102 bulletin: museum of compakative zoologYo 



summit of a peak of elevated coralliferous limestone, which have been 

 denuded to the level of the sea and then subjected to submarine erosion, 

 forming first a flat, then an incipient atoll of which the lagoon has been 

 gouged out, in one case to twelve, in the other to sixteen fathoms. 

 Upon the outer edge of the flats, corals have grown, protecting the rims 

 to a great extent from further denudation and erosion. 



The depth of the plateau from which Tova rises is probably very 

 considerable. The depth of water at a distance of seven or eight miles 

 is, judging from soundings to the eastward, perhaps as much as 1,200 or 

 1,500 fathoms ; while Thakau Momo apparently rises from a shallower 

 depth of less than 500 fathoms to the north, and about 900 fathoms at 

 a distance of about five miles. 



Such atolls as Thakau Momo, Tova, and a host of others occurring in 

 Fiji, are identical in tlieir mode of formation with such an atoll as the 

 Hogsty in the Bahamas,^ the lagoon of which 1 believe to be due to 

 mechanical causes similar to those which have shaped the above named 

 lagoons in Fiji. While the former are recognized by writers on coral 

 reefs as true atolls, the latter are regarded as pseudo atolls. It is juggling 

 with words to represent structures as diS'erent because the one is in the 

 Pacific and the other in tlie Atlantic, and because the one is in an area 

 recognized as stationary or as one of elevation, while the other is in an 

 area formerly supposed to be one of subsidence and which is now found 

 to be one of elevation. In both cases the coral rims of the atolls are 

 shown to be of little thickness. The same authors refuse to recognize 

 as true barrier reefs, and call them patch reefs, barrier reefs occun-ing 

 in other districts than those examined by Dana and Darwin, because 

 they have been shown to be of comparatively moderate thickness. We 

 can now show, in the very districts which have been selected as typical, 

 that neither the coral reefs of the atolls nor those of the barrier reefs 

 have the thickness attributed to them. 



Thakau Lekaleka. 



Plates 21, 111. 



On our way from Oneata to ^Mothe we steamed close to Thakau 

 Lekaleka (Plate 21), a very narrow reef flat of polygonal outline, some- 

 what more than a mile in diameter, enclosing a shallow lagoon, judging 

 from the light blue color of the impounded water. The reef flat rises 



1 Bull. Mus. Comp. ZoGL, Vol. XXVI. No. 1, p. 103, 1894. 



