AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 105 



Leudenfeld ^ gives altogether too great prominence to the part played 

 by dry atolls. They do not occur anywhere except as most diminutive 

 atolls. The only atoll in Fiji which answers at all to the theoretic 

 requisitions of Lendenfeld is that of Vuata Vatoa (Plate 23% Fig. 4), 

 near Turtle Island, in Lau. I have not visited the island, and my 

 impression is taken from the chart" and the Sailing Directions where 

 it is described. Yet even that atoll has outfalls on the northwest side, 

 and the margin is awash at high Avater, evidently allowing large masses 

 of water to be poured into the lagoon, so that further denudation and 

 erosion must eventually so modify this atoll that there will be a larger 

 and more open lagoon than formerly, and it will not fill up as is required 

 from the other point of view. 



EXTINCT CRATERS ANT> ATOLLS. 



There is, however, still another phase in the formation of atolls 

 which has received but little attention, and that is the influence of the 

 denudation and erosion of volcanic summits or ridges, or of extinct 

 craters, in the formation of atolls. There are in the Fijis two extinct 

 craters which are most interesting. One of these is the small extinct 

 crater of Thombia in the Ringgold Islands (Plates 18, 70). The highest 

 point of its rim, the extei'ior cii'cumference of which is nearly two miles, 

 is nearly 600 feet, and it is continuous with the exception of a small 

 part of its eastern edge, about a fifth of a mile, across which a coral reef 

 extends, the extension of the fringing reef surrounding the island closing 

 the entrance to the crater ; the enclosed basin has a depth of 24 

 fathoms. The other extinct crater is that of the island of Totoya 

 (Plates 16, 22, 66-69), an isolated peak in the southern part of the 

 group. It is about six miles in diameter, with an inner basin of three 

 miles in diameter and a depth of over thirty fathoms. The highest 

 point of the rim is 1,200 feet, and at two points the rim is low, forming 

 in one case a narrow isthmus separating the crater basin from the outer 

 lagoon. The horns of the open rim are connected by a coral reef, over 

 which thunders the Pacific swell, piling up the water into the great 

 basin of the crater. This water finds its way out through an opening 

 called the Gullet, which, though narrow, forms an excellent passage into 

 the anchorage inside of the crater. This island has not only a fringing 



1 Nature, June 12, 1890, p. 148. 



2 Admiralty Cliart, Plan 742. 



