40 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



thus forms a shallow sink, the outer Nullipore edge forming as it were the 

 diminutive barrier reef of a shallow and narrow lagoon on the outer part of 

 the shore platform (PI. 4, fig. 2). The outer wall and outer edge of the plat- 

 form are cut into by deep gullies, into which the breakers rush with great 

 violence, forcing the sea into the lagoon area of the shore platform, and at 

 contracted spots throwing up huge columns of water in the air.' There 

 is usually a considerable current formed in this platform lagoon, towards 

 the nearest lowest part of the Nullipore ridge. 



The shore platform is nearly devoid of growing corals. A few small 

 patches and heads are found here and there in the platform lagoon ; 

 towards the outer edge they become somewhat more numerous, and on the 

 sea face or the summit of the Nullipore ridge they often cover quite large 

 areas. The inner part of the platform lagoon is often covered for long 

 reaches with masses of recent beach rock, or masses torn off from the Nulli- 

 pore ridge which finally have become isolated and undercut by the encroach- 

 ment of the deep gullies into the shore platform. These deposits often 

 completely conceal the underlying old tertiary reef platform. The islet 

 about 2^ miles south of Mohican Islet shows this well ; there huge masses of 

 modern reef rock and beach rock have been forced by the waves of the 

 lagoon over the old reef rock and into the spaces between the ridges of this 

 rock which constitute the substratum of the islet. 



Mohican Lslet (PI. 7, fig. 2) is also composed of old reef rock which has 

 gradually been decomposed and eroded to the level of the lagoon, in which 

 it forms a flat of considerable size ; on Brander Island there is considerable 

 building up going on from the washing up of the ledge sand into bars and 

 islets and from the formation of recent beach rock on the imderlying 

 plateau of old reef rock, thus preventing the further disintegration and 

 erosion of the island. On the lagoon slopes of both these islands corals are 

 growing in from one to five fathoms or more, but they diminish rapidly 

 after six fathoms. 



In the channel to the west of Brander Island corals grow as far as nine 

 to ten fathoms ; they are mainly Madrepores, Porites, and Astrasans. 



Following the shore line of the lagoon from Avatoru to Tiputa Pass, we 



^ See the figures of the blow-holes of Tongatdbu (Pis 1 17, 118). 



