14 "ALBATROSS" TROTICAL PACIFIC EXPEDITION. 



meet them, not only in the Paumotus, but also as they are modified 

 by the local conditions existing in the Marquesas, in the Society Islands, 

 the Tonga group, and the many regions where atolls and coral reefs 

 flourish. In not one of these can it be said that the explanation adapted 

 for the structure of any one group can be applied to that of another 

 without some modification, and often witli very great changes. Nor can 

 we say that a universal law such as that of subsidence can explain the 

 conditions now existing in the formation of atolls and of barrier reefs 

 in the coralliferous regions of the Atlantic and of the Pacific. We must 

 seek in the many mechanical causes due to the trades, to the geological 

 composition of the islands, to the general geodetic and orogenic condi- 

 tions, to chemical action and to minute local conditions, the varied and 

 numerous factors at work in building up an atoll or a barrier reef. 



After striking Ahe we made for Rangiroa, the largest atoll of the Pau- 

 motus. As Rangiroa was the first typical atoll we visited characteristic of a 

 supposed area of depression, it was of course with great interest that we 

 examined the conditions of the land rim and became acqiiainted with a tvpe 

 of formation so common in the Paumotus, the Ellice, the Gilbert, and the 

 Marshall Islands as to become monotonous at the end of a few months' sail- 

 ing. Rangiroa is characterized by the extreme narrowness of the land rim, 

 the width of the reef flats on the weather side of the atoll, and the nearly 

 uniform depth of the lagoon. 



At Rani'iroa we obtained the first intimation of the immense amount of 

 water which flows in and out through the main passes, and, according to the 

 strength of the trades, continues sometimes to flow two or three days in one 

 direction, and is forced by them over the narrow part of the reef flats, 

 which is bare at low water, or between the gaps separating the islands and 

 islets which have been thrown up on the weather or lee side of the atoll. 

 During the strong trades both wind and seas follow the trend of the shores 

 of the islands, and little or no lee is found under them. 



It was fortunate that our first examination of the Paumotus was made 

 among the northwestern atolls and islands of the group. Rangiroa, Tikahau, 

 Matahiva, and Makatea readily gave us the key to the structure of the rest 

 of the group. I doubt if an examination of the southern and eastern Pau- 



