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AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 101 
Reef to Motua lai lai, Motua Levu, and to Adolphus Reef. They all 
owe their origin to the disappearance through atmospheric agencies of 
the peaks or ridges of greater or less heights which they represent, and 
to the formation of lagoons by submarine erosion, passing finally into 
such sunken banks as the Penguin, Alexa, 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 off 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 in shape, 
from such atolls as Wailangilala. 
Thakau Momo. 
Plates 34, 12, 14, 233, Fig. 6. 
On our way from Nairai to Moturiki 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 surface. This flat 
being edged on the outside by the narrow white 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 
water pouring in over the weather edge. 
Tova Reef. 
Plate 23°, 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 
