MAROKAU AND RAVAHERE. Ill 



The eastern part of the atoll is a wide reef flat, partly covered with beach 

 rock boulders and shingle on the sea face, and edged in part with low sand 

 bars on the lagoon side of the reef flat. All the islands of the atoll, as well 

 as the islets on the outer reef flat, are faced with coral sand beaches on the 

 lagoon side. The lagoon is from 12 to 18, and, in places, 22 fatlioins deep; 

 though it is said to have deep spots of 30 fathoms, the average depth is 15 

 fathoms. 



The old ledge is, as in other atolls, generally planed off to the level of the 

 outer reef flat, and overlaid by recent conglomerate and beach rock, which 

 supplies the material to check the erosion of the old ledge, and to build 

 up the sand bars, islets, and islands heaped up on the rim of the atoll, 

 according to its position with reference to the gaps and the prevailing 

 winds. 



Hikueru is, like Makerao, full of shoals. Corals seem to thrive remark- 

 ably well in this lagoon. There are great masses of them on the slopes of 

 the shoals and flats, and they supply a large amount of the material which 

 has built up the islands from the lagoon side. It is said that there are 

 in this lagoon, as well as in Hao, deep holes of very limited area, somewhat 

 like the ocean holes of the Bahamas,' but I was unable to obtain any definite 

 information regarding them. 



On the wide flat of the southwest point, which must be 1200 to 1500 

 feet in width, there are long concentric lines of boulders and of shingle and 

 of beach rock and heads of old ledge. 



Marokau and Ravahere. 



Plates 70, figs. 1, S ; 201, 203; S04, fig. 5. 



The Two Groups, as the islands of Marokau and Ravahere are called, are 

 interesting ; they probably represent the denuded summit of a ridge with a 

 valley of a depth of not more than 270 fathoms separating these two summit 

 ridges. The islands are both triangular, the northern island (Marokau) 

 being somewhat the larger ; the second or southern island (Ravahere) is 

 elongated, and is characterized by its secondary lagoon, which extends nearly 



1 See A. Agassiz, Bull. M. C, Z., Vol. XXVI. p. 42. 



