thrown upwards and may bring to the surface mud or ooze covering the bed of the sea.

In the North Sea near the mouths of the Scheldt, in the pass of Wielingen, where there is a large deposit of alluvial matter, in heavy gales from the north-west the water is stated to become so turbid that vessels sailing through it become coated with mud, and require scrubbing and cleaning when in port.

During heavy north-east gales, the author has known the water in the offing off the Lincolnshire coast, for a short distance from the shore, from the Humber to the Wash, to be rendered turbid.

Deposits of alluvium and the growth of salt marshes only occur at the mouth of rivers and in sheltered estuaries protected from the action of the waves. The upper soil of which these salt marshes consist is deposited during the period of slack water at or about the time of H.W.S.T. Up to the level of high water of neap tides sand and silt only are deposited, and the fine warp in which the marine grasses of the salt marshes grow is never deposited below the level of H.W.N.T.

Large areas of marshes formed from the alluvial matter brought down in suspension are to be found in the estuaries of this country, and where they have been reclaimed from the sea by embankments, form very fertile agricultural land.1

The largest area of salt marshes to be found on any sea-coast is that which extends along the east coast of the United States. The rivers that drain this long stretch of country have, as already described, been cut off from discharging into the sea by barriers of sand which have been thrown up along the coast, between which and the main land are large lagoons, into which the rivers discharge their water, and also the débris taken up in suspension from the land which they drain. These sand barriers form a natural embankment to vast areas of salt marshes many hundreds of square miles in extent, which are covered with only one or two feet at high water. Mr. Shaler estimates that along the coast south of New York there are not far from two million acres of marshes fit for reclamation, and which, owing to the small rise of the tide, which does not exceed from 4 to 5 feet, and owing to the banks already formed by the sand along nearly

"The Reclamation of Land from Tidal Waters," by A. Beazeley, M. Inst. C.E. Crosby, Lockwood & Co., 1900. "The History of the Fens of South Lincolnshire," chapter xvi. "On Geology and Land Reclamation." Simpkin, Marshall & Co. 1897.

their whole length, could be enclosed at a comparatively small expense.1

It has been considered by some geologists that material eroded from sea-cliffs can be carried along the shore for considerable distances, and into estuaries and up rivers; and schemes for the reclamation of tidal land have been brought forward on the supposition that an unlimited amount of alluvial matter is to be derived from the sea. One of the most extensive of these schemes was for the reclamation of 150,000 acres of land in the Wash. This scheme was supported by engineers of eminence, and more recently received the approval of the survey or of the Government Geological Survey, who expressed the opinion that the sands in the Wash could be warped by material brought in by the sea derived from the erosion of the Yorkshire coast.2 It has also been frequently stated that the warp in the Humber, even as far up as Hull and Goole, is due to the same cause.

A more careful consideration of the facts relating to coast erosion and littoral drift would show the physical impossibility of material from the sea-coast being carried for 50 miles to the Wash, and from 20 to 30 miles to the Humber, and 40 miles up that channel.

Where the tidal currents run strongly through shifting sandbanks into a river, sand may be driven along the bottom of the channel for some distance up it; and in dry seasons, owing to the diminished downward fresh-water flow, shoals of sand may be left at the head of the tide, but these are again removed as soon as the ebb current is strengthened by the ordinary discharge of the river.

The foot of the Yorkshire cliffs is only reached at spring tides, and the average distance from the mouth of the Humber may be taken at about 20 miles. It is, therefore, only during the last part of the flood that the material eroded from the cliffs is reached by the tides. While the sand and stones from the cliffs are drifted along the beach, the finer particles become placed in suspension, and are carried by the flood current southward at the rate of about 21⁄2 knots for say from two to three hours. The tide then turns, and the current sets northward away from the Humber.

1 "Sea and Land," by N. S. Shaler. Smith, Elder & Co. London, 1895. "The Geology of the Fenland." Memoir of the Geological Survey of England and Wales. 1877.

Even if the turbid water reached the Humber, it is contrary to the laws of gravity for it to be permanently carried upward against the preponderating influence of the ebb current, strengthened by the downward flow of the land water. This question will be found more fully dealt with in the description of the Yorkshire coast in Chapter VII.

The Settlement of Solid Matter in Fresh and Salt Water.-Since the publication of the report of Mr. Slidell on the deposits of the Mississippi delta,1 containing the remarkable statement that, while the deposit contained in the river-water of the Mississippi took from 10 to 14 days to settle, with solutions of salt, sea-water, or sulphuric acid the water became limpid in from 14 to 18 hours, it has generally been taken as an accepted fact that alluvial matter settles more rapidly in salt than in fresh water. Professor Archibald Geikie, in his "Text-book of Geology," endorses this theory; and in a recent article in the American Engineering Magazine on the transportation of solid matter by rivers, Mr. Starling, one of the Government geologists, states that a small quantity of salt or other foreign material dissolved in water will diminish the suspending power and increase the rapidity of subsidence to a marked degree, sometimes even many hundredfold.

On the face of it, the result naturally expected would be that, as sea-water is of greater specific gravity than fresh water, and more viscous, the grains of solid matter would sink more slowly in salt than in fresh water. The very great distance over which solid matter brought down by rivers remains in suspension after reaching the sea, extending from 6 miles from the mouth of the Rhone to 35 from the outlet of the Nile, up to 300 miles, over which the sea-water is stated to be discoloured by the effluent of the Amazon, appears to indicate that salt water is capable of retaining solid matter in suspension for a longer time than fresh

water.

Experiments made by Professor Vernon Harcourt with alluvial matter placed in suspension in sea-water and fresh water, and in solutions containing different strengths of salt and other foreign material, although not of a conclusive character, show that there is little difference between the rate of deposit in sea or in fresh water. Of samples from different estuaries which were allowed to settle in sea-water and pond-water respectively, the particles of the former took about 9 per cent. more time to subside than Report on the Mississippi River," by Humphreys and Abbot. 1861.

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the latter. The general conclusion he arrived at was, that though sea-water promotes the deposit of "very light clayey matter contained in river-silt under favourable conditions, there are no grounds for regarding it as exercising the very preponderating influence on the formation of deltas attributed to it by geologists.": The writer some time ago investigated this subject in connection with researches he was then making as to alluvial deposits in estuaries, and has again more recently conducted a series of experiments the mean results of which are given in the following table.

It will be seen from this table that the rate of settlement depends on the minuteness of the particles in suspension, and varies nearly in proportion to the square of the diameter of these.

With sand and silt, there was practically no difference in the rate of settlement in fresh as compared with salt water. When the particles of the solid matter were very fine, as in the case of what is generally known as mud or ooze, the rate of settlement was slightly more rapid in salt than in fresh water; but there was nothing to justify the conclusion arrived at by Mr. Slidell.

Mr. H. S. Allen, of the Blytheswood Laboratory, commenting on these experiments carried out by the author, says that it is only in the case of very finely divided solid matter in suspension that the addition of salt solution causes increased precipitation; and that the results given are confirmed by the investigations carried out by Carl Burns and Bodlander. The precipitation of such suspensions by the addition of an electrolyte is accompanied by the coagulation or flocculation of the solid matter; and clay suspensions that will pass through a filter-paper can be filtered if coagulated by a salt solution.

All the material was first screened through a sieve having 90 meshes to the lineal inch.

The proportion by weight of solid matter to water was that which was found to exist on the average of fourteen large rivers when in flood, or 079 lb. to a cubic foot, equal to o part in weight of the water in the tube.

Both sea-water and water saturated with ordinary salt were

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'Investigation on the Action of Sea-water in accelerating the Deposit of Riversilt," Min. Proc. Inst. C.E., vol. cxlii.

2 Nature, July 18, 1901, "On the Settlement of Solid Matter in Fresh and Salt Water," where references are given to a list of German and American papers dealing with this subject.

tried, the latter in the proportion of one pound of salt to a cubic foot of water. There was no appreciable difference between these. The samples were placed in glass test-tubes 1 foot long and inch in diameter, filled with clean water up to the 10-inch mark.

The material was well shaken and incorporated with the water, and the time given for settling is that taken by the particles to settle through 10 inches and become visible in a solid form at the bottom of the tube, and when no more particles could be discerned as settling when the tube was held up to the light.

The column "water clear" is that in which the water in the tube had become sufficiently transparent for black marks on a white ground to be discerned through it.

Practically all solid matter had settled in the time given in the first column. The quantity deposited between the interval of "settling" and "clear" was almost inappreciable, but still sufficient to keep the water discoloured. With the specimens containing the coarser material, the water became bright again in the time given in the second column; but with the very fine material, intervals varying from 2 to 3 hours up to as many days elapsed before the water became as bright as it was before the solid matter was added, partly depending on the fineness of the material, but due more to the staining quality of some of the ingredients contained in the sample. Thus the material taken from Tilbury dock basin turned the water a black colour, which took some time to clear. The salt water took much longer to become bright again than the fresh.

Samples were selected as fairly representing the material brought down in suspension by rivers or eroded from the seacliffs, and deposited either in the form of salt marshes or transported to the bed of the sea.

Thus numbers 1, 2, and 3 represent the sand found on the foreshores of the sea-coast and covered at every tide; 4 and 5, material derived from chalk cliffs; 6 and 7, the material in suspension in the rivers Ouse and Trent, and of which the warp lands bordering on those rivers are composed, 6 being the material first deposited and near the river, and 7 that further away where the water remains quiescent for some time; 8 and 9 represent the material of which salt marshes are composed, 8 being the silt deposited on the sand, and on which, when it rises to about neaptide level, or 668 above ordnance datum, samphire begins to grow, 9 the finer warp deposited from about the level of mean