THE ORIGIN OF CORAL REHES. oF 
viously shown to be but an adaptation of Chamisso’s explana- 
tion of the growth of an atoll to the origin of barrier-reefs, 
A very gradual submarine slope and the presence of sediment 
in suspension in the shore waters are the determining causes 
of the lagoon-channel; the agencies of solution, diminished 
food-supply, organic degradation, and tidal scour, being, as I 
think, auxiliary causes which come into play after the reef 
has begun to grow at that distance from the shore where the 
suitable conditions for reef-growth exist. 
A circumstance, hitherto not satisfactorily explained, is to 
be found in the usual apparent position of a barrier-reef at the 
margin of a submarine plateau, beyond which the slopes 
descend rapidly to great depths. I have employed the 
epithet “‘ apparent’ because I do not desire to commence my 
line of argument by assuming the point at issue. Let us 
examine for a moment the submarine profile of coasts 
bounded by barrier-reefs. I will first take the east end of 
the large island of Bougainville, in the Solomon Group, a 
locality with which I am personally acquainted. Its sub- 
marine profile is that of a submarine ledge, 15 miles in width, 
and possessing a scarcely recognisable slope represented by a 
total drop of about 300 feet, or at the rate of 20. feet per mile: 
at its edge lies a barrier-reef; and beyond, the slope descends 
very rapidly to the 100-fathom line at angles varying 
between 15 and 25 degrees. ‘Take the barrier-reef of Tahiti, 
distant about a mile from the shore and with a depth of on 
the average 20 fathoms inside it, which gives the ledge, at 
the margin of which the reef lies, a gentle slope of somewhat 
over a degree. Beyond the reef, the submarine slope for the 
first 250 yards descends at an angle of 15 or 16 degrees to 
depths of from 30 to 40 fathoms; during the next 100 yards 
the slope is very steep, and sometimes exceeds 45 degrees ; 
between 350 and 500 yards from the reef, the slope has an 
angle of about 30 degrees. Beyond this distance the angle of 
slope decreases, until at about a mile from the reef the angle 
is 6 degrees and the depth 590 fathoms. Such are the results 
of the soundings made by Mr. Murray and Lieutenant Swire 
off the Tahiti barrier-reef. I come now to the line of barrier- 
reef lying at a distance of between 20 and 30 miles off the 
north-west coast of Viti Levu, Fiji. ‘The broad ledge, at the 
border of which the reef lies, has an average drop only of 
from 10 to 15 feet in the mile; it is, therefore, for all practical 
purposes, a level surface. Outside the reef, however, judging 
from the soundings hitherto made in this locality, the sub- 
marine slope descends to depths of between 300 and 
400 fathoms at an angle of about 25 degrees, and the average 
