Sand, one of the products of the disintegration of rocks, is composed mainly of grains of quartz—some sands being more purely quartzose than others. Speaking broadly, we may say that all rocks undergoing disintegration are eventually resolved into two kinds of sediment—viz. sand and clay or silt—the former representing the practically insoluble quartz of the original rock, the latter the insoluble constituents of the other minerals. Sand is formed in various ways. It is, as every one knows, one of the most common sediments of rivers, lakes, and seas. Sea-sand exactly resembles river-sand—one cannot be distinguished from the other except by means of included organic remains. As a rule the grains of aqueous sands are angular and subangular in form, especially in the case of fine-grained deposits. When the grains are large they may be more or less well rounded. These last have been rolled over each other and pushed forward in the bed of stream or sea, while the smaller particles, carried in suspension, have in some measure escaped trituration. Aqueous sands are very widely distributed. They are commonly met with forming terraces along the courses of streams and rivers—not infrequently they occupy the sites of ancient lakes and estuaries—and now and again they form what are known as raised beaches in maritime districts. Most of these sands are of recent geological age; others, however, such as the sands of the Tertiary basins of England, France, Belgium, Austria, &c., represent the sea-floors of much more remote times. Sands of wind-blown origin occur frequently in maritime regions and in dry desiccated desert countries. In coastlands the material of the dunes is obviously washed up by the sea; while in certain inland tracts, as in Poland, the sand which is there blown about by the wind is derived from wide-spread fluvio-glacial deposits—relics of the ice age. But in other countries, as in the Libyan Desert, the sand has resulted from the subaerial degradation of granites, schists, sandstones, and other rocks. As the rock-ingredients are swept forward over the ground they are subjected to much attrition, so that eventually even the smallest grains become well rounded, and when seen under a magnifying glass resemble little pebbles. Sand-deposits are also the result of volcanic action. These consist of the very finely comminuted debris of volcanic rocks, and are readily distinguished from sedimentary and æolian sands. Now and again, however, volcanic sands are sifted by the winds and heaped up into dunes.
Sand varies in texture from extremely fine-grained, almost dust-like material, up to coarse granular grit. Indeed all gradations occur from sand through coarse grit into fine gravel. Pure white sands are not uncommon, but shades of yellow, brown, and red predominate, especially in the case of æolian and aqueous sands. The colour is generally due to the presence of iron. Gray, dark-brown, green, and black sands are also met with. The latter are often largely composed of magnetite, and have been derived from the disintegration of certain igneous rocks such as basalt. Green sands usually owe their colour to glauconite. Volcanic sands are generally dingy—chiefly dull gray or black. Some sands are rich in gold, others in precious stones and gems. These are alluvial deposits which have been derived from the disintegration of crystalline igneous rocks, schists, &c. Pure white sands are in demand for the manufacture of glass, while others are employed as abrasives in sawing marble, &c., and in smoothing the surfaces of that and other ornamental stones. Sharp sand, again, is largely used for mixing with mortar. See DRIFT, GRAVEL, PLIOCENE SYSTEM, QUICKSAND, SAHARA, SANDSTONE.
MUSICAL SAND.—Some kinds of sand, which consist of well-rounded and polished grains of tolerably uniform size, and which are clear or free from dust and small particles, exhibit remarkable sonorous qualities when struck or subjected to friction. The well-known 'musical sand' of the island of Eigg (Inner Hebrides) is a good example, and was at one time believed to be almost unique; but, as Professor Bolton of Hartford, Connecticut, and Dr A. Julien of New York have shown, sonorous sands are widely distributed in Europe and America. The sounds emitted are often decidedly musical, and distinct notes can be produced, high or low, according to the nature of the friction and the quantity of sand operated upon. When one walks over a bed of strongly sonorous sand a tingling sensation is perceived even through the boots. After being subjected to friction for some little time musical sand gradually loses its peculiar qualities, and the same result is produced when the sand is wetted. There is nothing in the appearance of musical sand to distinguish it from mute sand—sonorous and non-sonorous sand of precisely similar aspect lying side by side on the same beach. No satisfactory explanation of the phenomenon has been given.
SANDBANKS.—These are met with in the beds of rivers and estuaries and shallow seas. In rivers the banks are usually elongated in the direction of the current, and are liable to constant changes as the force and direction of the current become modified. Opposite the mouths of rivers sandbanks tend to accumulate. Much of the material of which these bars are composed is brought down by the rivers, but a large proportion is also swept up by the sea itself. Such banks are constantly changing their form, and oscillating to and fro, according as the sea or the river is the more active. The sea also tends to form sandbanks across the mouths of shallow inlets and other indentations of a coast-line, so that eventually a secondary coast-line may come to be formed in this way—shallow lagoons separating the new from the old coast-line. Islands are in like manner converted into peninsulas by the heaping up of sandbanks by tidal- currents between them and the mainland. The Eye peninsula in the Island of Lewis is an example, and there are many islets off British coasts which in time will be converted into similar peninsulas by the growth of sandbanks, which in some cases has proceeded so far that the islets become peninsulas at low tide. In the shallow seas that surround the British Islands sandbanks are of common occurrence. Some of these are doubtless due to tidal action, as is the case with the sandbanks of similar shallow seas all the world over. Others again probably mark the sites of undulating land-surfaces submerged during a recent geological period. It is thought by some that the Dogger Banks of the North Sea may consist largely of the morainic debris laid down by the great Scandinavian ice-sheet of the glacial period, now more or less modified by current-action.