1851.] HOPKINS ON CHANGES OF CLIMATE. 89 



accepted in opposition to any other theory which 'may be free'from 

 objections of so grave a character. 



We may also observe, that any theory of the production of cold 

 solely by the elevation of the regions presenting glacial phaenomena 

 would be insufficient to account for many of these phsenomena. It 

 would be necessary that such a theory should embrace also the de- 

 pression of such regions beneath the level of the sea, either before or 

 after their elevation, for some of the phsenomena in question may be 

 referred to floating ice and currents of water with quite as much 

 certainty as others can be to the action of glaciers. 



32. Again, I have shown that the requisite degree of cold for the 

 production of glaciers might arise from the diversion of the Gulf- 

 stream into some other channel, the submergence of a great portion 

 of the existing European continent, and a cold current from the 

 north. This diversion of the Gulf-stream might be produced by the 

 elevation of a portion of the bed of the Atlantic so as to form con- 

 necting land between the most western part of Africa and the most 

 eastern portion of South America. But this would require an enor- 

 mous movement, of which, I believe, not the slightest geological 

 indication has been recognized, and the hypothesis is therefore liable 

 to the same objection as that which may be made against the suppo- 

 sition of the more northern portion of the Atlantic having been 

 elevated into dry land during the glacial period. But there is another 

 mode in which the diversion of this great current may, as it appears 

 to me, have been effected, and to which I would especially direct the 

 attention of geologists. 



33. On the west of the continent of North iVm erica, a continuous 

 and lofty range of mountains, the Rocky Mountains, extends from 

 Mexico to the Arctic Sea. Another, but far less lofty chain, the 

 Alleghanies, runs parallel to the eastern coast from near the Gulf of 

 Mexico to the St. Lawrence. The great valley of the Mississippi and 

 its tributaries, extending over some 30° of longitude, occupies the 

 southern portion of the space between these two mountain-chains, 

 being bounded on the south by the Gulf of Mexico, into which the 

 Mississippi discharges its waters. In proceeding from the mouth to 

 the source of this great river, we ascend about 1500 or 1600 feet*. 

 Proceeding northward from its source, we descend into the great 

 valley extending from the Gulf of St. Lawrence, on the eastern coast, 

 along the great chain of North American lakes, to the mouth of the 

 Mackenzie River, which discharges itself into the Arctic Sea. Thus a 

 depression of 2000 feet would convert the valley of the Mississippi 

 into a great arm of the sea, of which the present Gulf of Mexico 

 would form the southern extremity, and which would communicate 

 at its northern extremity with the waters occupying the submerged 

 district above-described as the great valley now occupied by the chain 

 of lakes. A direct communication would thus be produced between 

 the Gulf of Mexico and the Arctic Sea along the eastern base of the 

 Rocky Mountains. 



* See Sir John Richardson's paper " On some points of the Physical Geology 

 of North America," Quart. Journ. GeoL Soc. 1851, vol. vii. p. 212. 



