72 



FOYE. 



This flat is covered with toad-stool islets whose overhanging ledges 

 are 4 to 5 feet above the surface of the flat and project 8 to 10 feet from 

 the main body of the rock. 



As shown in the accompanying sketch map and cross-section 

 (Figure 34) a narrow, steep-sided ridge, 270 feet in height, runs along 

 the eastern side of the island. A dismembered ridge, 220 feet high, is 

 located on the extreme western side. Between the ridges there is a 

 flat, pitted plain, dotted with caves. The roofs of the caves bear 

 stalactites and often one portion has collapsed, leaving the other 

 portion as a niche in the side of a cliff. East of the seaward base of 

 the eastern ridge, a flat 500 feet wide occurs at an elevation of 10 to 

 15 feet above sea-level. 



There exists, therefore, in Ongea a flat, central plain at a slight 

 elevation above the sea which has been produced largely by atmos- 

 pheric solution. If the ocean level should rise but a few feet, this 

 plain would be converted into a region similar to the sandy plain at 

 its southern border. Neither atmospheric solution nor wave-erosion 

 within a lagoon can, however, produce a level surface of this type 

 10 to 11 fathoms below sea-level. In other words, the present lagoon 

 about Ongea must have been submerged since its formation. As the 

 elevated corals are Pleistocene or Recent in age, it is believed they 

 were uplifted in post-Pleistocene time, and hence the lagoon flat about 

 the island cannot be ascribed to Pleistocene wave-cutting, nor the 

 depth of the water above the flat to the rise of the ocean after the 

 melting of the Glacial ice. A recent period of subsidence is, there- 

 fore, inferred. 



Vatoa (Figure 35). 



Vatoa, or Turtle Island, is an isolated mass of limestone lying 50 to 

 60 miles south of the Ongea Group. The single island within the reef 



Figure 35. East- West and North-South Profiles of \';\toa. 



resembles Ongea Levu in structure, as is shown by the following cross- 

 sections (Figure 35). There are high ridges on the eastern and western 



