BY H. B, GUPPY, M.B., SURGEON, R.N. 955 



elevated barrier-reefs on its weather coasts. Such a process is in 

 actual operation at the present time in the passages, the encroach- 

 ment of the mangrove on either side and the upward growth of 

 coral in the channels being the agencies at present effecting this 

 operation. These remarks may be made more clear by a reference 

 to the section of the Shoi'tland Islands. 



It follows from this view of the formation of barrier-reefs in 

 this region that the lagoon channels inside the reefs should never 

 be deeper than the zone in which reef building corals are stated to 

 thrive, a depth from which my soundings in different parts of the 

 Solomon Group I place at fifteen fathoms, but which has been 

 variously estimated in other parts of the world, where coral reefs 

 occur, at from ten to thirty fathoms. The passages inside the 

 reefs of the Shortland Islands and Choiseul Bay, comply with this 

 condition. Depths however of forty to fifty fathoms occur, as 

 stated in the commencement of this paper, inside the line of 

 barrier reef that skirts the eastern extremity of Bougainville. 

 Similar depths are not uncommon in the lagoon channels of barriei'- 

 reefs in other regions of the Pacific ; and thus this view of the 

 formation of barrier-reefs apparently breaks down. There, 

 however, appears to be no " a priori " reason why reef-building 

 corals should not thrive beyond the belt' of calcareous sand and 

 gravel that apparently marks the limit of their zone, and therefore 

 in depths greater than those which are usually accepted as 

 favouring the growth of reefs. Soundings off the outer edge of 

 barrier-reefs have rarely been extended (in the Pacific at least) 

 much beyond fifty fathoms, the presence of the sand and gravel, 

 which I hold to be merely gathered together into a belt, having 

 been considered as marking the lower limit of the reef coral zone. 

 I refer not to the soundings taken in a nautical survey which fail 

 to particularize the nature of the bottom with sufficient accuracy, 

 but to such lines of soundings as are taken by observers with a 

 specific object before them. 



My observations on the recently elevated calcai-eous formations 

 of this group enable me to approach this subject by another road ; 

 and in passing f)-om the consideration of a probable cause of the 



