456 
SAND IN THE MEDITERRANEAN. 
tance of 475 feet in thirty-three years.* Besides this, we have no 
evidence of the existence of deep-water currents in the Medi¬ 
terranean, extensive enough and strong enough to transport 
quartzose sand across 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 impalpable slime, long before they could be swept 
over to the African shore. 
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,f and 
* Bottgek, Das Hittelmeer, p. 128. 
t The testimony of divers and of other observers on this point is con¬ 
flicting, as might he 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. An- 
dresen quotes Bremontier as stating that the movement of the waves some¬ 
times extends to the depth of five hundred feet, and he adds that others 
think it may reach to six or even 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 neces¬ 
sarily a movement 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. Sandbanks sometimes 
recede from the coast, instead of rolling toward it. Beclus informs us 
that the Mauvaise, a sandbank near the Point de Grave, on the Atlantic 
coast of France, has moved five miles to the west in less than a century.— 
jRevue des Deux Mondes , for December, 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 
6and 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 
