AGASSIZ: FIJI ISLANDS AND CORAL EEEFS. 143 



cessantly, and the hydraulic head obtained is timply sufficient to account 

 for the scouring of the lagoons after the I'eef has once established it- 

 self as a bank, and amply sufficient to wear away from the slope of 

 the islands the platform upon which the coral reef is built. The to- 

 pography of this platform is naturally much varied, depending upon the 

 character of the shore line, the direction of the valleys of the shore 

 hills, and their composition. A glance at the charts accompanying this 

 Bulletin will show all possible conditions of submarine erosion in the 

 cutting down of the submarine platforms of the islands of Fiji, and 

 in the manner in which islands, islets, and rocks have been left, attesting 

 their former greater extension in the various clusters of the Archipelago. 



When the principal openings are not on the lee side of the lagoons, as 

 is the case with Vanvia Mbalavu (Plate 19), and the Argo Reef or Totoya 

 (Plate 23), Fulauga (Plate 22), and a few others, there is usually a sim- 

 ple reason, such as the lower elevation of the island once covering the 

 area of the lagoon at some point not on the lee side, or the fact that the 

 lagoon has been formed on a steep volcanic slope looking eastward or 

 northward, so that deep ravines or tongues of deep water cut into tlie 

 lagoons, and intercept the coral patches forming its rim on the weather 

 side, and thus leave a windward passage. It is by some such orogenic 

 condition that we must explain the existence of deep soundings within 

 atolls, — soundings which in no way indicate a subsidence, as has been 

 assumed by Darwin, and which according to him were not to be ex- 

 plained by any other hypothesis. Such deep ravines are of coui'se also 

 to be traced on the slopes of the larger islands where we find, crossing the 

 shallow plateaus on which coral patches grow, valleys of considerable 

 depth, which appear as deep soundings within the area of an outer reef 

 flat such as in the great plateau off Viti Levu and Yanua Levu (Plates 

 3% 4), or of Kandavu (Plates 10, 11) and Taviuni (Plate 4), which 

 according to Darwin would indicate a subsidence, while, on the con- 

 trary, they are a part of the results of the elevation and lifting up of 

 that region of Fiji. 



Xor are the great depths found close to narrow lines of corals an 

 indication that the corals have grown up as a nearly vertical wall from a 

 depth of two to three hundred fathoms or more. It merely indicates 

 tliat the corals foi'm a thin crust, at most 120 feet in thickness, over a 

 sharp volcanic ridge, the summits or crest of which have either reached 

 by elevation the depths at which corals can grow, or have been denuded 

 by submarine erosion to form a platform below the level of the sea, 

 where corals have found a footing upon them. 



