I42 THE FLOOR OF THE OCEAN 



the Dardanelles, where the denser, more saline Mediterranean 

 water keeps running along the bottom, through the strait, the 

 Sea of Marmora, and the Bosporus, into the basin of the Black 

 Sea. Overhead the less dense Black Sea water runs steadily in 

 the opposite direction, ultimately mixing with the water of the 

 Aegean Sea. A double salinity current on a much larger scale 

 persists at the Strait of Gibraltar, where a layer of Mediter- 

 ranean water, hundreds of feet thick, runs westward, while 

 above it a thick return stream of the less dense Atlantic water 

 flows into the basin of the Mediterranean Sea. Each of the 

 currents at the Strait of Gibraltar has measured velocity reach- 

 ing two to three miles per hour, although the difference of 

 density is only about 1/500 of the density of either Atlantic 

 or Mediterranean water. We note at once that the velocity of 

 this density current along the sea floor suffices to move even 

 gravel, and a fortiori to channel loose sands and muds. The 

 lower current at Gibraltar is warmer than the Atlantic water, 

 under which it glides, and, with the deep-sea thermometer, the 

 current has been followed for hundreds of miles out under the 

 open Atlantic. Because of the fact that the density-raising salts 

 are in complete solution, and also because of the slowness of 

 diffusion, it is not surprising that the lower current preserves 

 its individuality to a great distance. The endurance of a silty 

 current running down a continental slope represents a more 

 difficult problem. For, as already remarked, the current must 

 lose velocity as the weighting particles of rock, originally sus- 

 pended in the current, obey an inevitable tendency and gradu- 

 ally settle out of the water. For this reason particular interest 

 attaches to observations made by Forel on the Swiss lakes. 

 More than half a century ago he studied the behavior of the 

 Rhone River, where its water, milky with suspended rock-flour 

 brought from the high glaciers, enters Lake Geneva. 13 



Any summer visitor on the heights above Montreux can sec 



