374 ALEXANDER AGASSIZ 



to show that the formation of the atolls is due to very 

 much the same causes which have shaped the limestone 

 islands of the Fijis, namely, to elevation and a subse- 

 quent eating away of the elevated islands. The fact that 

 the Paumotus are all of nearly the same height he ex- 

 plained by supposing them to have been (before erosion) 

 only slightly and about equally elevated. In most of the 

 atolls of this group the Tertiary limestones have been 

 cut down to the water's edge. The appearance of the 

 old Tertiary ledge and of the modern reef rock is so 

 strikingly different, that it was a comparatively simple 

 matter to distinguish the two, even where only relatively 

 small fragments were found. 



Whereas in the Fijis the islands on the atoll rims are 

 the remnants of an elevated island that once covered 

 the entire area of the atoll, in the Paumotus the orig- 

 inal islands have been, in most cases, completely cut 

 away, and the land rim has been formed by the refuse 

 of the old ledge, combined with the fragments of the 

 coral that has established itself upon it, which gradually 

 build up small islands on the reef flat ; these slowly 

 grow, become fused, and form a more or less connected 

 rim about the lagoon. 



The reef shelves of the Paumotus, far wider than 

 those in the Fijis, supply great masses of material from 

 the breaking up of the outer and inner edges of the 

 Tertiary limestone platforms. These, together with the 

 fragments of coral growing upon the flats, are thrown 

 up on the reef flats and form a pudding stone or brec- 

 cia. This pudding stone or beach rock is found on all 

 the atolls of the group. It forms great bars, generally at 

 right angles to the shore line. The sea and wind usually 

 follow the trend of the shores, and the bars of beach 



