576 



SCIENCE. 



[N. S. Vol. XI. No. '276. 



Gilbert, seem to be somewhat higher than 

 the Paumotus, but this difference is only 

 apparent and is due to the difference in the 

 height of the tides, which is very small in 

 the Paumotus, while in these groups it may 

 be five and even six feet. 



From Jaluit we visited among the Caro- 

 lines, the islands and atolls of Kusaie, 

 Pingelap, Ponapi, Andema, Losap, Namu, 

 the Royalist Group, Truk and ISTamonuito, 

 obtaining thus an excellent idea of the 

 character of the high volcanic islands of 

 the group from our examinations of Kusaie 

 and of Ponapi, while the others represent 

 the conditions of the low atolls, having 

 probably a volcanic basis, but this was not 

 observed at any of those we examined. 



The reefs of the volcanic islands of the 

 Carolines are similar in character to those 

 of the Society Islands, though there are 

 some features, sucli as the great width of 

 the platforms of submarine erosion of Ponapi 

 and of Kusaie, and the development of a 

 border of mangrove islands at the base of 

 the volcanic islands, which are not found 

 in the Society Islands. 



The Truk Archipelago was perhaps the 

 most interesting of the island groups of the 

 Carolines, and it is the only group of vol- 

 canic islands surrounded by an encircling 

 reef which I have thus far seen in the Pa- 

 cific which at first glance lends any support 

 to the theoi'y of the formation of such 

 island-groups as Truk by subsidence. This 

 group was not visited by either Darwin or 

 Dana ; and I can well imagine that an in- 

 vestigator seeing this group among the first 

 coral reefs would readily describe the 

 islands as the summits, nearly denuded, of 

 a great island which had graduallj^ sunk. 

 But a closer examination will readily show, 

 I think, that this group is not an exception 

 to the general rule thus far obtaining in 

 all the island groups of the Pacific I have 

 visited during this trip ; that we must 

 look to submarine erosion and to a multi- 



tude of local mechanical causes for our ex- 

 planation of the formation of atolls and of 

 barrier and encircling reefs and that, on the 

 contrary, subsidence has played no part in 

 bringing about existing conditions of the 

 atolls of the South and Central Pacific. 



ISTowhere have we seen better exemplified 

 than at Truk how important a part is played 

 by the existence of a submarine platform in 

 the growth of coral reefs. The encircling 

 reef protects the many islands of the group 

 against a too rapid erosion, so that they 

 are edged by narrow fringing reefs, and no- 

 where do we find the wide platforms so es- 

 sential to the formation of barrier reefs. 

 The effect of the northeast trades blow- 

 ing so constantly in one direction for the 

 greater part of the year is of course very 

 great; the disintegration and erosion of 

 islands within its influence is incessant, and 

 their action undoubtedly one of the essen- 

 tial factors in shaping the atolls of the 

 difi'erent groups, not only according to the 

 local positions of the individual islands, 

 but also according to the geographical po- 

 sition of the groups. Thus far I do not 

 think any observer has given sufficient 

 weight to the importance of the trades in 

 modifying the islands within the limits of 

 the trades, nor has an3'^one noticed that the 

 coral reefs are all situated practically within 

 the limits of the trades both north and 

 south of the equator. 



The soundings made going west from 

 Jaluit to jSTamonuito indicate that there is 

 no great plateau from which the Carolines 

 rise, but that the various groups are, as is 

 the case with the neighboring groups of the 

 Marshalls and Gilberts, isolated peaks with 

 steep slopes rising from a depth of over 

 2000 fathoms. The line we ran from the 

 northern end of Namonuito to Guam de- 

 veloped the eastern extension of a deep 

 trough running south of the Ladrones. The 

 existence of this trough had been indicated 

 by a sounding of 4475 fathoms to the south- 



