in the Pacific, ^c. 81 



suppose the face of the mountain to have presented an angle 

 with the horizon of say SO*^, it is evident that for every 

 foot of subsidence there would have happened three feet 

 of recession from the reef's original Hmit. By the time 

 it had sunk two thousand feet, allowing as above, the lateral 

 and upward growth of the reef to be equal in rate, and that 

 rate sufficient to maintain it at its primary level, a channel 

 would thus be formed four thousand feet in width, between 

 reef and shore. The steeper the mountain, the narrower 

 would be the lagoon formed by the same amount of subsi- 

 dence ; and the reverse. 



Now this difference is precisely that which is really exhib- 

 ited by the encircling and barrier reefs, according to the na- 

 ture of their coasts. In the abrupt and lofty volcanic islands 

 of Polynesia, the lagoons seldom exceed three fourths of a 

 mile or a mile in breadth, while on the gently ascending 

 coast of New Holland, the reef is in some places fifty miles 

 from shore. 



I shall notice at present but one more feature in these 

 lagoons, which is their small depth, in comparison with what 

 the assumed subsidence would at the first glance lead us to 

 expect. But the wash from the beach, which in every in- 

 stance under my observation, spite of the protection aiforded 

 by an outlying reef, was very considerable ; and the detritrus 

 of the reef itself, together with the alluvium deposited by 

 streams, would be sufficient to raise the bed of the lagoon 

 very materially. If in addition to this we suppose, what may 

 well have been the case, that there were intervals of time 

 during which the land was stationary, while these causes con- 

 tinued in full operation, or that any considerable time has 

 elapsed since a cessation of the subsidence, there is no longer 

 any difficulty in accounting for this comparative shallowness 

 of the lagoons. 



As the general character and aspect of the low coral islands 

 is not very clearly understood by some of our number, I 

 may be pardoned for venturing to occupy a few moments in a 

 hasty sketch of their structure. This seems the more called 

 for, because in the recent course of Lectures delivered for the 



11 



