of Edinburgh, Session 1885-86. 
901 
which reef-corals thrive ; but that in the case of reefs possessing a 
gradual seaward slope, i.e., less than 5°, the lower margin of this 
band of detritus will lie within the zone of reef-building corals, 
and in consequence a line of barrier-reef will be ultimately formed 
beyond this band with a deep water channel inside. 
6. 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 growing upward along the lower 
margin of the band of detritus derived from the line of reef inside 
it. In such a manner have the Shortland Islands been formed. 
7. That the forgoing explanation of the formation of barrier-reefs 
in these islands is substantially the same as that proposed thirty 
years ago by Professor Joseph Le Conte in the instance of the 
Florida reefs. He then pointed out that since corals will not grow 
on muddy shores or in water upon the bottom of which sediment is 
collected, the favourable conditions can only be obtained at some 
distance from the shore, where a barrier-reef would ultimately be 
formed limited on one side by the muddiness and on the other by the 
depth of the water. 
Since I was not acquainted with Professor Le Conte’s view, my 
explanation has all the more value. The independent agreement of 
the two views is itself a third argument in favour of this mode of 
formation of barrier-reefs, which may now be briefly stated. 
These reefs will only grow on gradual submarine slopes, i.e., on 
those less than 5° ; and since corals cannot thrive in the shallower 
depths on account of the accumulation of sand and the presence of 
sediment in the water, a reef can only be formed in the depths at 
some distance from the shore where these unfavourable conditions 
do not exist. If such depths are within the reef-coral zone, then a 
barrier-reef will be produced. The foregoing conditions may be 
described as the determining causes of a barrier-reef. After the reef 
has been formed, other agencies, such as solution, organic degradation, 
and the scouring action of tidal currents, will keep the lagoon- 
channel open. 
The circumstance that barrier-reefs often exist at the margin 
of a submarine plateau, beyond which the slopes descend rapidly 
to great depths, has hitherto not been satisfactorily explained. I 
