358 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



on the lagoon side. Nearly all the islands of the outer reef flat are 

 well covered with cocoanut trees and with the usual belt of low bushes 

 characteristic of the outer barrier reef flat islands. 



The summits of Truk vary from 100 to 1200 feet in height. The 

 outline of some of the islands clearly shows the extent of denudation to 

 which they have been subjected. Patches of beach rock conglomerate 

 flank some of the islands ; small narrow belts of mangroves also line the 

 shores, as in Kusaie and Ponapi, only on a very small scale. We see 

 in this atoll, perhaps better than in any other we have visited, that its 

 present configuration is mainly due to orogenic conditions ; the great depths 

 of parts of the lagoon represent valleys formed at the time of upheaval 

 of the original mass of Truk, the spurs separating them having been 

 planed off to form the great platform of Truk. 



We must look upon the deep parts of some lagoons as due to the sub- 

 marine erosion and denudation of the outer slopes of the spurs of the 

 original land masses. Volcanic slopes often extend far across the barrier 

 reef platforms ; when denuded and eroded they become islands and islets 

 and bars, shaved down to the level of the sea and covered with coral 

 sand, or with large coral boulders thrown up between them on the outer 

 barrier reef flat. Here and there an islet may be the remnant or the 

 outlier of one of the volcanic spurs, once extending close to the outer 

 edge of the reef. 



The points of the islands we passed on the way to Uola are covered 

 with short reaches of coral. At the northern extremity of Tsis Island 

 a long line of volcanic boulders extends far out into the lagoon, and is 

 the remnant of a spur of the central ridge of the island. 



Between the islands the lagoon is full of patches, some of which are 

 volcanic ledges broken into all kinds of fantastic shapes, while others are 

 coral reef flats which have grown up on the volcanic ledges after they have 

 been eroded to below the level of the sea. A long line of such ledges ex- 

 tends between Periadik towards the northern point of Ruk. 



The shore of Uola is edged by a fringing reef belt, varying in width from 

 500 to 800 feet, extending out in a gradual slope into the lagoon to a depth 

 of from five to six fathoms. Where coral heads growing to the surface have 



