224 THE PHYSICAL GEOGRAPHY OF THE SEA. 



come equal to the amount rained down again, precisely in the 

 same way that the amount of water evaporated from the sea is ex- 

 actly equal to the whole amount poured back into it by the rains, 

 the fogs, and the dews.* Thus the great lakes of this continent 

 would remain inland seas at a permanent level ; the salt brought 

 from the soil by the washings of the rivers and rains would cease 

 to be taken off to the ocean as it now is ; and finally, too, the 

 great American lakes, in the process of ages, w^ould become first 

 brackish, and then briny. 



622. Now suppose the water basins which hold the lakes to be 

 over a thousand fathoms (six thousand feet) deep. We know 

 tkey are not more than four hundred and twenty feet deep ; but 

 suppose them to be six thousand feet deep. The process of evap- 

 oration, after the St. Lawrence had gone dry, might go on until 

 one or two thousand feet or more were lost from the surface, and 

 we should then have another instance of the level of an inland 

 water-basin being far below the sea-level, as in the case of the 

 Dead Sea ; or it would become a rainless district, when the lakes 

 themselves would go dry. 



623. Or let us take another case for illustration. Corallines 

 are at work about the Gulf Stream ; they have built up the Flor- 

 ida Reefs on one side, and the Bahama Banks on the other. Sup- 

 pose they should build up a dam across the Florida Pass, and ob- 

 struct the Gulf Stream ; and that, in like manner, they were to 

 connect Cuba with Yucatan by damming up tlie Yucatan Pass, so 

 that the waters of the Atlantic should cease to flow into the Gulf 

 of Mexico. What should we have ? 



624. The depth of the marine basin which holds the waters of 

 that Gulf is, in the deepest part, about three quarters of a mile. 

 The officers of the United States ship Albany have run a line of 

 deep-sea soundings from west to east across the Gulf; the great- 

 est depth they reported was about six thousand feet. Subsequent 

 experiments, however, induce the belief that the dejDth is not quite 

 so great. 



625. We should therefore have, by stopping up the channels 

 between the Gulf and the Atlantic, not a sea-level in the Gulf, but 



* The quantity of dew in England is about five inches during a year. — Glaisher. 



