95G FORMATION OF BARRIER REEFS IN BOUGAINVILLE STRAITS, 



origin of barrier-reefs to the study of such reefs when upraised, with 

 their foundations above the sea, I at once enter a domain of greater 

 certainty. These investigations have shown that coral-reefs are 

 based usually on a partially consolidated calcai'eous ooze, often 

 foraminifei'ous, generally abounding with recent shells, and now 

 and then laden with pteropod-shells in considerable numbers, the 

 thickness of the overlying coral-rock rarely exceeding a hundred 

 feet. That the reef-corals commence to grow on such a bottom, and 

 not on a layer of detritus of sand and gravel, is shewn by the 

 fact of my finding at Santa Anna two massive corals of the 

 Astrseidae, the largest four feet in diameter, imbedded in the 

 position of growth, at a height of forty feet above the sea, in the 

 base of a coral-limestone cliif where they almost rested on the 

 subjacent partially consolidated ooze. It is a noteworthy circum- 

 stance that in my numerous soundings off the outer edge of reefs in 

 this group. I.e., extending to fifty fathoms, the armings never 

 brought up any other indication of the nature of the bottom, 

 outside the usually accepted coral-zone, than that of calcareous sand 

 and gravel. In truth my soundings down to depths of fifty 

 fathoms failed to reach the ooze. It would therefore appear that 

 such reefs as those of the Shortland Island commenced to build in 

 depths greater than fifty fathoms. If elevation had brought the 

 ooze within these depths uncovered by the calcareous detritus, the 

 armings would probably have recorded such an occurrence amongst 

 some of my numerous soundings. The following question then 

 .seems pertinent to the subject in hand. How is it that since 

 coral-reefs base their foundations on calcareous ooze, it is necessary 

 to go far beyond the depths in which reef-corals are usually stated 

 to thrive to reach the ooze. The reply to such a query may furnish 

 a more satisfactory explanation of the depths of forty and even 

 sixty fathoms, which have been found in the lagoon channels of 

 barrier-reefs and in the lagoons of atolls, than those which have 

 been hitherto advanced. Mr. Darwin admitted that an objection 

 to his theory of subsidence might be found in " the circumstance 

 of the lagooQS within atolls and within barrief-reefs never having 

 become in any one instance during prolonged subsidences of a 



