THE PAUMOTU PLATEAU. 25 



side, and to outliers of the elevated coralliferous limestone exposed in the 

 gaps of the land rim on the lee face of the atoll. The lagoon of Rangiroa 

 represents a huge sound, the marginal land rim of which has been denuded 

 and eroded to attain its present condition. 



In all elevated coralliferous limestone islands it is difficult to deteruiine if 

 their sink is the remnant of a former lagoon of the island, or of a sound 

 formed during its elevation, or if it has been formed by the action of rain 

 and atmospheric agencies. The amount of denudation and erosiou to which 

 some islands have been subjected is very great, as is clearly indicated by the 

 small caiions. pinnacles, spires, and walls of limestone, as well as crevasses 

 which occur in all directions over the surface of the basin or sink. The 

 extent to which this action has penetrated into the mass of the island is 

 plainl}'^ shown by the great number of caverns which crop out along the 

 .sea face of the cliffs of elevated islands. Some of. the caverns are of great 

 length, and extend as long galleries far into the interior of the island. It 

 is of course difficult, in the face of this extensive denudation and erosion, 

 to state positiveh' what may be part of the ancient lagoon, or sound, and 

 what has been carried awaj' by atuiospheric and other agencies since the 

 elevation of the island. 



The Paumotu Plateau. 



Plates 201, 2.34, and Fiys. 1-12. 



Between the Marquesas and the northwestern extremity of the Paumotus 

 (Fig. 2) we occupied nine stations; close to the Marquesas the depth was over 

 1900 fathoms, the greatest depth on that line being 2700 fathoms at Station 

 31, in lat. 12° 20' S., and long. 144° 15' W. The depths varied between 2451 

 and 2527 fathoms, and diminished to 1208 fathoms off the west end of Ahe, 

 and then to 706 fathoms when about sixteen miles N. E. off Avatoru Pass 

 in Rangiroa atoll. 



We developed to a certain extent the width of the Paumotu plateau by 

 a line of soundings in continuation of the direction of Avatoru Pass, extend- 

 ing a little less than nine miles seaward, where we obtained a depth of 819 

 fathoms. Sub.sequeutly we ran a similar line normal to the south shore 



