animalcules which form the corals at the bottom of the ocean cease to live, their structures adhere to each other, by virtue either of the glutinous remains within, or of some property in salt water; and the interstices being gradually filled up with sand and broken pieces of coral washed by the sea, which also adhere, a mass of rock is at length formed. Future races of these animalcules erect their habitations upon the rising bank, and die in their turn, to increase, but principally to elevate this monument of their wonderful labours. The care taken to work perpendicularly in the early stages would mark a surprising instinct in these diminutive creatures. Their wall of coral, for the most part in situations where the winds are constant, being arrived at the surface, affords a shelter, to leeward of which their infant colonies may be safely sent forth; and to this, their instinctive foresight, it seems to be owing, that the windward side of a reef exposed to the open sea is generally, if not always, the highest part, and rises almost perpendicular from the depth of many fathoms. To be constantly covered with water seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark; but the coral, sand, and other broken remnants thrown up by the sea, adhere to the rock, and form a solid mass with it, as high as the common tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their adhesive property; and remaining in a loose state, form what is usually called a Key, upon the top of the reef. The new bank is not long in being visited by sea-birds: salt plants take root upon it, and a soil begins to be formed; a cocoa-nut, or the drupe of a pandanus, is thrown on shore; land birds visit it, and deposit the seeds of shrubs and trees; every high tide, and still more every gale, adds something to the bank; the form of an island is gradually assumed; and last of all, comes man to take possession.

"Half-way Island is well advanced in the above progressive state; having been many years, probably some ages, above the reach of the highest spring tides, or the wash of the surf in the

heaviest gales. I distinguished, how-
ever, in the rock which forms its basis,
the sand, coral, and shells, formerly
thrown up, in a more or less perfect
Small pieces of
state of cohesion.
wood, pumice stone, and other extra-
neous bodies which chance had mixed
with the calcareous substances when
the cohesion began, were inclosed in the
rock, and, in some cases, were still
separable from it without much force.
The upper part of the island is a mix-
ture of the same substances in a loose
state, with a little vegetable soil; and is
covered with the casuarina and a va-
riety of other trees and shrubs, which
give food to parroquets, pigeons, and
some other birds; to whose ancestors,
it is probable, the island was originally
indebted for this vegetation."

It has been generally believed that
the deep perpendicular reefs, very near
to which the sounding line finds no
bottom, consist wholly of coral; but
MM. Quoy and Gaimard have adduced
very satisfactory reasons, to prove that
the zoophytes, far from raising from
the depths of the ocean perpendicular
walls, form only layers or crusts of a
few fathoms thickness. They remark
that the species which always construct
the most considerable banks require the
influence of light to perfect them; and
it is well known, that all those steep
walls common in the equatorial seas,
are intersected with narrow and deep
openings, through which the sea enters
and retires with violence; whereas, if
they were entirely composed of madre-
pores, they would have no such open-
ings between them, since it is the pro-
perty of zoophytes to build in masses
that have no interruption. It is besides,
difficult to suppose that these animals
can support such different degrees of
pressure and temperature as they neces-
It is, therefore,
sarily must, if they exist at such different
depths in the ocean.
most reasonable to conclude that the
summits of submarine hills and moun-
up their
tains are the bases upon which the
zoophytes form layers and raise
fabrics; a supposition which perfectly
accounts for the great depth of the sea
close to the reefs and islands which
they have elevated to the surface of the
water.

The changes occasioned by the eruptions of volcanoes are very considerable near the seat of action, but they operate over a less extensive field than any of C those which have yet been mentioned.

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