328 J. D. Dana on American Geological History. 



have been produced over Europe, partly at least, by a diversion 

 of the Gulf Stream from its present position. He seems in his 

 paper to attribute too much effect to the Gulf Stream, and too 

 little to the prevailing currents of the atmosphere. But, setting 

 this aside, it is unfortunate for the hypothesis, that there is no 

 reason to suppose that America was not then as much in the way 

 of such a diversion as now. The small changes of level which 

 the Tertiary and Post-tertiary beds of the Gulf have undergone, 

 prove that the gate of Darien was early closed, and has since 

 continued closed. America, as facts show, has not been sub- 

 merged since the Tertiary to receive the stream over its surface. 

 If it had been, it would have given other limits to her own drift 

 phenomena; for it is an important fact that these limits in 

 America and Europe show the very same difference in the cli- 

 mates or in the isothermals as that which now exists.* 



On the question of the drift, we therefore seem to be forced 

 to conclude, whatever the difficulties we may encounter from the 

 conclusion, that the continent was not submerged, and therefore 

 that icebergs could not have been the main drift agents : that the 

 period was a cold or glacial epoch, and the increase of cold was 

 probably produced by an increase in the extent and elevation of 

 northern lands. Further than this, in the explanation of the 

 drift, known facts hardly warrant our going. 



If, then, the Drift epoch was a period of elevation, it must 

 have been followed by a deep submergence to bring about the 

 depression of the continent already alluded to, when the ocean 

 stood four hundred feet deep in Lake Champlain, and a whale — 

 for his bones have been found by the Eev. Z. Thompson of Bur- 

 lington — was actually stranded on its shores ; and when the upper 

 terrace of the rivers was the lower river flat of the valleys. This 

 submergence, judging from the elevated sea-beaches and terraces, 

 was five hundred feet on the St. Lawrence and Lake Champlain ; 

 eighty feet at Augusta, Maine ; fifty feet at Lubec ; thirty at 

 Sancoti Head, Nantucket ; over one hundred at Brooklyn, N. Y. ; 

 and two hundred to two hundred and fifty in Central New Eng- 

 land, just north of Massachusetts ; while south, in South Caro- 

 lina, it was but eight or ten feet. 



But whence the waters to flood valleys so wide, and produce 

 the great alluvial plain constituting the upper terrace, so im- 

 mensely beyond the capability of the present streams ? Perhaps, 

 as has been suggested for the other continent, and by Agassiz for 



* Moreover, the Gulf Stream is known to be a deep current, so deep as to be 

 turned around to the northward in part by the submarine slopes of the outer West 

 Indian Islands, and it would have required a submergence of many hundred feet, 

 and moreover a passage quite across the continent into the Arctic seas, to have 

 given the stream a chance over the land : and even then, if the West Indian Islands 

 were not also deeply sunk in the ocean, a large part of the current would still have 

 kept its present track in the Atlantic. 



