A very important and interesting thing about "beaches" is the way in which the pebbles of which they consist are assorted in sizes. Suppose that one prepares a trough some two or three yards long and twelve inches deep, and lets it fill with water from a constantly running tap, tilting it slightly so that the water will overflow and run away at the end farthest from the tap. Then if one drops into the trough near the tap handful after handful of coarse sand and small stones of varied sizes, they will be carried along by the stream, and the more rapid and voluminous the stream the farther they will be carried. But they will eventually sink to the bottom of the trough, the bigger pieces first, then the medium-sized, then the small, and the smaller in order, as the current carries them along, so that one gets a separation and sorting of the solid particles according to size, a very fine sediment being deposited last of all at the far end of the trough. The waves of the sea are continually stirring up and assorting the constituents of the beach in this way. Usually the largest pebbles are thrown up farthest by the advancing waves, and dropped soonest by the backward suck of the retreating water, so that one generally finds a predominance of big pebbles at the top of the beach. But on the flat shore of firm ripple-marked sand lying lower down than the sloping "beach" and only exposed at quite "low tide," one often finds very big pebbles of eight or nine pounds weight scattered here and there and little rubbed or rounded. They have gradually moved down the sloping beach and are too heavy to be thrown back again by the waves of the shallow sea which flows over the flat shores characteristic of much of our south-eastern and southern coast. On some parts of the coast huge banks, consisting exclusively of enormous pebbles as big as a quartern loaf, are piled up by the waves, forming a great ridge often miles in length, as at the celebrated Chesil pebble bank near Weymouth, and at Westward Ho! in North Devon. The presence of these specially large pebbles is due to the special character of the rocks which are broken up by the sea to form them, and to the specially powerful wave-compelling winds and tidal currents at the parts of the coast where they are produced.
One generally finds a selected accumulation of moderate-sized pebbles lower down the beach as the tide recedes, and then still lower down patches of sand alternating with patches or tracts of quite small pebbles not much bigger than a dried pea. They are always assorted in sizes, but the extent of each tract of a given size of pebble varies greatly on different beaches along the coast, and even from day to day on the same shore. The greater or less violence of the waves, and of the currents caused by wind and tide, is the cause of this variation and local difference. The pebbles of the "beach" are, of course, always being worn away, rounded and rubbed down by their daily movement upon one another, caused by the waves as the tide mounts and again descends over the shore. Even the biggest stones, excepting those which lie in deeper water beyond the beach, are eventually rubbed down, and become quite small; but a point is reached when, the weight of the pebbles being very small, they have but little effect in rubbing down each other, and consequently where the pebbles consist of very hard material—like flints—the smallest ones are not so much rounded, but are angular and irregular in shape.
Whilst a perfect gradation in size can be found from the largest flint pebbles some 6 inches or 7 inches long to the smallest, usually not bigger than a split pea (though sometimes a patch of even smaller constituents may be found), there is a real break or gap between "pebbles" and "sand." I am referring now to what is commonly known as "sand" on the southern part of the East Coast, much of the South Coast, and the shores of Holland, Belgium, and France. There are "sands" of softer material (limestone and coral sand), but the sands in question are almost entirely siliceous, made up of tiny fragments of flint, of quartz, agate, and hard, igneous rock. They are often called "sharp" sand. The particles forming this sand are sorted out by the action of moving water, and form large tracts between tide-marks looking like brown sugar, for which baby visitors have been known to mistake them, and accordingly to swallow small handfuls. The strong wind from the sea blows the sand thus exposed, as it dries, inland out of reach of the tide, to form sand-dunes, and it is also deposited, together with still finer particles (those called "mud"), on the shallower parts of the sea bottom. The curious thing about the particles of "sharp" sand is that they are angular, and for the most part without rounded edges. If you examine them under a microscope you will see that they do not look like pebbles—in fact, they are not pebbles, for they are so small and have so little weight, or, rather, mass, that they do not rub each other to any effect when moved about in water. They look like, and, in fact, are, for the most part broken bits of silica, unworn and sharp-edged splinters and chips, glass-like in their transparency and most of them colourless, a few only iron-stained and yellow. Amongst these are a few rounded, almost spherical pieces, which are no doubt of the nature of minute water-worn pebbles. Although these few minute pebbles exist among the sharp, chiplike particles of "sand," it is clear that we must broadly distinguish "pebbles" of all sizes down to the smallest—from the much smaller "sand particles." There is no intermediate quality of material between "sand" and the finest "shingle."
CHAPTER VIII
QUICKSANDS AND FIRE-STONES
THERE are curious facts about sand which can be studied on the seashore. There are the "quicksands," mixtures of sand and water, which sometimes engulf pedestrians and horsemen at low tide, not only at the Mont St. Michel, on the Normandy coast, but at many spots on the English, Welsh, and Scotch coasts. Small and harmless quicksands are often formed where the sand is not firmly "bedded" by the receding sea, and the sea-water does not drain off, but forms a sort of sand-bog. Then one may also study the polishing and eroding effect of dry blown sand, which gives a "sand-glaze" to flints, and in "sand-deserts" often wears away great rocks. The natural polishing of flints and other hard bodies by fine sand carried over them for months and years in succession by a stream of water, is also a matter of great interest, about which archæologists want further information.
A very interesting fact about the ordinary sand of the seashore is that two pints of dry sand and half a pint of water when mixed do not make two pints and a half, but less than that quantity. If you fill a child's pail with dry sand from above the tide-mark, and then pour on to it some water, the mass of sand actually shrinks. The reason is that when the sand is dry there is air between its particles, but when the sand-particles are wetted they adhere closely to each other; the air is driven out, and the water does not exactly take an equivalent space, but occupies less room than the air did, owing to the close clinging together of the wet particles. If you add a little water to some dry sand under the microscope, you will see the sand-particles move and cling closely to one another. "Capillary attraction"—the ascent of liquid in very fine tubes or spaces—is a result of the same sort of adhesive action. If you walk on the firm, damp sand exposed at low tide on many parts of the seashore when it is just free from water on the surface, you will see that when you put your foot down the sand becomes suddenly pale for some seven inches or so all round your foot. The reason is that the water has left the pale-looking sand (dry sand looks paler than wet sand), and has gone into the sand under your foot, which is being squeezed by your weight. The water passing into that squeezed sand enables its particles to sit tighter or closer together, and so to yield to the pressure caused by your weight. You actually squeeze water "into" the sand, instead of squeezing water "out" of it, as is usually the case when you squeeze part of a wet substance—say a cloth or a sponge. When you lift your foot up, you find that your footmark is covered with water—the water you had drawn to that particular spot by squeezing it. It separates as soon as the pressure is removed.
Quartz and quartzite pebbles occur on the South as well as the East Coast. They are sometimes called "fire-stones," because they can be made to produce flashes of flame. If you take a couple of these pebbles, each about as big as the bowl of a dessert-spoon (a couple of flint pebbles will serve, but not so well), and holding one in each hand in a dark room, or at night, scrape one with the other very firmly, you will produce a flash of light of an orange or reddish colour. And at the same time you will notice a very peculiar smell, rather agreeable than otherwise, like that of burning vegetable matter. It would seem that the rubbing together of the stones produces a fine powder of some of the siliceous substance of the stone and at the same time a very high temperature, which sets the powder aflame. I had the idea at one time, based on the curious smell given out by the flashing pebbles, that perhaps it was a thin coating of vegetable or other organic matter derived from the sea-water which burns when the stones are thus rubbed together; but I found on chemically cleaning my pebbles, first with strong acid and then with alkali, that the flame and the smell were produced just as well by these chemically clean stones as by those taken from the beach. The flame produced by the rubbing of the two stones seemed then to be like the sparks obtained by strike-a-lights of flint and steel, or the prehistoric flint and pyrites. Now, however, a new fact demands consideration. The supposition that the powdered silica formed, when one rubs the two pebbles together, is actually "burnt," that is to say, combined with the oxygen of the air by the great heat of the friction, is rendered unlikely by the fact that if you perform the rubbing operation in a basin of water with the stones submerged, the flash is produced as easily as in the air. My attention was drawn to this fact by a letter from the well-known naturalist the Rev. Reginald Gatty. I at once tried the experiment and found the fact to be as my correspondent stated. Not only so, but the smell was produced as well as the flash.
With the desire to get further light on the subject, I consulted the great experimental physicist, my friend Sir James Dewar, in his laboratory at the Royal Institution. He told me that the late Professor Tyndal used to exhibit the production of flame by the friction of two pieces of quartz in his lectures on heat, but made use of a very large and rough crystal of quartz (rock-crystal) and rubbed its rough surface with another large crystal. Tyndal's note on the subject in his lecture programme was as follows (Juvenile Lectures on Heat, 1877-78): "When very hard substances are rubbed together light is produced as well as heat." Sir James Dewar kindly showed me the crystals used by Tyndal, the larger was 16 inches long and 4 or 5 inches broad. We repeated the experiment in the darkened lecture room, and obtained splendid flashes. The same smell is produced when rock-crystal is used as when flint or quartz pebbles are rubbed together. All three are the same chemical body, namely, silica (oxide of silicon). We also found that when the crystals were bathed with water or (this is a new fact) with absolute alcohol, the same flashing was produced by the friction of one against the other.