674 LOUIS AGASSIZ. 



the low grounds east of the Alleghany range 

 is another indication of the permanence of the 

 ocean trough, on the margin of which these 

 more recent beds have been formed. I am 

 well aware that in a comparatively recent pe- 

 riod, portions of Canada and the United States, 

 which now stand six or seven hundred feet 

 above the level of the sea, have been under 

 water ; but this has not changed the config- 

 uration of the continent, if we admit that the 

 latter is in reality circumscribed by the two 

 hundred fathom curve of depth. 



The summer was passed in his beloved lab- 

 oratory at Nahant (as it proved, the last he 

 ever spent there), where he was still continu- 

 ing the preparation of his work on sharks 

 and skates. At the close of the summer, he 

 interrupted this occupation for one to which 

 he brought not only the reverence of a dis- 

 ciple, but a life-long debt of personal gratitude 

 and affection. He had been entreated to de- 

 liver the address at the Humboldt Centennial 

 Celebration (September 15, 1869), organized 

 under the auspices of the Boston Society of 

 Natural History. He had accepted the invita- 

 tion with many misgivings, for to literary 

 work as such he was unaccustomed, and in 



