i 4 2 CORAL-REEFS. 



MenchicofT atoll is formed of three loops, two of which (as 

 may be seen in Fig. 2, Plate I.) are connected by a mere 

 ribbon-shaped reef, and the three together are 60 miles in 

 length. In the Gilbert group some of the atolls have 

 narrow strips of reef, like spurs, projecting from them. 

 There occur also in parts of the open sea, a few linear and 

 straight reefs, standing by themselves ; and likewise some 

 few reefs in the form of crescents, with their extremities 

 more or less curled inwards. Now, the upward growth of a 

 barrier-reef which fronted only one side of an island, or one 

 side of an elongated island with its extremities (of which 

 cases exist), would produce after the complete subsidence 

 of the land, mere strips or crescent or hook-formed reefs : 

 if the island thus partially fronted became divided during 

 subsidence into two or more islands, these islands would be 

 united together by linear reefs ; and from the further 

 growth of the coral along their shores together with sub- 

 sidence, reefs of various forms might ultimately be produced, 

 either atolls united together by linear reefs, or atolls with 

 spurs projecting from them. Some, however, of the more 

 simple forms above specified, might, as we have seen, be 

 equally well produced by the coral perishing during sub- 

 sidence on part of the circumference of an atoll, whilst on 

 the other parts it continued to grow up till it reached the 

 surface. 



The Great Chagos Bank. — I have already shown that 

 the submerged condition of the Great Chagos Bank (Plate 

 IV., Fig. 1, with its section Fig. 2), and of some other 

 banks in the Chagos group, may in all probability be 

 attributed to the coral having perished before or during the 

 movements of subsidence, to which this whole area by our 

 theory has been subjected. The external rim or upper 

 ledge (shaded in the chart), consist of dead coral-rock thinly 



