174 COELENTBRATA. 



at any rate, have readied so near the surface that coral polyps could form a 

 settlement on it, and (2), as whole groups of islands have the atoll form, that 

 it should in the great majority of cases not rise above that level. 



Formerly the view was held that atolls were formed on submarine moun- 

 tains (volcanic) which reached to within the limit of growing coral distance 

 (40 fathoms), but this was objected to on the ground that it was exceedingly 

 unlikely that there should have been so many mountains reaching to just within 

 the right distance from the surface. But it has recently been pointed out* that 

 a large number of banks exist inthe ocean, springing from deep water and 

 rising to a wide level top at a depth of about 30 fathoms from the surface. 

 On many of these banks a rim of growing coral has been found raised a few 

 fathoms above the general level. Such banks have probably been produced by 

 a submarine volcanic eruption which has formed a mound, sometimes reaching 

 to the surface, sometimes reaching above it, but composed of loose scoriae in a 

 more or less finely divided state. Within the last few years the formation of 

 such mounds has been observed in different parts of the sea, and they have since 

 been, or are now in process of being reduced, by the action of the waves on the 

 loose material, to the condition of submarine banks such as are known to exist, 

 and such as are required for the foundation of atolls. 



Darwin, to whom the existence of these banks was unknown, pronmlgated 

 his well-known theory of coral reefs, which was as follows : fringing reefs are 

 formed within the coral-reef zone on the shores of places where the conditions 

 are favourable for coral growth. Barrier reefs and atolls are derived from fring- 

 ing reefs by the subsidence of the land on which the reef is placed. 



If we suppose an island bordered by a fringing reef to begin to subside (Fig. 

 144^), the corals, which as we have seen, are in the most vigorous condition on 

 the edge of the reef, no longer limited in their growth by the level of low water, 

 will grow upward ; the width of the lagoon will be increased by the extent to 

 which the sea now encroaches on the shore, and its depth by the amount of the 

 subsidence. Continue the subsidence and the reef will rise like a wall round the 

 land from which it is separated by the deepening lagoon, and the land itself, 

 lessened in area, will be reduced to the higher parts of the original mass, forming 

 an island or a group of islands in the centre of the wide lagoon (Fig. 144^). 

 The breaks in the fringing reef at the mouths of rivers still remain in the 

 barrier corresponding to the valleys, these parts of the shore line having been 

 originally free from coral growth. Although the lagoon deepens as the island 

 subsides, its depth will not be increased by the whole amount of that subsidence, 

 being partially filled up by detrital matter carried down by streams on the one 

 hand, and on the other by the material broken off by the wear and tear of the 

 breakers on the outside of the reef. 



Continue the subsidence further till the highest mountain peak disappears 

 below the waters of the lagoon and an atoll remains, "like a monument, marking 

 the place of the burial," of the island (Fig. 144e). 



In confirmation of his view as to the conditions under which fringing reefs 

 are formed, Darwin pointed out that their distribution is in many cases 

 coincident with lines of recent volcanic activity, with which elevation of the 

 earth's crust is often associated ; as well as with raised shore lines giving direct 

 evidence of elevation. Although not ignoring the possibility of atoll-shaped 

 islands being formed without the aid of subsidence, Darwin regarded this as 



* See "Foundations of Coral Atolls," by Admiral W. J. L. Wharton, F.R.S. 

 Nature 1426, Feb. 25, 1897, p. 390. 



