180 "AiBATROSS" TROPICAL PACIFIC EXPEDITION. 



which continued until the present epoch, that denudation and erosion took 

 place on the gigantic scale on which they have acted upon the land of 

 this elevated limestone plateau and it assumed existing conditions. In the 

 Tonga group we find all the conditions which characterize the so-called 

 elevated atolls, or plateaus of limestone, or high elevated limestone island.s, 

 like Eua. The terraces of this, the southernmost of the Tonga Islands, and 

 of the Haapai and Vavau Plateaus, clearly indicate a sort of synchronism, 

 marking the periods of rest which have taken place during the elevation of 

 this gigantic ridge of limestone. 



The sounding at Station 182 shows that the Aldrich deep, which cul- 

 minates off the Kermadec Islands in 5155 fathoms, extends much farther 

 north than has been indicated heretofore by the soundings. The greater 

 cold of the bottom also indicates the existence of a ridge separating the 

 Moser Deep from the eastern extension of this tongue of deep water of the 

 Southern Pacific. It was immediately off the entrance to Vavau, at Station 

 187, that sounding in 682 fathoms we brought up remarkable colored crys- 

 talline calcite and other crystals, some reddish in color, others gray, which 

 look like the weathered and w^ashed sand coming from the old coralliferous 

 limestone ledges. It is probable that much of this sort of bottom has been 

 called volcanic sand on the charts, and has given rise to the statements 

 that the underlying rocks of Vavau are probably volcanic in their structure. 

 While it is true that to the westward of Vavau true volcanic sand exists 

 near the line of volcanoes which stretches north and south a distance of 

 nearly 200 miles parallel with the Tonga Plateau, yet this volcanic sand is, 

 when closely examined, very different in its appearance and can readily be 

 distinguished from the colored calcite crystals. 



Eua Island. 



Plates 111-11',; l:li,fii].-l; il,i,fi(j. J, ; 213, 21 J^. 



Eua Island (PI. 212, fig. 4) is at least ten and a half miles in length, and 

 three miles broad ; it lies southeast of Tongatabu, about ten miles distant, and 

 is separated from it by a comparatively shallow chainiel, with not more than 

 120 fathoms in the shallowest part of the channel. The soundings on the 



