226 THE PHYSICAL GEOGRAPHY OF THE SEA. 



of drainage that has been cut off, and an illustration of the process 

 by which Nature equalizes the evaporation and precipitation. To 

 do this, in this instance, she is salting up the basin which received 

 the drainage of this inland water-shed. Here we have the appear- 

 ance, I am told, of an old channel by which the water used to flow 

 from this basin to the sea. Supposing there was such a time and 

 such a water-course, the water returned through it to the ocean 

 was the amount by which the precipitation used to exceed the 

 evaporation over the whole extent of country drained through this 

 now dry bed of a river. The winds have had something to do 

 with this ; they are the agents which used to bring more moist- 

 ure from the sea to this water-shed than they carried away ; and 

 they are the agents which now carry off from that valley more 

 moisture than is brought to it, and which, therefore, are making 

 a salt-bed of places that used to be covered by water. In like 

 manner, there is evidence that the great American lakes formerly 

 had a drainage with the Gulf of Mexico ; for boats or canoes have 

 been actually known, in former years, and in times of freshets, to 

 pass from the Mississippi River over into the lakes. At low wa- 

 ter, the bed of a dry river can be traced between them. Now the 

 Salt Lake of Utah is to the southward and westward of our north- 

 ern lake basin ; that is the quarter (§ 364) whence the rain-winds 

 liave been supposed to come. May not the same cause which 

 lessened the precipitation or increased the evaporation in the Salt 

 Lake water-shed, have done the same for the water-shed of the 

 great American system of lakes ? 



630. If the mountains to the west — the Sierra Nevada, for in- 

 stance — stand higher now than they formerly did, and if the winds 

 which fed the Salt Lake valley with precipitation had, as (§ 361) 

 I suppose they have, to pass the summits of the mountains, it is 

 easy to perceive why the winds should not convey as much vapor 

 across them now as they did when the summit of the range was 

 lower and not so cool. 



631. The Andes, in the trade- wind region of South America, 

 stand up so high, that the wind, in order to cross them, has to 

 part with all its moisture (§ 196), and consequently there is, on 

 the west side, a rainless region. Now suppose a range of such 



