BARRIER REEFS. 
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fathoms, with a submarine slope, varying from 12° to 2G°. 
The erosion line on the volcanic rocks indicates an up- 
heaval of some four or five feet, prior to the coral growth. 
The land is bordered by extensive fiats, covered by less 
than a fathom of water, on which the coral appears to be 
dead, and two basins or lagoons occur within them, about 
20 fathoms deep. 
Sections are given of some barrier reefs. As the result 
of his investigations of these, Mr. Guppy concludes that, 
from the edge of the reef flat for the first 70 or 80 yards, 
there is usually a gradual slope to a depth of from 4 to 5 
fathoms. On this but little living coral is found. Beyond 
it, there is generally a rapid descent to a depth ranging 
between 12 and 18 fathoms, on the face of which the corals 
flourish: ‘this is in fact the growing edge of the reef.’ 
Below this descent sand and gravel, produced by the action 
of the breakers at the margin of the reef, collect at a depth 
generally of from 15 to 20 fathoms, though sometimes this 
occurs at greater depths. One section, that of Santa Anna 
Island, exhibits two submarine cliffs, — the one, after a 
rapid slope, occurs between the depths of 1G and 32 
fathoms ; the other, after reaching a depth of about 42 
fathoms, gives a drop of 25 fathoms, after which, a slope at 
an angle of 18° or 19° descends to considerable depths ; 
corals thrive in this case at a depth of 30 fathoms. 
Another section exhibits a second but less strongly 
marked drop at about 25 fathoms. 
In an explanation of the formation of barrier reefs, 
which Mr. Guppy regards as produced successively while 
the ground is uprising, it is admitted that on this hypo- 
thesis, lagoon channels should never be deeper than the 
limit at which reef-building corals can grow. ‘ But as a 
matter of fact the depths inside barrier reefs as well as 
atolls, not unfrequently exceed ’ 25 fathoms — in corrobora- 
tion of which statement several cases of soundings in these 
