46 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



two miles before we obtain a depth of over five fathoms. The lagoon 

 slope of the old ledge as it extends eastward from the southern land belt is 

 quite shallow (Pis. 15, fig. 2 ; 16). About two miles from the south shore the 

 lagoon is not more than two to four fathoms deep, and the surface is covered 

 with huge patches of corals, mainly Porites and Astraeans, as well as Madre- 

 pores and Pocillipores, at a depth corresponding to that where flourish the 

 corals on the north side of the lagoon, both to the west and east of Avatoru 

 Pass. We landed on the south side of Funuarua Island, the largest of the 

 islands on the windward face of the lagoon (PI. 16, fig. 1). This island we 

 found, like those on the land belt of the north side, to be in reality a 

 chain of islands and islets, separated, like them, by cuts, only the cuts were 

 much more marked, and the process of formation of the islands of the land 

 belt was much more apparent than on the north side. At our landing-place 

 we found a cut fully 200 feet wide running directly across the island (Pis. 15, 

 fig. 1 ; 16, fig. 2) from the lagoon side to the great wall of ancient elevated 

 reef rock which shuts it off completely from the southern or windward side. 

 This wall was fully 12 feet high (Pis. 15, fig. 2 ; 17, fig. 2 ; 18, fig. 1), and 

 is the remnant of the ancient coralliferous limestone ridge which flanked the 

 southern side of Rangiroa, and which Dana^ described (vicariously) while 

 coasting along the southern face of Rangiroa in the " Peacock." 



This old ledge is deeply pitted and honeycombed and eroded into all 

 kinds of fantastic spires and pinnacles (Pis. 15, 17 ; 18, fig. 1) and walls (PI. 

 14, figs. 1, 2) cut through by crevasses extending from low-water mark to 

 the summit, which is more or less covered by the high sand beach accumu- 

 lated behind it on the lagoon side. This beach completely conceals the 

 extension of the old ledge (PI. 16, lig. 2) under the island ; it has been 

 formed from the disintegration on the lagoon side of the extension of the 

 ledge (PI. 15, fig. 1), and has been piled up to a height of from 10 to 15 

 or even 18 feet against the great sea wall of old reef rock which extends 

 almost unbroken from the northern point of Rangiroa to its southwestern 

 end. 



The island we examined (Funuarua), as well as the islands and islets 

 to the westward, are all built up at right angles to the trend of the coast. 



1 Dana, loc. cit. pp. 20,!, 369. 



