THE CAROLINE ISLANDS. 331 



tliat subsidence has played no part in bringing about existing conditions 

 in the atolls of the South and Central Pacific. This group was not visited 

 by either Darwin or Dana. 



Nowhere have we seen better than at Truk how important is the part 

 played by a submarine platform in the growth of coral reefs. The encir- 

 cling reef protects the many islands of the group against a too rapid 

 erosion, so that they are edged by narrow fringing reefs, and nowhere 

 do we find the wide platform so essential to the formation of barrier reefs. 

 The effect of the northeast trades blowing 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 essential factors in shaping the land rims of the 

 different atolls, not only according to their local position, but also accord- 

 ing to the geographical position of the groups. Thus far I do not think 

 any observer has given sufficient weight to the importance of the action of 

 the trades in modifying the islands within the limits of the trades; nor has 

 it been noticed that the coral reef areas are, with few exceptions, situated 

 within the limits of the trades, both north and south of the equator. 



We did not visit a number of the eastern and central Carolines, but, 

 judging from the Admiralty Charts, they present no features of special 

 interest. The small land rims of some of the low atolls are very striking ; 

 for instance, at the Mortlock Islands,' the land rim of Sotoan, its principal 

 atoll, nearly twenty miles in length and six in width, is indicated only by 

 a few islands and islets ; the greater part of the atoll is marked merely 

 by a reef flat with extensive coral patches on its western and northern 

 face. Lukunor, another atoll of the Mortlock Islands, has been surveyed 

 by Captain Lutke. It is slightly pyriform in outline ; its lagoon has a 

 depth varying from twenty to thirty fathoms, with an average depth of 

 about twenty. The land rim of the southern and of the southern part 

 of the east face of the atoll consists of a large number of islands and 

 islets; the narrow northern and western reef flats are awash. 



The Hall Islands resemble the Mortlock group, as do the Lamotrek and 

 the Uluthi Islands, all of which have been surveyed by Captain Liitke. 



1 A. Chart 772. 



