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atolls, it readily accounts for the existence of the numerous sub- 
merged reefs in the Solomon Group, which lie at a constant distance 
below the surface. Another circumstance favouring the view that 
detached sunken reefs cannot raise themselves to within the construc- 
tive power of the breakers is to be found in the fact that on the 
weather sides of reefs in this group corals do not thrive in the wash 
of the breakers ; there is here a gradual slope descending to a depth 
of 4 or 5 fathoms, which is largely bare of living coral {vide p. 858). 
The formation of the reef-flat is worthy of a passing remark. I have 
often observed that on the lee sides of islands, especially of those of 
small size, the corals grew in irregular patches at the sea-border, and 
did not form a reef-flat such as existed on the weather side which 
was exposed to the trade-swell. Professor Semper would apparently 
attribute the absence of the reef-flat on the lee side of an island to 
the circumstance of the corals not being exposed to strong impinging 
currents ; but in these islands, however, it is evidently due to the 
corals not being subjected to the action of the breakers. 
I have now come to the description of the reefs which have 
reached the surface. The three principal classes are to be found in 
this region, but of these the fringing and barrier-reefs are more 
commonly distributed, whilst the atolls are comparatively few in 
number and of small size. A line of barrier-reef, probably not 
much under 60 miles in length, and bearing innumerable islets on 
its surface, fronts the east coasts of the islands of New Georgia at a 
distance of from 1 to 3 miles from the shore. Extensive reefs of 
the same class, having a broad deep-water channel inside them, lie 
off the north side of the large island of Isabel, and off the south 
coast of Choiseul. Similar reefs of smaller extent skirt the west 
end of Guadalcanar ; and existing and elevated barrier-reefs occur 
in Bougainville Strait. Of this class of reef I was only able to 
examine those of the last locality : they will be subsequently referred 
to in my general description of the reefs of this strait. 
I will now proceed with the description of the reefs of the 
islands we visited.* 
St Christoval . — My observations were for the most part restricted 
to the north side of this large and mountainous island. The reefs 
* For the description of the elevated reefs and their foundations, I must 
refer the reader to my paper in the Trans. Edin. Roy. Soc., vol. xxxii. part 3. 
