of Edinburgh , Session 1885-86. 
887 
the lagoon channels should never be deeper than the zone in which 
reef-corals flourish ; but, as a matter of fact, the depths inside 
barrier-reefs as well as atolls not unfrequently exceed those in which 
reef-corals are believed to thrive. For instance, to take the case of 
barrier-reefs, with which we are at present concerned, depths of from 
40 to 50 fathoms occur inside the line of barrier-reef that skirts the 
eastern end of Bougainville Island in the Solomon Group. Sound- 
ings of between 50 and 60 fathoms have been obtained inside the 
Yanikoro reef in the Santa Cruz Group. Depths of 40 fathoms are 
found within the barrier-reef lying west of the Fiji Islands, and 
inside the great Australian barrier-reef the depth increases to 60 
fathoms. It is the depths of from 35 to 60 fathoms which are 
found occasionally within both barrier-reefs and atolls that lend 
the greatest support to the theory of subsidence. However I have 
found a simpler explanation of the difficulty. 
In the Solomon Group the depth of the reef-coral zone on the 
outer slopes of reefs varies greately in different localities. In most 
plases its lower limit is 12 to 15 fathoms ; in others again it is from 
20 to 25 fathoms ; whilst off the reef of Choiseul Bay I did not 
seem to have reached this lower limit in soundings of 40 fathoms.* 
This variation I found to depend upon the disposition of the 
sand and detritus on the seaward face of the reef, and this was 
itself determined by the degree of inclination of the slope and by 
the presence and position of submarine declivities. In the pro- 
tected waters of lagoons and lagoon-channels, the main determin- 
ing condition is to be sought in the degree of clearness of the 
water. The estimates of observers in other regions have been 
equally varied. MM. Quoy and Gaimard placed the lower limit 
of the zone in the Pacific at 5 or 6 fathoms. Ehrenberg formed 
a similar estimate with reference to the Bed Sea corals. In 
the Florida Seas, Professor A. Agassiz has ascertained it to be less 
than 10 and usually not more than 6 or 7 fathoms. Professor Dane 
would place it at 20 fathoms in the Pacific. Mr Darwin’s limit is 
20 to 30 fathoms for all reef-regions. In the Red Sea, again, large 
beds of living coral were found in depths of 25 fathoms by Captain 
* These depths and other particulars are to be found in my paper in the 
Ann. & Mag. Nat. Hist., June 1884. More recent data have led me to change 
some of the views there expressed. 
