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AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 143 
cessantly, and the hydraulic head obtained is amply sufficient to account 
for the scouring of the lagoons after the reef 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 Vanua Mbalavu (Plate 19), and the Argo Reef or Totoya 
(Plate 23), Fulanga (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 the 
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 course also 
to be traced on the slopes of the larger islands where we fiud, 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 Vanua 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. 
Nor 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 
that the corals form 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. 
