240 Prof. A. Agassiz on the 



being quite small, with a circumference of 2 miles and a 

 depth of 24 fathoms, %Yithout patches in the central lagoon ; 

 the other being much larger, more than 25 miles in circum- 

 ference, having a depth of 34 fathoms inside the lagoon. The 

 lagoon of the I'otoya atoll would be dotted with ])atches, some 

 of which formed parts of the rim, others being the remains of 

 eroded spurs extending towards the centre of the extinct 

 crater. 



There are in Fiji a number of small atolls from 1 to 3 or 

 more miles in circumference, the formation of which, it seems 

 to me, can only be satisfactorily explained on the theory that 

 they have been formed upon the eroded summits or rims of 

 extinct craters, the rim of the volcano having been eroded 

 either to a continuous flat or to flats separated by deeper 

 passages (as in the case of the low parts of the rim of Totoya) 

 forming entiances into the enclosed lagoons. Such atolls are 

 Motiia Levu, Motua Lailai, the Adolphus reef. Bell reef, 

 Williamson, Pitman, and the Horseshoe reefs, and Thakau 

 Lakaleka. Of course it is possible that some of these atolls 

 may have been formed from the erosion and denudation of 

 isolated peaks or ridges. It is also possible that some of the 

 larger atolls in which are enclosed volcanic islands, like 

 Vanua ]\Iba]avu, Komo, Motha, Lakemba, I\lbenga, the Ring- 

 gold Islands, and others, may represent parts of the rim or 

 ridges and spurs of volcanic peaks and extinct craters which 

 have disappeared by erosion and have left the outer flats upon 

 which the barrier-reef corals have grown ; while the deeper 

 valleys and gorges of these now eroded volcanic islands repre- 

 sent the undulations in depths of the lagoons. The depths 

 inside the lagoons vary greatly ; in the case of Vanua Mbalavu 

 we find 72 fathoms on parts of the eastern slope of the lagoon. 

 These great depths, far beyond any at which corals can grow, 

 represent the elevated gorges and slopes of the volcanic peaks 

 which probably once extended over the v/hole area enclosed 

 by the outer reef, during the elevation of which the reef 

 which covered a part of tlie same area was lifted to its present 

 or even to a greater heiiiiit. 



Such large volcanic centres with extensive craters of con- 

 siderable depth are not unknown. We can reconstruct 

 conditions from Totoya which would give us an atoll open to 

 the west, with a few islands on the outer rim and a greatest 

 depth of 250 fathoms inside the lagoon. Again, Haleakala 

 in the Sandwich Islands has a crater with a depth of nearly 

 250 fathoms, while many small volcanic peaks, some fully 

 1200 feet in height, rise from its bottom. The diameter of 

 Haleakala is fully as great as that of any of the atolls in 



