243 Mr J. D. Dana on the Origin of Continents. 



water, so that, under a climate peculiarly genial, coal vege- 

 tation might have grown luxuriantly. But had it continued 

 thus flat to a later period, it would have had but small 

 streams, and probably, for want of a mountain barrier to in- 

 tercept the drying Pacific winds, the desert regions of the 

 west would have traversed the land, as Sahara has spread 

 over Africa. As if to prevent these results, and give a vast- 

 ness scarcely equalled to its resources, the land was raised 

 into mountains on either coast, those of the west, where the 

 barrier was most needed, ascending even to the regions of 

 perpetual snows. The whole interior is now inclosed by the 

 Rocky Mountains on the one side and the Appalachians on 

 the other, and a thousand streams are set in motion over the 

 wide land from either bound, all to contribute to a common 

 trunk, the great highway of the country. Thus the largest 

 possible extent of intercommunicating inland waters has 

 been secured ; and for the same reason a great part have 

 been made to flow so nearly on a plain as to afford naviga- 

 tion almost from one end of the territory to the other, and 

 extend their fertilizing influence over the whole surface. A 

 similar result has been produced on the narrow ocean side of 

 the main chains by the succession of parallel coast ranges ; 

 for the waters have been compelled to flow far north and 

 south between these ranges, and fertilize an extended country 

 before the sea was reached. Thus the noble Columbia, with 

 its wide spread tributaries, was made for Oregon ; and in the 

 same manner were formed the Willammet, the Sacramento, 

 and the Joachim, which run in long courses between the 

 Cascade and Coast ranges of heights. Thus on the Atlantic 

 side, we have the Shenandoah and other head waters to the 

 Potomac, and at the north, a Hudson, Conecticut and Merri- 

 mack flowing in parallel lines. 



Note. — In connection with this article, it should have been 

 earlier mentioned that the theory of " secular refrigeration" 

 has been presented with much force, in many points of view, 

 by W. W. Mather, in the American Journal of Science, 

 vol. xlix., p. 284 (1845), and the foldings of the Appalachians 

 are attributed by him to this cause. 



