96 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



only a couple of hundred yards wide at the outside. We find on the 

 islets the usual coral shingle on the sea face beaches and coral sand on the 

 lagoon side. The shore platform is from 100 to 200 feet in width, and is 

 edged by high Nullipore knolls ; it slopes quite sharply from the base of the 

 shingle beach to the sea face. The boulders and ledges at the base of the 

 shingle beach are made up of beach rock and recent conglomerate which at 

 one time must also have covered the greater part of the old ledge reef flat. 

 But the material which covered the sea face of the old ledge has all been 

 washed away and has been deposited either as boulders at the foot of the 

 beach where the beach rock still crops out, or has been ground into the coral 

 shingle forming the shingle beaches, or has together with the smaller rolled 

 coral fragments supplied the material for building up the sea side of the 

 islands of the outer rim of the lagoon. 



A similar condition of things has been described at Fakarava and on the 

 north side of Rangiroa. 



On the west face the gaps separating the islands are often quite wide, 

 and from the color of the water passing through them a large quantity of 

 water must pass in and out at every tide ; or else the gaps may be closed 

 either on the lagoon side or on the sea face by shingle banks or sand bars, 

 forming thus diminutive bays opening, as the case may be, towards the 

 sea or communicating freely with the lagoon. 



About three or four miles north of the West Pass there is a long line of 

 huge boulders of beach rock, some of them of colossal size ; one of them must 

 have been eighteen by twelve feet and nearly as high (twelve feet). They 

 formed a striking dark mass at the base of a narrow beach, with a narrow 

 reef platform of yellowish rock edged with a brilliant band of pinkish and 

 orange Nullipore and Pocillipore knolls slightly raised above the level of the 

 edge of the reef platform. Where the boulders were still a component part 

 of the stratum of rock overlying the old ledge, close to the base of the beach, 

 they are more or less undercut. 



All the islands on the east end show fine coral sand beaches on the 

 lagoon side and beach rock boulders and coral shingle on the sea face. 

 Occasionally we come across a stretch of beach or coral shingle from five to 

 six feet hiojh. 



