GEOLOGY. 265 



range and the lakes, they would have no longer any moisture to deposit 

 at the supposed lake temperature of 50. They could not yield the 

 dew point to anything above 30. Consequently the precipitation in 

 the lake country would fall off; the winds which feed the lakes would 

 cease to bring as much water as the lakes now give to the St. Lawrence 

 that river and the Niagara would drain them to the level of their 

 bed. Evaporation would be increased by reason of the dryness of the 

 atmosphere and the paucity of rain ; and the lakes would sink to that 

 level, at which, as in the case of the Caspian Sea, the precipitation 

 and evaporation would become equal. Thus, our great lakes would 

 remain inland seas at a permanent level. The salt brought from the 

 soil by the washing of the rivers and rains, would cease to be taken off 

 to the ocean as it now is ; and, finally, the great lakes too, in the pro- 

 cess of ages, would become first brackish and then briny. Now, suppose 

 the water basins which hold the lakes to be over a thousand fathoms 

 (6,000 feet) deep. We know they are not nearly so deep. But suppose 

 they are 6,000 feet deep. The process of evaporation, after the St. Law- 

 rence 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 might become a rainless district, 

 when the lakes themselves would go dry. 



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

 the Florida Reefs on one side, and the Bahama Banks on the other. 

 Suppose they should build up across that pass and obstruct the Gulf 

 Stream, and that in like manner they were to connect Cuba with Yucatan, 

 by damming up the Yucatan pass, so that the waters of the Atlantic 

 should cease to flow into the Gulf. What should we have ? The Ma- 

 rine Basin, which holds the waters of the Gulf, is in the deepest parts 

 about a thousand fathoms. The officers of the U. S. ship Albany have 

 run a line of deep-sea soundings, from west to east, across the Gulf. 

 The greatest depth they obtained was 960 fathoms, (5,760 feet.) We 

 should, therefore, have, by stopping up the channels between the Gulf 

 and the Atlantic, not a sea level in the Gulf; but we should have a 

 mean level between evaporation and precipitation. If the former were 

 in excess, the level of the Gulf waters would sink down until the sur- 

 face exposed to the air would be just sufficient to return to the atmos- 

 phere as vapor the amount of water discharged by the rivers, the Mis- 

 sippi and others, into the Gulf. As the waters were lowered, the ex- 

 tent of evaporating surface would grow less and less, until nature should 

 establish the proper ratio between the ability of the air to take up and 

 the capacity of the rain to let down. Thus we might have a sea whose 

 level would be much further below the water-level of the ocean than 

 is the Dead Sea. 



There is still another process by which the drainage of these inland 

 basins may, through the agency of the winds, have been cut off from 

 the Great Salt Seas, and that is by the elevation of continents from the 

 bottom of the sea in distant regions of the earth and, consequently, 

 the substitution of a dry land for a water surface, as the sources of 

 vapor supply to the winds that blow over the place. From what part 



23 



