A glance at the map of the island, where these inland plains are indicated by the 300 feet of the contour-line, will make this point more clear. These plains are traversed by the Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; and so slight is the fall that cutters usually ascend the rivers for several miles, whilst the tide extends for a considerable distance up their courses. That the emergence of the inland plains of Kalikoso in the eastern part of the island is comparatively recent there can be but little doubt. In that locality as described on page 224, the low marshy land, surrounding the fresh-water lake of Vakalalatha, although five miles inland, is only elevated 20 to 30 feet or less above the sea, and silicified corals are scattered over its surface.
There is one other method of ascertaining the character and amount of elevation that may be still in progress in this island namely the comparison of the results of surveys of the coasts at different periods. In this manner data may be obtained as regards the growth of the mangrove belt, changes in size of the low reef-islets and islands, and alterations in depth. For this purpose I have employed the charts of the north and west coasts of the island made by Commodore Wilkes in 1840[10] and the Admiralty charts 379 and 382 as completed from the survey of these coasts by Commander Combe in 1895-96.
It was not easy to make many good comparisons in the case of the advance of the mangrove-belt of the main coast. There certainly has been no great advance seaward of the margin of the mangroves in this half century. The average amount probably lies between the estimate obtained for the coast opposite Mathuata Island, where there has either been no change or an advance of only 100 yards or so, and that for the advance seaward of the mangrove promontory of Lekutu which amounts to 500 or 600 yards. In this last case, however, much of the extension may be due to the advance of the mangroves on the mud brought down by the Lekutu river, so that, as far as these data show, the average advance of the belt of mangroves on this coast between 1840 and 1895 would appear to be slight.[11]
On the other hand, the mangrove-borders of the several low islands and islets, mainly formed of reef-_débris_, that lie off the coast, have often extended themselves during this period in a marked degree. The results of my comparisons are given below, the rate of advance being obtained by halving the increase in length or breadth as measured between the mangrove-borders, the breadth being used in the long islands.