150 LEISURE-TIME STUDIES. 



depth, must of necessity die ; a new sphere of operation 

 being at the same time afforded by the subsiding operation 

 for the uppermost corals. These latter will therefore con- 

 tinue to produce new polypes, and an upward growth of the 

 coral will accordingly accompany the downward movement 

 of the land. If the land-subsidence continues, the increase 

 of the sea-wall or outer aspect of the reef will be greater 

 than that of its shore side or inward portion, seeing that on 

 the former surface the conditions of life are more suitable 

 for the growth of the massive reef-building corals. The 

 inner part or shore aspect of what was once the fringing reef 

 thus becomes deeper and deeper as subsidence proceeds, 

 and in due time we find a great coral-ridge growing up in 

 front of the sinking land, and separated therefrom by a belt 

 of deep water. In this way, the barrier reef is evolved by 

 the subsidence of the fringing reef. But the land may be 

 depressed to a still greater extent, and as before, the upward 

 coral-growth will keep pace with the subsidence. If we 

 suppose that we are dealing with the case of an island or 

 with land of limited extent, we may conceive that in time 

 the last island peak or surface of original land will sink 

 beneath the waves. The coral-growth has, however, been 

 proceeding uninterruptedly as before; and the lost land 

 becomes ultimately surrounded by a great wall or cup of 

 coral, enclosing a quiet lake, the atoll or lagoon of the Pacific 

 voyager. 



The formation of coral-reefs may be readily understood 

 from an inspection of the appended theoretical figures (Figs. 

 15 and 1 6), by the late Mr. Jukes. The sea-levels are repre- 

 sented at s s ; and in the first figure A B and c D are two shores 

 sloping seawards at different inclinations or angles. On 

 these shores corals grow at their own limited depth, repre- 

 sented by the line F F, and thus form a fringing reef (R E G). 

 The greater growth of the reef at E G, that is, furthest from 

 the land, may produce a sheet of water between R and E ; 

 and the breadth of the reef is seen to be greatest where the 

 sea-bed slopes least abruptly. If the shores (A B and c D) 



