530 SANDS OF NOETHEEN AFRICA. 



and it is equally certain tliat 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 can not at present assign. The wind does not stir water to 

 great depths with sufficient force to disturb the bottom,* and the 

 sand tlirown upon the coast in question must be derived from a 

 narrow belt of sea. It must hence, in time, become exhausted, 

 and the formation of new sand-banks and dunes upon the south- 

 ern shores of the Mediterranean will cease at last for want of 

 material, f 



* The testimony of divers and of other observers on this point is conflict- 

 ing, as might be expected from the infinite variety of conditions by which the 

 movement of water is effected. 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 Bremontier as 

 stating that the movement of the waves sometimes extends to the depth of 

 five himdred feet, and he adds that others think it may reach to six or even 

 seven himdred 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 Mondes for De- 

 cember, 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 at a point much exposed to the wash from Galata and Pera, a 

 number of bronze guns supposed to have belonged to a ship-of-war blown up 

 about a hundred and fifty years before. These guns were not covered by 

 sand or slime, though a crust of earthy matter, an inch in thickness, adhered 

 to their upper surfaces, and the bottom of the strait appeared to be wboUy 

 free from sediment. The current was so powerful at this depth that the 

 divers were hardly able to stand, and a keg of nails, purposely dropped into 

 the water, in order that its movements might serve as a guide in the search 

 for a bag of coin accidentally lost overboard from a ship in the harbor, was 

 rolled by the stream several hundred yards before it stopped. 



f Few seas have thrown up so much sand as the shallow Grcrman Ocean ; 

 but there is some reason to think that the amoimt of this material now cast 



