338 THE POPULAR SCIENCE MONTHLY. 



continents were raised from the sea, the lake-basins had been already 

 formed, and came up, therefore, brimful of water. In the northern 

 and eastern part of the continent, where the supply from rain and 

 snowfall exceeds the loss by evaporation, the salt, being continuously 

 carried away through their outlets, has become so diluted as to be an 

 imperceptible quantity. In arid regions, as the Pacific slope and the 

 country about the Caspian, where the evaporation was in excess of the 

 supply, the water-level of the lakes continuously sank until, on ac- 

 count of the diminished extent of surface, the equilibrium of loss and 

 gain was attained. Hence the exceeding saltness of Great Salt Lake, 

 the Dead Sea, etc. For a like reason the water of the Mediterranean 

 contains more salt relatively than that of the ocean. Evaporation 

 exceeding the supplies from the rivers and rainfall, it requires a con- 

 stant current through the Strait of Gibraltar. The same is true of 

 the Red Sea, causing a like current through the Strait of Bab-el- 

 Mandeb. Other salt or brackish lakes probably owe their saltness to 

 the supplies from the land. Water being the most general of all 

 solvents, the rains gather up the chloride of sodium from the soils and 

 the disintegrating rocks, and where the streams fall into lakes whose 

 only outlet is evaporation, the land itself must be a constant source 

 of saline supply, and their waters must become more and more salt, 

 until their capacity as a solvent has been reached. 



The Utah Basin must once have been filled to its brim with ocean- 

 water. The outlet has been evaporation. The lake, receding to its 

 present level, has left many evidences of its former extent. 



To the drying up of salt lakes is probably due the presence of 

 rock-salt, often found in great quantities in regions of little rainfall. 



I come, lastly, to the trend of the North American lakes. A good 

 map, and especially one on the Mercator projection, will show that 

 lakes are not dotted promiscuously here and there, with no regard to 

 system. They have with each other a trend of direction often as 

 well defined as mountain-ranges, or the coast-lines of continents. As 

 already shown, the great American depression bifurcates at about the 

 fortieth parallel, and nearly at right angles, into northeastern and north- 

 western branches, whose lines of direction lie respectively in approxi- 

 mate parallelism with the far-off Appalachian and Rocky Mountain 

 ranges, and with the still farther-off Atlantic and Pacific shore-lines. 



Geologists, and especially physical geographers, have noted the 

 fact that the mountain-ranges, the shore-lines of continents, and the 

 islands with each other, have lines of trend mostly northeastward or 

 northwestward. The lakes of North America have similar trends of 

 direction, and therefore form an integral part of the great system 

 upon which the planet itself is built. This is as should be expected. 

 That the line of greatest depression should have an approximate 

 parallelism with the adjacent greatest upheaval is but a physical 

 necessity. 



