AGASSIZ : FIJI ISLANDS AND CORAL REEFS. 101 



Reef to Motua lai lai, Motua Levii, and to Adolphus Reef. They all 

 owe their origin to tlie disappearance through atmospheric agencies of 

 the peaks or ridges of greater or less heights which they represent, and 

 to the formation of lagoons hj submarine erosion, passing finally into 

 such sunken banks as the Penguin, Alesa, and other banks discovered 

 by Captain Field in the " Penguin," and figured in Admiralty Chart 

 1431 (see Plate 23*). Possibly, as I have suggested in the case of 

 the smaller atolls, the lagoons may represent the extinct craters of 

 small volcanoes of which the rim has been planed oiT by the action 

 of the sea. 



We did not visit Duff Reef (Plate 18), which, judging from the chart, 

 presents no points of special interest or of difference, except iu shape, 

 from snch atolls as Wailangilala. 



Thakau Momo. 



Plates 3a, 13, 14, 33a, Fig. 6. 



On our way from Nairai to ^loturiki Channel we examined the Horse- 

 shoe Reef (Thakau Momo) about nine miles northwest of Nairai. Owing 

 to the high stage of the tide we did not find it awash, and could 

 only trace the very narrow ring of the yellowish green flat upon which 

 violet patches of corals could be seen dotted over its surfoce. This flat 

 being edged on the outside by the narrow wdiite rim of breakers, and 

 the dark blue water of the deeper soundings on the inner edge being 

 fringed by a narrow belt of light green passing into light blue and into 

 the somewhat darker blue water of the central part of the lagoon. 



The plan of Thakau Momo (Plate 23*, Fig. 6) shows the narrow rim 

 forming the outer edge of the lagoon awash, with a depth in the centre 

 of twelve fathoms. The western edge is worn away, leaving the greater 

 part of the rim of the lee edge more or less open to the outflow of the 

 \vater pouring in over the weather edge. 



Tova Reef. 



Plate 33% Fig. 5. 



The plan which is given of Tova Reef is more detailed, and shows a 

 lagoon with a more complete rim than that of Thakau Momo, with boat 

 passages on the lee side through the northern and western rim of the 

 encircling reef. These two atolls are sharp peaks, the one, Thakau Momo, 

 probably the summit of a volcanic ridge, the other, Tova, perhaps the 



