274 "ALBATEOSS" TROnCAL PACIFIC EXPEDITION. 



The land rims of all the atolls hi the Marshall Islands are really 

 msignificant in comparison with the wide expanse of the lagoons they 

 enclose ; their depth as well as that of the passes is also remarkably 

 uniform. The conditions existing over the Marshall group are less varied 

 than they are in any of the other groups of atolls we have examined ; 

 there is not even the same diversity which exists among the atolls in the 

 Paumotus. The amount of moisture which falls in the Marshall Islands is 

 not only great, but most constant, and, owing to the varying position of 

 the faces of the atolls with reference to the trades, we find in consequence 

 the vegetation far more luxuriant than on the Paumotus, although tjie 

 extent of the land rim is much more limited. Dana has made an approxi- 

 mate estimate of the land areas of some of the atolls of the different 

 island groups;^ they show remarkably well the conditions which have 

 resulted from the peculiar geographical position of the Paumotus as 

 contrasted with the Marshalls, or the Marshalls as contrasted with the 

 volcanic atolls of the Carolines and of the Society Islands. It is evident 

 that in the case of volcanic islands, like those of the Society Islands, the 

 Fijis, the Carolines, Samoa, and the Cook Islands, the land areas are, in 

 consequence of the greater hardness of the volcanic rocks, far greater in 

 proportion to the extent of the lagoons than in such districts as Lau 

 in Fiji, where tlie atolls are either elevated coralliferous limestone islands, or 

 where they are partly limestone and partly volcanic ; or in the extensive 

 areas of the Paumotus, where they are coralliferous limestone. When we 

 come to groups like the Marshall, Ellice, and some of the Gilbert Islands, 

 there is no such connection between the dimensions of the lagoon and the 

 land rim ; the foundation has been eroded and much of the material which 

 forms the land rim is constantly shifting backward and forward during the 

 existence of the trades ; it is thrown up in one direction on the narrow land 

 rim in favorable exposures, while it is eaten away from the opposite side 

 during the prevalence of the westerly winds. In the Marshall Islands, 

 where the lagoons present such a long reach to the prevailing trades, we 

 often find on the lagoon face, either on the eastern or western sides of the 

 atoll, an amount of mechanical work performed during the season of the 



1 Dana, loc. cit., p. 169. 



