188 THE PHYSICAL GEOGRAPHY OF THE SEA. 



a higher water-level be unmistakable and conclusive, it does not 

 follow^, therefore, that there has been a subsidence of the lake 

 basin itself, or an upheaval of the w^ater-shed drained by it. 



384. The cause which has produced this change in the water- 

 level, instead of being local and near, may be remote ; it may 

 have its seat in the obstructions to " the winds in his circuits," 

 which have been interposed in some other quarter of the world, 

 which obstructions may prevent the winds from taking up or from 

 bearing off their wonted supj^lies of moisture for the region whose 

 water-level has been lowered. 



385. Having therefore, I hope, made clear the meaning of the 

 question proposed, by showing the manner in which winds may be- 

 come important geological agents, and having explained how the 

 upheaving of a mountain range in one part of the world may, 

 through the winds, bear upon the physical geography of the sea, 

 affect climates, and produce geological phenomena in another, I 

 return to the Dead Sea and the great inland basins of Asia, and 

 ask. How far is it possible for the elevation of the South American 

 continent, and the upheaval of its mountains, to have had any ef- 

 fect upon the water-level of those seas ? There are indications 

 (§ 374) that they all once had a higher water-level than they now 

 have, and that formerly the amount of precipitation was greater 

 than it now is ; then what has become of the sources of vapor ? 

 What has diminished its supply ? Its supply would be diminished 

 (§ 381) by the substitution of dry land in those parts of the ocean 

 which used to supply that vapor ; or the quantity of vapor depos- 

 ited in the hydrographical basins of those seas would have been 

 lessened if a snow-capped range of mountains {^ 376) had been 

 elevated across the path of these winds, between the places where 

 they were supplied with vapor and these basins. 



386. A chain of evidence which it would be difficult to set 

 aside is contained in the chapters beginning severally at p. 66, 

 97, and 104, going to show that the vapor which supplies the ex- 

 tra-tropical regions of the north with rains comes, in all probabil- 

 ity, from the trade-wind regions of the southern hemisphere. 



387. Now if it be true that the trade-winds from that part of 

 the w^orld take up there the water which is to be rained in the 

 extra-tropical north, the path ascribed to the southeast trades of 

 Africa and America, after they descend and become the prevail- 



