958 FORiMATION OF BARRIER REEFS IN BOUGAINVILLE STRAITS, 



according to the observations of Professor A. Agassiz, 

 the southern extremity of Florida is growing westward, 

 but with this distinction, that in Florida the area seems 

 to have remained stationary for a long period, the lagoon 

 channels between the concentric lines of i-eef being 

 merely silted up into dry land. 



(4). That the calcareous detritus, which covers the outer 

 slopes of reefs in this group in depths usually of twenty 

 fathoms and beyond, is probably a band dividing the 

 zone of reef-building corals into two sub-zones where the 

 slope is gradual, but where the slope is of a more rapid 

 character extending far beyond the coral zone. 



(5). That in the case of reefs which possess such a gradual 

 slope that the lower margin of this band of detritus lies 

 within the zone of reef building corals, a line of barrier- 

 reef will be ultimately formed beyond this band with a 

 deep channel inside : but that in the case of reefs, which 

 possess a more x'apid submarine slope so that the lower 

 limit of the band of detritus extends far beyond the 

 depths in which the reef corals thrive, no such line of 

 barrier-reef will be formed, 



(G). That where the area is undergoing elevation, a succession 

 of concentric lines of barrier-reefs would thus originate, 

 line after line being advanced, as fresh portions of the 

 sea bottom are brought towards the surface, each line 

 gi^owing upward along the lower margin of the belt of 

 detritus derived from the line of reef inside it. 



(7). That inasmuch as my observations go to show that the 

 elevated reefs in this group repose on a partially consoli- 

 dated calcareous ooze which is not found in depths under 

 fifty fathoms on the outer slopes of the present reefs, it 

 is probable that coral reefs may commence to build in 

 depths greater than those usually assigned. 



(8). That on such a view may be readily explained the 

 circumstance, that the depths of the lagoons inside 

 barrier-reefs and atolls so frequently exceed the depths 

 in Avhich reef corals are stated to thrive. 



