66 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



The reef flat on the west side is from 100 to 150 yards wide ; its founda- 

 tion consists of coralliferous limestone of the old ledge rock ; but along the 

 higher parts of the beach the old ledge rock is often topped with a bed of 

 conglomerate made up of beach rock, fragments of pieces of broken coral, 

 and of old ledge rock (PI. 33), the whole cemented together. This material 

 hides the higher parts of the vmderlying ledge rock;^ it can, how- 

 ever, be seen cropping out again wherever the upper part of the old 

 ledge buttresses has not been obliterated, and on the top of the land rim 

 back of the beach (PL 34, fig. 1). All the way across the land rim from 

 the beach to the lagoon the old ledge crops out in the vegetation and 

 rises in small blocks and pinnacles. On the lagoon side the old ledge is 

 fully as well exposed as on the sea flice of the atoll (PI. 35). There 

 are no corals in the lagoon, and sea water probably percolates through 

 the porous rock of the land rim. It is rarely washed over the high land 

 rim of the atoll except in a hurricane, as in the cyclone of 1878, when the 

 lagoon was to a certain extent filled with sand and coral fragments washed 

 in by the sea. The density of the water in the lagoon is 1.0216 at 

 28° C. The water is very salt. 



Wherever the old ledge cropped out, either on the sea face of the atoll, 

 the summit of the land rim, or on the lagoon side, it was pitted and honey- 

 combed and covered with diminutive spires and points, as is all the old ledge 

 rock of Makatea; in fact, it was like it in all respects, full of tertiary fossils, 

 and was not composed in any way of recent material except in those places 

 where the conglomerate mentioned above had been formed and deposited on 

 the surface of the old ledge rock or between its buttresses. 



At Niau, as is the case at Rangiroa, we find the beach sand is gradually 

 encroaching upon the old ledge, covering it in places, until finally it will 

 obliterate it along the sea face, and eventually the sand will completely hide 

 the old ledge both on the land rim and lagoon side. With the decomposi- 

 tion and disintegration of the reef platform, the undercutting of the old 

 ledge, and the grinding up of the fragments and blocks of dead coral, more 

 material is constantly supplied to build up a higher and higher coral sand 



1 This conglomerate or beach rock has also comparatively recently been elevated to a height of from 

 six to eight feet above high-water mark. 



