II EVIDENCE OF RECENT EMERGENCE 17 



this period. The difference between Thukini in 1840 and in 1895 

 is very noticeable. In the time of Wilkes the mangroves only 

 occupied about one-third of the reef-patch. Now they occupy 

 about two-thirds, the area of the reef-patch remaining much about 

 the same. Taking the minus and plus values of all the islands 

 here measured, the average rate of the advance of the mangrove- 

 margins during this half-century may be placed at about 250 yards 

 in the case of these reef-islands, which would amount to a mile in 

 400 years. 



It is probable that a long island like Ndongo, which is about 

 four miles in length, has been formed by the union of smaller man- 

 grove islands. Therefore, taking half its maximum breadth of a 

 mile as a guide, it would at this average rate of growth require 

 two centuries for its formation. But since the extension of the 

 mangroves depends on the growth of the reef-patch, which takes 

 place on the average at a much slower rate, it follows that this 

 can only be a minimum limit for the age of this island. We can 

 only assume that if the reef-patch had suddenly appeared 200 

 years ago, Ndongo Island could by this time have acquired its 

 present dimensions. It does not follow that the mangtove border 

 has been continuously advancing. A hundred years ago there 

 may have been a state of equilibrium between the growth of the 

 mangrove and the reef-patch, which does not now exist. All we 

 can say of some of these low islands is that the mangroves have 

 been rapidly extending their margins during the last half century, 

 and that the normal adjustment between reef-growth and 

 mangrove-growth, which must have once existed, does not now 

 prevail. 



There is evidence of the shoaling of the ship channel amongst 

 these islands to the extent of about a fathom during this period.^ 

 The usual depth immediately around the patches, on which the 

 islands have been formed, is 8 to 10 fathoms. If, therefore, the 

 shoaling is a general process, it is to be inferred that although the 

 outward growth of the reef-patches would be usually very slow, 

 probably not over fifty yards in a century, there must be times 

 when, in shallowing depths, the growth of the reef-patch would be 

 comparatively rapid ; and it is at such times that the adjustment 

 between the relations of mangrove and reef-patch would be upset 



1 Between Mathuata Island and the coast a change is indicated from 9—10 

 fathoms to 8—9 fathoms, north of Motua Island 12—13 to 11— 12, and between 

 Nangano and Thakavi 16 to 14 fathoms. 



C 



