952 FORMATIOX OF BARRIER REEFS IS BOUGAINVILLE STRAITS, 



The profile, which I have appended, of the seaward slope of this 

 barrier-reef, has been drawn on a true scale, partly from my own 

 soundings which extended to forty fathoms, and partly from those 

 made by the officers of the survey as far as the " hundred fathom 

 line." As shewn in this section, the submai'ine portion of the reef 

 at first slopes gradually to a depth of four or five fathoms, when it 

 plunges down by a steep declivity another nine or ten fathoms, 

 from the foot of which there is a less precipitous talus-like slo})e to 

 a depth of about twenty fathoms from the surface. Beyond, there 

 extends a broad ledge covered by from 23 to 25 fathoms of water 

 which terminates in another rapid slope to a depth of a hundred 

 fathoms, which is the limit of the section. Living corals flourish 

 on the upper part of the submarine slope down to the cliff or 

 declivity above referred to. In depths of fifteen to twenty fathoms 

 at the foot of this submarine clifi' there appeared to be very little 

 living coral, since out of twelve casts in these depths the armings 

 brought up calcareous sand and gravel on eight occasions. Carry- 

 ing the soundirigs further seaward on the broad ledge previously 

 described, I found that the armings of my lead gave much less 

 frequent indications of the occurrence of sand and gravel, whilst the 

 greater proportion of the casts shewed the presence of living coral. 

 Out of eleven soundings in depths between 23 and 40 fathoms, 

 seven shewed a perfectly clean indentation on the arming as of 

 living coral ; but on account of the swaying movement of the lead 

 only two of these impressions were recognisable : from a depth of 

 23 fathoms the prints of the contiguous stars of an Astreea were 

 preserved ; and in a subsequent cast of 31 fathoms the impressions 

 of a rounded knob of a Porites with its characteristic small cells wei-e 

 similarly displayed. A reference to the section in profile of this 

 reef will explain this distribution of the detritus and of the living 

 coral. 



Sand and gravel, derived from the constant action of the rollers 

 breaking on the edge of the I'eef-flat, would naturally tend to collect 

 at the foot of the first declivity in depths of fifteen to twenty 

 fathoms ; in such a situation living coral would be scarcely 

 expected to thrive ; but in the more level region beyond, as the 



