planted and a large forest been called into existence, which not only has moderated the force of the wind and given solidity to the movable sand, but has also converted it into valuable ground. These pine forests have greatly improved the health of the district, and form one of the chief attractions for visitors to Arcachon.

In Denmark the dunes cover an area of more than 260 square miles, which vary from a narrow row of hills to a width of 6 miles and a height of 20 feet. The average rate of advance landward is from 3 to 20 feet a year. As the sand advances the ruins of the buildings which in former times had became buried are exposed. Various laws were passed against stripping these dunes of the Arundo arenaria (Klittetag in Danish), and about the middle of the eighteenth century a regular system was adopted of preserving and planting this grass, and also of conifers and birches, which proved a valuable aid to the fixing of the sand and rendering it productive.1

On the south shores of the Baltic the dunes extend in an almost unbroken chain from the North Point of Jutland to the Elbe, a distance of 300 miles.

The same

In Prussia they occupy an area of 100,000 acres. system of protection and planting has been pursued here as in Denmark, and between the mouth of the Vistula and West Prussia 14,000 acres have been planted and secured from drifting. The island of Heligoland depends on its sand-dunes for its defence from the sea.

In the middle of the eighteenth century the dunes which protected the island of Syet, on the west of Schleswig, began to move eastward, and the sea followed. The church of the village of Rantum was obliged to be taken down, and thirty years afterwards the sand-hills had passed the site, the waves had swallowed up its foundations, and the sea was gaining so rapidly that fifty years later the site where they were was nearly 300 yards from the shore.2

Sand-dunes are to be found in several parts of the coast of America. Upon the Atlantic coast the prevalence of westerly winds blowing off shore is not favourable to their formation. Where, however, by the irregularity of the coast-line, the beach faces west, dunes are to be found. At Hatteras and Henlopen they attain a height of 70 feet, and have filled up a swamp and

"Man and Nature."

Andresen, "Om Klit-formationem."

buried houses and land, and it is anticipated that in a few years the island lying north of Cape Hatteras will be rendered uninhabitable.

The seaward extremity of Cape Cod is largely made up of wind-blown sands, which are covered with whortel-berry bushes; and there are extensive dunes on the mainland in the Essex district.

There are also ranges of dunes on the Pacific Coast and on the shores of San Francisco.

On the south-eastern shores of Lake Michigan dunes rise to a height of from 100 to 200 feet, and have buried a considerable area of forest land, the tops of the trees in places being just visible above the sand.

In the island of Bermuda, which is composed of coral and shell fragments, the shell sand blown from the beach in gales has accumulated in dunes, in some cases 250 feet high. At Elbow Bay the sand has filled up a valley, covering houses, gardens, and woodlands, leaving behind only the trunks of dead trees standing partially exposed in the midst of a sandy plain.1

The removal of existing forests near the coast has in some places had a disastrous effect on the country behind them. In the last century the estate of Conbin, near Forres, Scotland, was overwhelmed with irruptions of blown sand caused by the cutting down of trees and grubbing up the star-grass which grew on the sand-hills.

The cutting down of the timber in the forests on the west coast of Jutland, by removing the resistance to the lower currents of the westerly winds, led to the moving inland of the sand-dunes and the burying of the cultivated land with sand, and as the sand was blown from the shore, the sea followed closely behind. Remedial measures were taken about the end of the eighteenth century, and the replanting of the land has since stopped this advance.

The formation of artificial dunes for the protection of the land from the inundation of the sea has been successfully practised in some places. On a tide-washed flat lying between the Zuiderzee and the North Sea, a sand-dyke was thrown up between 3 and 4 miles in length, enclosing 1600 acres of land, and against this the wind-blown sand has accumulated to the width of a mile; and a similar result followed the construction of a bank called 1 "Rocks and Rock Weathering," by G. P. Merrill. Macmillan & Co. 1897.

the Zyperzeedyk. On the north coast of Holland, at Ameland, the sea was encroaching on the south side, and the sand was being stripped from a low flat connecting the two higher parts of the island, allowing the water to invade the island in storms. Between 1749 and 1809 the low hills were driven back 150 yards, or at the rate of 10 yards a year. The further progress of the sand was stopped by the construction of a sand-dyke and breakwater. The dunes were also regulated both as to slope and general direction, deep indents were filled in, and some grasses planted. In exposed places the foot of the dunes was protected by fascine-work weighted with stones and the face covered with faggots. The length of foreshore thus protected was 4 miles, and the result entirely satisfactory.1

100

Alluvium consists of detritus eroded from the cliffs or brought down the rivers in suspension, and consists of the decomposition of material from rocks of many different kinds of which cliff's are composed, or through which the rivers and their tributaries have flowed. While particles of sand of from 1 to 6 of an inch in diameter rapidly settle in water and do not discolour it, the size of the particles of which alluvium is composed are so minute that they remain in suspension for a considerable time, and in proportion to the minuteness of the particles cause the water to remain turbid for a longer or shorter period. When sand is mixed with a sufficient amount of chalk or clay to give the particles a certain amount of adherence, the mixture is termed silt; and mud consists of particles finer than those of which silt is composed. When the size of the particles suspended in the water is so small as to be from 0 to 1000 of an inch down to sizes so minute as not to be counted except by the aid of a powerful microscope, they constitute the richest kind of alluvium, and when settled on the sea-bed this material is known as ooze.

Clay consists of very fine grains of silica to the extent of from 70 to 90 per cent., the alumina giving it the peculiar sticky and tenacious character by which it is known. The particles of silica in clay vary from about of an inch down to sizes too small to count.

800

The specific gravity of alluvial soil may be as great as that of sand, but owing to the minute size of the individual particles these are kept longer in suspension by the viscosity of the

water.

1 Min. Proc. Inst. C.E., vol. lxxxix., 1886.

Alluvium mixed with water makes it turbid, and a period varying from a few minutes to several hours elapses, when the water is undisturbed, before the action of gravity overcomes the frictional resistance and the particles of solid matter rest on the bottom.

As the constituents of which the earth is composed are few in number, and as the varying kinds of rocks are dispersed generally over wide areas of the country, the ingredients of the alluvium, although influenced to a limited degree by the preponderance of some special element, are practically the same in all estuaries.

The mean of twenty-one samples taken from places so far apart as the estuaries of the Humber, the warp from the river Trent, a polder in Holland, the Danube, the Mississippi, the Nile, the Rhine, the coast of North America, boulder clay in cliffs, brick and china clay, gives the following results :

The maximum quantity of silica in these samples was 92-22; of alumina and iron, 22:32; lime, 19-24; potash, etc., 778; salt, 3.87.

Some rocks under the action of the sea rapidly decompose at once into a state of mud without passing through the form of sand. In the experiments conducted by Daubré, already referred to, he found that fragments of feldspar under attrition resulted in an impalpable mud of such tenuity as to remain for many days in suspension.

Alluvium derived from the erosion of sea-cliffs is never drifted along the beach in the same way that shingle and sand are.

Every wave that breaks on the beach carries away in suspension a certain amount of eroded soil. Oscillating backwards and forwards with the waves and tidal currents, it becomes diffused over an ever-increasing area, the particles gradually gravitating downwards at a rate proportionate to their size, and being drifted along the sloping bed of the sea until they attain a depth where there is no longer any agitation from the waves.

Beds of alluvial matter or marine ooze are to be found in the bed of the sea adjacent to the mouths of all great rivers. While the bed of the shallower part of the English Channel consists of sand, in the 'deeper part it is covered with ooze and broken shells. In the Bay of Biscay, off the mouths of the Loire and the Gironde, deep-water soundings show beds of soft mud 20 miles in breadth over a length of 150 miles. On the bed of the North Sea, off the outfall of the Rhine and the Scheldt, there exists a very large deposit of mud.

As a rule, the water of the sea is clear and transparent even near the shore. Its appearance, however, especially when the sky is overcast with clouds, is deceiving. Water taken by the author from the estuary of the Mersey near the bar, which in bulk in the sea appeared turbid, when brought up in sample glasses, was perfectly bright, and so clear that small print could be read when placed on the opposite side of the glass that contained the water. So also samples taken on the flood tide from the entrance to the Ribble in Liverpool Bay, from the Humber off Spurn Point, the Colne near Brightlingsea, were all perfectly clear and contained no solid matter in suspension, except a few grains of fine sand.

A very small amount of solid matter, if the particles are very minute, is sufficient to give water a turbid appearance.

Close to the shores during on-shore gales, owing to the waves breaking on the beach, the water becomes charged with solid. matter, rendering it turbid; but this appearance does not extend far from the land, the particles gradually gravitating downwards and becoming diffused over such a wide area as no longer to affect the colour of the water. Captain Washington found in Dover Bay a tidal current of 2 knots held in suspension 473 grains of material in a cubic foot; a half-knot current, 30 grains; and at slack-water calm, from 20 to 30 grains. The material was composed of 50 per cent. of fine sand, 25 per cent. chalk, and the remainder organic matter. The maximum quantity was found at high water spring tides during off-shore winds at 20 feet below the surface, and 16 feet from the bottom.1

In shallow seas, water is sometimes rendered turbid by the stirring up of mud settled on the bottom by waves of unusual height, or by ground swells due to storms. Also where a strong tidal current runs and encounters a shoaling in the bottom, or an obstruction due to projecting rocks, the particles in the water are

1 Report, Tidal Harbour Commissioners.