i8 A NATURALIST IN THE PACIFIC chap. 



so that the advance of the mangroves would be for a time 

 unrestricted. 



It is, therefore, apparent that the rate of growth of one of 

 these low islands is not to be determined by the rate of growth of 

 the mangrove-tract occupying the surface. The subject is a com- 

 plicated one ; but I think enough has been said to show that the 

 destructive agencies do not prevail on this great submarine platform 

 on the north coast of Vanua Levu. 



If the data here adduced of the increase of the low islands, of 

 the shoaling of the channels, and of the advance of the delta of 

 the Lekutu river,^ are well founded, all the islands, islets, and reef- 

 patches that lie along this north coast will be united to each other 

 and to the main island within a thousand years. 



The facts here produced do not directly indicate a movement of 

 upheaval but they are quite consistent with the conclusion that 

 the great movement of elevation which has built up Vanua Levu 

 by the union of several smaller islands is still in operation at its 

 coasts. To assume that there is now in progress at the sea-border 

 the same process of island-building which has produced Vanua 

 Levu, as we now see it, is to assume a uniformity in nature's 

 methods which is disregarded by the hypothesis that the great 

 submarine platform, from which the large islands of Viti Levu and 

 Vanua Levu now arise, represents the work of marine erosion into 

 the flanks of the upheaved islands since the last elevation. The 

 origin of this submarine platform is dealt with in Chapter XXVII. 

 Here it may be remarked that 1 regard it as older than the islands 

 that rise from it. 



However, this movement of upheaval is so gradual that the 

 utmost one can expect to do by the comparison of surveys made 

 half a century apart is to show the lack of evidence of the 

 destructive agency of erosion. As far as the comparison admits 

 of judging, there seems to have been no important change on the 

 coasts of the western end of the island during this period. The 

 low neck of land connecting Naivaka with the main island, if we 

 take the low-water line in the Admiralty chart as the limit, had 

 much the same breadth at the time of both surveys. The depths 

 in Mbua Bay remain about the same, with perhaps a shoaling of 

 less than a fathom in places. There are two cays awash in the 



* By referring to the chart it will be seen that extensive mud-flats occur at 

 the mouths of the Sarawanga and Ndreketi rivers, where the land-margin is 

 slowly advancing. 



