GEOLOGICAL OHSEllVATIONS IN FIJI. 



73 



sides of the island and hetwcen is a low, pitted plain. A sea-cut heneh 

 at an elevation of 10 to 12 feet is well defined along the northeastern 

 side, but disappears to the south, where its absence may be due to 

 more active atmospheric solution or to a tilting of the island. An 

 extensive sand flat projects southward as a long reef-point which 

 would perhaps suggest that the latter view is correct. 



Dismembered portions of the elevated limestone mass of Vatoa 

 dot the lagoon about the island. They have largely disappeared from 

 the eastern coast, probably because of the greater abrasive efficiency 

 of the lagoon sands to the windward. In the western lagoon they are 

 found within 300 yards of the edge of the barrier reef. 



The northern end of the island is in process of dismemberment from 

 the southern part. A strait 20 to 30 feet in width separates the two 

 remnants. This strait is bare at low tide and the basin-shaped depres- 

 sion near its center contains a pool filled with small coral heads. 



There is no good passage across the reef at Vatoa. The anchorage 

 is a protected bay in the reef to the westward. In rowing across 

 the reef flat and lagoon to the main island a generalized section was 

 drawn which holds good for most of the reefs studied (Figure 36). 



Fat HO MS 



3a of a. mile 

 Figure 36. Tj-pical Cross-section of a Barrier Reef. 



(1). Sandy platform at an average depth of 7^ fathoms with some 

 coral waste. 



(2). Overhanging ledge of honeycombed coral through which are 

 openings looking down into the depths. Mushroom heads of coral. 



(3). 'Holes in the coral formation are here filled with sand. Coral 

 growth is less abundant and stag-horn coral prevails. 



(4). The sand increases in amount till the flat is nearly barren of 

 coral and in the lagoon only an occasional head is seen. 



(5). Coral patch with fairly vigorous growth of rounded heads. 



(6). Bar connecting toad-stool islets; coral growth is here vigorous. 



(7). This zone varies with conditions. If the lagoon is wide and 

 the sand, carried back across the barrier reef, settles before it reaches 

 this zone, a fringing reef develops; otherwise a sandy bottom is found. 



