AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 119 



row Avails to separate them (Plate 30) ; and finally, near the outermost 

 edge begins the belt of Madrepores and Pocillopores (Plates 28, 29) 

 which extends into the line of breakers and beyond. There are few of 

 the massive type of corals, like Porites, Astraea, and the like, and these 

 are of diminutive size. 



The sui'face of the reef is kept from too rapid wearing by the growth 

 of marine algee and of corallines which carpet it. The algee are mainly 

 species of Udotea, of Caulerpa, and of Turbinaria, which in sheltered 

 pools grow to a considerable size. One finds also ou the surface an occa- 

 sional large black Holothurian, a blue Linckia, a green Goniaster, a Mu- 

 reena darting from a pool, a large Cancer trying to hide under a shelving 

 piece of coral, or a Squilla, or a few specimens of small fish of a brilliant 

 blue color. There are few Mollusks under the negro-heads and dead 

 masses of coral scattered upon the reef ; the lower surface of these is 

 often carpeted with many species of brilliantly colored Sponges, in which 

 small Mollusks, Crustacea, and Annelids find refuge. The coral masses 

 themselves are perforated in all directions by boring Mollusks and 

 Annelids. 



The substratum of the eastern and the western reef is made up of 

 elevated limestone, extending from the shore close to the outer edge of 

 the reef, independently of the thin crust of corals which grows upon the 

 outer edge of the reef flat. So it is also with the mushroom-shaped heads 

 which are found upon the outer edge of the reef flats of Vatu Leile, Mango, 

 and other islands consisting wholly or in part of older limestones. They 

 are all composed of the same rock as that of the shores and of the sub- 

 structure. The outer sea face of the reef flats is steep in all cases, irre- 

 spective of the increase due to the corals growing upon them, or of the 

 character of the underlyiug rock, which is only an unimportant factor 

 in determining the thickness of the sea face edge of the outer reef 

 flats. 



We also examined the outer face of the barrier reef forming the east- 

 ern edge of the passage leading into Suva Harbor. This reef flat, like 

 that of the western side of the passage, is entirely made up of ancient 

 elevated limestone planed down by the sea to a level flat, with small shal- 

 low pools scattered over the surface. Negro-heads and fragments of the 

 ancient limestone are scattered over considerable areas on the reef flat ; 

 many of them have been thi'own up by the sea from the undermining of 

 the outer edge, others have been torn off from the reef flat itself, and both 

 are gradually wearing away, leaving fragments of branches of corals and 

 of coral heads scattered over the reef. The belt of the reef flat close to 



