378 ALEXANDER AGASSIZ 



pelagoes are in striking contrast to the Paumotus, the 

 Society Islands, the Tonga, and the Fiji Islands, where 

 the character of the underlying foundations of the land 

 rims is readily ascertained. But on the other hand, the 

 first groups gave him the means of studying the forma- 

 tion of the land rims in a most satisfactory manner. He 

 was nowhere else able to trace so clearly the results of 

 the various agencies at work in shaping the endless 

 variations produced in the islands and islets of the 

 rims of the different atolls by the incessant handling 

 and rehandling of the material in place, or of the fresh 

 material added from the disintegration of the faces of 

 the rims, or of the corals on the slopes. 



In many of these atolls he was also able to observe 

 how the luxuriant growths, on the reef flats, of such 

 corals as Porites, are gradually changed into dead reef 

 flats, with a surface cemented by Nullipores ; they thus 

 become the base upon which a land rim of bars or islands 

 is gradually thrown up. 



In concluding, Agassiz mentions his opinion that thus 

 far no observer has given sufficient weight to the action 

 of the trade winds in modifying the islands within their 

 limits; or has noticed that the coral-reef areas are, 

 with few exceptions, situated within the limits of the 

 trades, both north and south of the Equator. 



In 1839, Wilkes, wishing to give some future voy- 

 ager an opportunity of measuring the growth of a coral 

 reef, set up a monument at Point Venus on the island 

 of Tahiti. A bench mark showed the height above a 

 certain point on the adjacent Dolphin bank. Agassiz 

 was naturally anxious to avail himself of this chance 

 to measure the growth of coral in sixty years. After all 

 the misinformation that had been published about coral 



