354 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



exist by which the lagoon can be entered, not only on the north side, 

 but also on the east, as well as the southeast side (PI. 231), the principal 

 passes being on the north face to the west of Pis Island (PI. 193, fig. 4) 

 and on the east coast opposite Gregoire Island (PI. 193, fig. 3). On the 

 southeast face are also numerous passages, as well as along the southern 

 face of the lagoon, where the barrier reef flat is narrow, with only here 

 and there a low island thrown up on the face. Between the disconnected 

 reef patches on the southern face there are no less than seventeen passes, 

 all of considerable depth. The western point of the reef flat forming the 

 western extremity of the atoll is broad ; on it are four low islands (PI. 

 231). The greater part of the lagoon varies from fifteen to twenty-five 

 fathoms in depth ; a few soundings indicate depths of thirty-five to thirty- 

 six fathoms. The northwestern part of the lagoon is nowhere more 

 than thirty fathoms, the deepest part being towards the southern face 

 of the atoll. 



The islands on the outer margin of the immense atoll of Truk are 

 very irregular in size ; they have been washed up on the outer barrier 

 reef flat, both from the sea and from the lagoon face. Numerous sand 

 keys, more or less covered with vegetation, have also been thrown up 

 on the barrier reef flats (PI. 231). Truk is helmet-shaped and not less 

 than thirty-five miles in diameter ; as we approached the atoll, we could 

 see gradually rising from the central parts of the lagoon the moun- 

 tains of Umol, of Toloas, of Ruk, and of other islands, the character of 

 their slopes clearly indicating their volcanic origin (Pis. 191, figs. 3, 4; 

 193, figs. 1, 2). The islands were flanked at the base by sand and coral 

 shingle beaches, derived from the fringing reefs which encircle them (PI. 

 231). The fringing reefs have grown upon the disintegrated spits of the 

 islands and their shores, and have formed extensive reef flats. The outer 

 barrier reef rim of the atoll differs in no wise from the rim of en- 

 circling reefs such as we found in the Society and Fiji Islands, where 

 the central land mass of the lagoon, or the land mass of the rim, was 

 made up of volcanic rocks. 



Enclosed by the encircling barrier reef are a number of high volcanic 

 islands, at a distance of from two to fifteen miles from it ; twelve of 



