xvi INTRODUCTION. 



of low bushes, separating the barrier reef lagoon from the sea. Erosion 

 and denudation have been great factors in shaping the volcanic slopes of 

 the islands of the group, and the wide platform on which the encircling 

 reefs have obtained a foothold has been cut by submarine erosion. 



In the Paumotus there is a great development of buttresses of tertiary 

 coralliferous limestone ; they run across the beaches of the sea face and of 

 the reef flat platforms, and are the last remnants of the former elevated 

 land once covering a large area of the atolls. 



In tlie Ellice and the Marshall Islands, and some of the Gilbert Islands, 

 we find similar buttresses of modern reef rock and conglomerate indicating 

 an elevation of a few feet. In the Gilbert Islands we find buttresses of both 

 modern and tertiary limestone, as we also do in the Paumotus and Fiji. 



We should remember the existence of many reefs, elevated to a few feet 

 only, where the elevated mass consists of recent reef rock material or of 

 recent conglomerate or breccia. These reefs are not to be confounded with 

 elevated coralliferous limestones of tertiary age, though they may have 

 been lifted to a moderate height only and may also constitute the under- 

 lying base of the elevated modern reef rock. 



In the Paumotus, as in other groups like the Marshall, Ellice, and Gil- 

 bert, the width of the land rim varies greatly. We may state in general 

 that a part of the land rim is built up on the sea face, a part may simi- 

 larly be built up fi-om the lagoon side where the atolls are large enough 

 to be swept by the trades, or the land rim may be increased simultaneously 

 both on the lagoon side and the sea face. Finally the land rim may be 

 widened by the material blown or swept over it from the sea face towards 

 the lagoon side, the latter process rapidly filling small lagoons, or in larger 

 atolls shoaling the lagoons and restricting their area by the formation of 

 extensive flats on the lagoon side of the land rim ; or, as in some of the 

 Marshall Islands, the sand dunes of the lagoon face are blown over the land 

 rim and encroach upon the outer reef flat platform. Add to this the changes 

 due to the solvent action of the sea, both on the sea face and lagoon side of 

 the land rim, and we get an idea of the complicated factors at work in 

 maintaining an equilibrium between the destructive or constructive agencies 

 acting on the land rim of an atoll. Any one of these factors may obtain 



