86 BULLETIN : MUSEUM OF COMPAKATIVE 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 fovuid 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 stud}' 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 obtahi 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, but 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 seoired 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 circu- 

 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. AVe 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. We also know that on the sea face of the outer 



