14 A NATURALIST IN THE PACIFIC CHAP. 



upheaval or emergence of the sea-border. It is not always easy 

 to see why there should be this fine adjustment between the 

 rapidly-growing mangrove and the slowly-growing reef. Under 

 normal conditions, however, that is to say, when the land is 

 stationary or when the change of level is of a very gradual nature, 

 the reclaiming agency of the mangrove receives a check, and this 

 relation between the mangrove-belt and the outer reef-flat is 

 maintained. 



Actual acquaintance with such localities soon forced me to the 

 conclusion that whilst a gradual emergence or upheaval of 3 or 

 4 feet in a century would not materially affect the relation between 

 the mangrove-belt and the reef-flat, a sudden or rapid change of 

 level of that amount would destroy the mangroves around the 

 whole island. There is some evidence, however, of there having 

 been a rapid upheaval of this kind in different parts of the coast : 

 and it follows, therefore, if this movement was general, that the 

 present mangrove-belts date only from the last upheaval. But this 

 elevation may have occurred ages ago ; and the equilibrium 

 between mangrove-belt and reef-flat may have been long since 

 established. Accordingly, the breadth of the mangrove-belt can 

 afford no indication of the period that has since elapsed. From 

 data referred to below, it is evident that the mangrove-belt, taking 

 its average width, away from the estuaries, at about 500 yards, 

 might have been formed in two or three centuries, whilst a thousand 

 years or more may have passed since it assumed its present 

 relation to the reef-flat. If, therefore, upheaval is in progress, it 

 must be of a very gradual character, since the normal relation 

 of mangrove-belt to reef-flat now prevails. 



There are indeed signs of such a gradual movement ot emergence 

 or of elevation being in operation on the north coast of Vanua 

 Levu at the present time. I have before referred (page 1 1 ) to the 

 extensive bare mud-flats in the midst of the mangrove-belt between 

 Nanduri and Lambasa, which are well represented on the Tambia 

 coast and in Nanduri Bay. They are only covered by the higher 

 tides, and in the intervals their surfaces are dried and cracked by 

 exposure to the sun. Here we have the central decay and the 

 marginal growth which would be expected in a mangrove tract 

 situated in a gradually rising area. 



An indirect indication of such a slow upheaval on the north 

 coast is to be found in the circumstance that the great submarine 

 platform, which reaches seaward to the line of barrier-reefs, 15 to 

 20 miles away, passes gradually, as it extends landward, into the 



