1898.] Funafuti, Rotuma and Fiji. 477 



Of the structure of the surface of a reef Dana gives the follow- 

 ing account : — " The reef rock, when broken, shows commonly its 

 detritus origin. Parts are of compact, homogeneous texture, a 

 solid white limestone without a piece of coral distinguishable and 

 scarcely an imbedded shell. But generally the rock is a breccia, 

 or conglomerate, made up of corals cemented into a compact 

 mass, and the fragments of which it consists are sometimes many 

 cubic feet in size 1 ." 



The last sentence certainly will not apply to the reefs of Fiji. 

 The parts of "compact homogeneous texture " are very numerous, 

 and are formed, I believe, mainly of the carbonate of lime secreted 

 by incrusting nullipores. The importance of the incrusting nulli- 

 pores, in the formation of the reefs of the Central Pacific, cannot 

 be overestimated. The greater part of the rim of the reef and its 

 outer slopes is covered by these alga?. A coral dying is at once 

 grown over by them ; sand grains and even loose boulders are 

 enclosed, and fissures as well as small pits are bridged. These 

 incrusting nullipores grow close to the surface they are on, and 

 spread gradually outwards, fixing themselves as they grow, so that 

 they form a very strong bulwark against the sea. At the bottom 

 of the sea, outside the reef in 7 — 15 fathoms, they often form on 

 the tops of the ridges large nodular masses round fragments of 

 coral and rock ; the smaller nodules (which alone could be loosened 

 by the use of Priestman's grab) showed thicknesses of upwards of 

 4 inches of compact white homogeneous limestone secreted by 

 them, and I have no doubt that by their agency the greater part 

 of these large masses was formed. 



The incrusting nullipores of the reef belong to the genus 

 Lithothamnion, which Walther has shown covers and has ap- 

 parently formed the greater part of certain shoals in the Bay of 

 Naples 2 . 



Walther afterwards investigated the tertiary "Nulliporen- 

 kalk " of Syracuse, and found all stages from well-preserved 

 specimens of Lithothamnion to a structureless limestone. Of the 

 lias of " Todten Gebirges " it is remarked that " the strata consist 

 of coral rock, detrital calcareous deposits, and associated with 

 these, masses of limestone, in which microscopic examination fails 

 to detect either vegetable or animal structure. These structure- 

 less beds are considered to have been Lithothamnion banks from 

 which percolating water has removed all traces of algal cells 3 ." 



The outer reefs of Fiji, Rotuma, and Funafuti seem as if they 



1 Loc. cit. p. 175. 



2 Die Gesteinbildenden Kalkalgen des Golfes von Neapel, etc. Zeitsch. deutsch. 

 Geol. Ges. xxxvn. 1885, p. 329. 



3 Algsa as Kock-building Organisms. A. C. Seward. Sci. Progress, Vol. n. 

 1894, p. 19. 



VOL. IX. PT. VIII. 38 



