of Edinburgh , Session 1885 — 86 . 
867 
reefs ( vide p. 877. That they have remained at much the same 
depth from the surface for a considerable number of years, is shown 
by the circumstance that in 1768 a shoal covered by 5 fathoms was 
found near the middle of the strait by the “ Etoile,” one of Bougain- 
ville’s vessels.* During the 116 years that have elapsed between 
the visit of the French navigator and the recent survey of this strait 
by Lieut. Oldham, neither the upward growth of the coral nor any 
movement of elevation have been sufficient to raise the surface of 
this shoal within the limit of constructive breaker-action ; and in 
consequence it is still submerged. 
It is certainly a remarkable circumstance that in this group we 
found no detached sunken reefs which would render the navigation 
of these seas dangerous for a vessel of light draught (9 or 10 feet). 
All detached reefs that had not reached the surface were below the 
limit of constructive breaker-action, their depth varying between 4 
and 10 fathoms, according to the sheltered or exposed character of 
their situation. It therefore appeared to me that a movement of 
elevation was first necessary to bring these submerged reefs within 
the power of the breakers, since the upward growth of the corals 
was arrested a little below their limit of constructive action. 
We may thus find a partial explanation of the submerged condi- 
tion of the leeward sides of many atolls, and of the total submergence 
of a few other reefs belonging to the same class, in the inability of 
detached submerged reefs to raise themselves within the constructive 
power of the breakers without the assistance of a movement of 
elevation. It may appear somewhat bold to suggest that atolls owe 
their appearance at the surface to such a movement; but it is a 
singular circumstance that I found this to be the case with Oima 
Atoll ( vide p. 879) ; and thisis the very movement which Professor 
Dana and Mr Couthouy have shown to be in operation amongst the 
numerous atolls of the Low Archipelago, many of which have 
experienced elevations varying between 2 or 3 and 250 feet.t In 
the Fiji and Pelew Groups, upraised reefs having the characters of 
atolls coexist with reefs of the same class that have been formed 
at the present sea-level. However this suggestion may apply to 
* Voyage autour du Monde , 2nd edit., Paris, 1772, tome ii. p. 183. 
t Dana’s Corals and Coral Islands, and Couthouy’s Remarks on Coral 
Formation, &c. 
