78 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



shoals, islands and islets, most of which are outcroppings of the old ledge 

 or are underlaid by the old ledge (PI. 47, figs. 2-4), which has been partially 

 covered by beach rock or recent conglomerate or blocks of dead corals. 

 Some of these ledges and shoals rise to nearly the same height — five to 

 seven feet above high-water mark — as the islands found in the lagoon 

 (PI. 47, fig. 1). 



The island forming the land rim at Rotoava varies in width from 900 to 

 120.0 feet ; it rises very gradually from the lagoon shore, where the beach 

 is from four to five feet at high water, towards the top of the beach on the 

 sea face. Its highest point is from 250 to 300 feet from the outer edge of 

 the ledge platform, which is itself quite narrow, only from 50 to 75 feet wide ; 

 then comes the steep beach face of coral sand or of shingle of broken coral, 

 next a slight depression, an outside beach of a lower level — a sort of break- 

 water, as it were — followed by a gradual rise to the highest point of the 

 outer beach (PI. 51, fig. 2). From the edge of the outer depression com- 

 mences the fringe of the characteristic Paumotu atoll vegetation which acts 

 as a shelter for the cocoanut trees (PI. 50). 



On the lagoon side the shore platform is in great part made up of beach 

 rock and recent conglomerate, which has covered the old ledge and its 

 shore outcroppings at most places (Pis. 46, fig. 3 ; 49). The lagoon plat- 

 form is from 60 to 100 feet in width, but often juts far out into the lagoon 

 in isolated patches (PI. 58, fig. 1). The platform slopes gradually into six 

 or seven fathoms of water, the slope and flats being full of coral patches, 

 some of which extend as far as our anchorage. 



Modern beach rock (PI. 51, fig. 2) and conglomerate are also deposited 

 on the sea face at the base of the steep beach on the top of the old ledge. 

 Here and there the old ledge crops out from the beach rock at some point 

 high upon the sea face of the beach, bej^ond the reach of the point where 

 recent conglomerate is formed. The narrow sea-face platform is exposed to 

 the full force of the breakers, which pound with their whole swing upon the 

 raised outer edge of Nullipore knolls (PL 51, fig. 2). These are constantly 

 broken off and forced up towards the base of the sea beach along the shore 

 platform, and go to add to the mass of coral rubble which is constantly being 

 rolled and ground into shingle or sand (PL 50). Together with the 



