276 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



than twenty to twenty-five feet in width ; the outer reef platforms like- 

 wise are often very narrow, in marked contrast with the wide reef flats of 

 some of the Paumotu or Fiji Islands. Again the land rim is often com- 

 posed of a number of small islets separated by gaps of greater or less depth, 

 nearly as large as the islands they separate, allowing additional circulation 

 into and out of the lagoon. In fact, as has been noticed before, there is no 

 group of atolls where the land rims are reduced to such an insignificant 

 area as in the Marshall Islands, and where what are usually shallow gaps 

 become deep passes with sufficient water to allow the entrance of large 

 ships. In the atolls of the Marshall Islands the number of passes suitable 

 for navigation is in marked contrast with those of other atolls (Pis. 226, 

 227), especially in the Paumotu and Fiji Islands. It is not uncommon to 

 find passages varying from eight to twenty fathoms, or in fact as deep as 

 the deepest part of the lagoon itself. This would seem to indicate that 

 the material forming the outer rim of the Marshall Islands atolls rested 

 upon a submarine platform of a depth varying from eight to twenty 

 fathoms, and that the mass of material kept in motion and affected, accord- 

 ing to the local conditions, by the sea and by the winds, forms the special 

 feature of each atoll in the Marshall Islands. 



Although the trades are the principal winds at work shaping the 

 land rims of the Marshall Islands, yet during three months westerly 

 winds prevail during the change of the monsoons. When the length and 

 trend of some of the lagoons is taken into consideration, we may have 

 acting upon the lagoon sides of the land rim, especially on the beaches 

 of the western islands, waves nearly as powerful as those beating on the 

 eastern side of the same islands daring tlie trades. Thus the movement 

 of loose material on the lagoon side is far more extensive on the exposed 

 patches and reef flats of the western slope of the eastern islands of the 

 land rim, than is usually the case in lagoons of a smaller size. The 

 reach which the sea obtains during westerly winds, or during the season 

 of the trades, is often from forty to sixty miles, a distance great enough 

 to act with considerable force on the movable material of the land rim of 

 these atolls. 



The level at which some of the dead coral heads still in place are found, 



