434 On American Geological History. 



Sancoti Head, Nantucket ; over one lumdred 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 

 this, from the melting snows of the declining glacier epoch. 



The frequent absence of fine stratification, so common in the 

 material of this upper terrace, has often been attributed to a 

 glacier origin. 



According to this view, the events of the Post-tertiary Period 

 in this country make a single consecutive series, dependent 

 mainly on polar or high-latitude oscillations: — an elevation for 

 the Jirsi or Glacial Epoch ; a depression for the secojid or Lauren- 

 tian Epoch ; a moderate elevation again, to the present height, 

 for the third or Terrace Ep)och. 



The same system may, I believe, be detected in Europe ; but, 

 like all the geology of that continent, it is complicated by many 

 conflicting results and local exceptions ; while North America, 

 as I have said, is like a single unfolding flower in its system of 

 evolutions. 



There is the grandeur of nature in the simplicity to which 

 we thus reduce the historical progress of this continent. The 

 prolonged oscillations of the crust, caused by pressure from the 

 southeast beneath the Atlantic, which reach on throuo-h the Pa- 

 Iteozoic ages, producing the many changes of level in the Silurian 

 and Devonian, still others of greater frequency in the Carboni- 

 ferous, and then, as in an outburst of long emprisoned energy, 

 throwing up the range of the Appalachians, with vast effusions 

 of heat through the racked and tortured crust, next go on de- 

 clining as the Jurassic and Cretaceous Periods pass, and finally 

 fade out in the Tertiary. The northern oscillations, perhaps be- 

 fore in progress, then begin to exhibit their effects over the high 

 temperate latitudes, and continue to the Human Era. The sink- 

 ing of Greenland, now going on, may be another turn in the 

 movement ; and it is a significant fact, that, while we have both 

 there and in Sweden northern changes of level in progress, such 

 great secular movements have nowhere been detected on the 

 tropical parts of the continents. 



