550 SANDS OF NORTHERN AFRICA. 
the sea. It may be added that much of the rock from which 
the torrent sands of Southern Europe are derived contains little 
quartz, and hence the general character of these sands is such 
that they must be decomposed or ground down to an impalpa- 
ble slime, long before they could be swept over te the African 
shore. 
Sands of Northern Africa. 
The torrents of Europe, then, do not at present furnish the 
material which composes the beach sands of Northern Africa, 
and it is equally certain that those sands are not brought down 
by the rivers of the latter continent. They belong to a remote 
geological period, and have been accumulated by causes which 
we cannot at present assign. The wind does not stir water to 
great depths with sufficient force to disturb the bottom,* and 
* The testimony of divers and of other observers on this point is conflicting, 
as might be expected from the infinite variety of conditions by which the 
movement of water is affected. It is generally believed that the action of 
the wind upon the water is not perceptible at greater depths than from fifteen 
feet in ordinary to eighty or ninety in extreme cases; but these estimates are 
probably very considerably below the truth. Andresen quotes Brémontier as 
stating that the movement of the waves sometimes extends to the depth of five 
hundred feet, and he adds that others think it may reach to six or eyen seven 
hundred feet below the surface.—ANDRESEN, Om Klitformationen, p. 20. 
Many physicists now suppose that the undulations of great bodies of water 
reach even deeper. But a movement of undulation is not necessarily a move- 
ment of translation, and besides, there is very frequently an undertow, which 
tends to carry suspended bodies out to sea as powerfully as the superficial 
waves to throw them on shore. Sand-banks sometimes recede from the coast, 
instead of rolling towards it. Reclus informs us that the Mauvaise, a sand-bank 
near the Point de Grave, on the Atlantic coast of France, has moved five 
miles to the west in less than a century.—Revue des Deux Mondcs for Decem- 
ber, 1862, p. 905. 
The action of currents may, in some cases, have been confounded with that 
of the waves. Sea-currents, strong enough, possibly, to transport sand for 
some distance, flow far below the surface in parts of the open ocean, and in 
narrow straits they have great force and velocity. The divers employed at 
Constantinople in 1853 found in the Bosphorus, at the depth of twenty-five 
fathoms and ata point much exposed to the wash from Galata and Pera, a 
