86 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
examination of the outer edge of a coral reef formed upon a substratum of 
volcanic rocks, has given us in Fiji any evidence of the great thickness 
of a modern coral reef. On the contrary, all the evidence I have gath- 
ered there tends to prove that a coral reef forms only a comparatively 
thin crust upon the platform of submarine erosion, whatever be its geo- 
logical structure, upon which it may have found a footing, — a crust of 
modern corals of no greater thickness than that within the bathymetrical 
range of which reef building corals can flourish. 
The examination of the conditions under which beds of coralliferous 
limestone of great thickness have been laid down has little in com- 
mon with the study of the conditions under which a modern reef is formed. 
The study of the conditions under which a modern reef—a reef of the 
present period — is formed and increases in thickness has yet to be made, 
and such a reef must be carefully selected. Probably a broad fringing 
reef resting upon a substratum readily distinguished from the reef rock, 
and one on which there existed a sand key close to the outer edge for 
boring until the substratum was encountered, would be the most suitable. 
But as it would be most difficult to judge from the core obtained at that 
or any other point where the growing reef ceased and the talus began to 
add to the thickness of the reef, it would be necessary to supplement 
this work with soundings taken close together, as well as with an examina- 
tion of the bottom by dredging, either with large or small dredges or 
with trawls and tangles ‘to obtain material in as many different ways as 
possible. Of course we can hardly expect to bring up in this way many 
specimens of reef building corals, hut we can surely, by combining the 
information obtained from the nature of the soundings, the character 
of the bottom as shown in the larger samples secured by the dredge, 
and the occasional fragments of living corals we are sure to bring 
up, form some idea of the exact bathymetrical range to which reef 
building corals extend, and this is not so difficult a problem as is sup- 
posed by Professor Sollas. We should in determining this remember 
that in the interior of large lagoons in which there is an abundant cireu- 
lation of pure sea water close to the inner edge of the outer reef flats we 
have very positive information as to the lower limit to which corals grow 
in abundance. We know that under these conditions the most flourish- 
ing belt of reef builders grows in depths of from three to eight fathoms, and 
beyond that the coral patches decrease very rapidly in number and size, 
that the patches are with increasing depth separated by wider lanes or 
greater areas of coral and coralline sand, or by masses of fragments of 
corals and of dead corals. Wealso know that on the sea face of the outer 
