CORAL REEPS. APPENDIX. 211 



1. Reefs appeal' at the surface as the result of growth at 

 about the sea-level or through upheaval. 



2. The numerous detached submerged coral-shoals, which 

 represent the early condition of reef-structures, are not able to 

 raise themselves to within the constructive power of the break- 

 ers without the aid of a movement of elevation. Being arrested 

 in their upward growth, at depths varying between 5 and 10 

 fathoms, according to the exposed or protected character of 

 their situation, they form flat shoals of no great size. 



3. Atolls of small size, i.e., a mile or so across, do not as- 

 sume their characteristic form until they have reached the 

 surface. A small flat-topped shoal is first brought by up- 

 heaval to or above the sea-level; lateral extensions or wings 

 grow out on either side, so as to ultimately form a horse-shoe 

 reef. Such a reef presents its convexity against the prevailing 

 surface-currents, to which in truth it owes its shape. 



4. The larger atolls have probably assumed their fdrm be- 

 neath the surface, " since, according to the principle laid down 

 by Mr. Murray, they would then have a relatively smaller 

 periphery for the supply of food and sediment to the interior 

 than would be possessed by the small submerged shoals above 

 described." 



5. The true " growing edge " of a reef is the seaward slope 

 which extends outward between the depths of 4^5 and 12-18 

 fathoms; where this submarine slope is more than 10° or 12°, 

 "as is usually the case," the sand and gravel arising from 

 oceanic degradation — which, with a more gentle slope, accumu- 

 lates at its base — is carried far beyond the depths in which reef- 

 corals thrive. In the case of reefs possessing a gradual sea- 

 ward slope, i. e., less than 5°, the lower margin of this band of 

 detritus will lie within the zone of reef-building corals, aid in 

 consequence a line of barrier-reef will be ultimately formed 

 beyond this band with a deep water channel inside. Succes- 

 sive series or belts of barrier-reefs thus formed may be brought 

 to the surface through a progressive rise of the sea-bottom. 



6. Reef-building corals are not restricted to a superficial 



