II EVIDENCE OF RECENT EMERGENCE 17 



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

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

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

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

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

 liere 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 

 ^.CXD 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 

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

 t wo centuries for its formation. But since the extension of the 

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

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

 c m 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 

 his been continuously advancing. A hundred years ago there 

 n ay have been a state of equilibrium between the growth of the 

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

 c.n say of some of these low islands is that the mangroves have 

 b ;en rapidly extending their margins during the last half century, 

 aid that the normal adjustment between reef-growth and 

 rr angrove-growth, which must have once existed, does not now 

 p evail. 



There is evidence of the shoaling of the ship channel amongst 

 tl- ese islands to the extent of about a fathom during this period.^ 

 T le usual depth immediately around the patches, on which the 

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

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

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

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

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

 cc Tiparatively rapid ; and it is at such times that the adjustment 

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



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

 fat 10ms to 8 — 9 fathoms, north of Motua Island 12 — 13 to 11 — 12, and between 

 N mgano and Thakavi 16 to 14 fathoms. 



C 



