THE BENTHAL 213 



may be seen, as on Tapamano, about 100 km. west of Tahiti, where 

 the central volcanic part is reduced to two hills, the highest of which 

 is about 50 m. high; the volcanic peaks at Motu-iti, near Bora-bora, 

 have disappeared, leaving only a low platform surrounded by a wreath 

 of small islets. 61 



The Darwinian explanation, on account of its simplicity, found 

 wide acceptance. Dana 64 believed that he could support it from his 

 studies of Pacific reefs, and it still has numerous proponents. Exact 

 investigations of many reefs, however, have shown that the situation 

 is more complex. It is true that barrier reefs and atolls are in general 

 absent along rising coasts, and that these have fringing reefs, as from 

 Dar-es-Salaam to the Red Sea, 65 and in the Sunda Islands. Barrier 

 reefs and atolls, however, are not confined to areas of submergence, 

 but may be formed where the shore line is stationary. 



Careful observers who have dealt with this problem 66 " 73 agree that 

 Darwin's theory is inadequate in this respect. They emphasize most 

 the effects of aerial and wave erosion and chemical solution of the 

 calcium carbonate on the determination of the height of the reef and 

 on the formation of lagoons. Semper 66 called attention to the fact that 

 atolls, barrier reefs, and fringing reefs may occur in the same region 

 in relatively close juxtaposition. On Tahiti the north, west, and south 

 sides are surrounded by a broad fringing reef, while on the east side, 

 under the influence of the strong trades, a deep lagoon has been formed 

 between reef and coast, whose origin through erosion may be plainly 

 seen through its gradual transitions. 68 Similarly the broad fringing 

 reefs of the Fijis may merge into barrier reefs with a narrow lagoon. 

 The great effects of marine erosion are shown by the Metisbank in the 

 Tonga Islands; in 1875 it was an island of 9 m. height above sea level; 

 it was later raised to 40-50 m., and is now reduced to a submarine 

 platform 30 m. below sea level. 



These eroding influences are counteracted on one side by the growth 

 of coral and, on the other, by the accumulation of coral sand brought 

 in by waves and dust blown by the wind, which tend to fill the lagoons 

 and produce extensions of the shore, shallows, islets, and secondary 

 lagoons. 



The abundance of concomitant factors and the various ways in 

 which the balance may be established between the constructive and 

 destructive agencies has as a consequence a great variety of appear- 

 ance in the reefs of a single region, especially in the Indo-Pacific, 

 according to local conditions. The variety of material of the shore, 

 solid or loose volcanic material, tertiary limestone or modern coralline 

 limestone, conglomerates, and breccias is a factor which affects the 



