IN this chapter I propose to give a general sketch of the physical geography of the Aru Islands, and of their relation to the surrounding countries; and shall thus be able to incorporate the information obtained from traders, and from the works of other naturalists with my own observations in these exceedingly interesting and little-known regions.
The Aru group may be said to consist of one very large central island with a number of small ones scattered round it. The great island is called by the natives and traders "Tang-busar" (great or mainland), to distinguish it as a whole from Dobbo, or any of the detached islands. It is of an irregular oblong form, about eighty miles from north to south, and forty or fifty from east to west, in which direction it is traversed by three narrow channels, dividing it into four portions. These channels are always called rivers by the traders, which puzzled me much till Ipassed through one of them, and saw how exceedingly applicable the name was. The northern channel, called the river of Watelai, is about a quarter of a mile wide at its entrance, but soon narrows to abort the eighth of a mile, which width it retains, with little variation, during its whole, length of nearly fifty miles, till it again widens at its eastern mouth. Its course is moderately winding, and the hanks are generally dry and somewhat elevated. In many places there are low cliffs of hard coralline limestone, more or less worn by the action of water; while sometimes level spaces extend from the banks to low ranges of hills a little inland. A few small streams enter it from right and left, at the mouths of which are some little rocky islands.
The depth is very regular, being from ten to fifteen fathoms, and it has thus every feature of a true river, but for the salt water and the absence of a current. The other two rivers, whose names are Vorkai and Maykor, are said to be very similar in general character; but they are rather near together, and have a number of cross channels intersecting the flat tract between them. On the south side of Maykor the banks are very rocky, and from thence to the southern extremity of Aru is an uninterrupted extent of rather elevated and very rocky country, penetrated by numerous small streams, in the high limestone cliffs bordering which the edible birds' nests of Aru are chiefly obtained. All my informants stated that the two southern rivers are larger than Watelai.
The whole of Aru is low, but by no means so flat as it has been represented, or as it appears from the sea. Most of it is dry rocky ground, with a somewhat undulating surface, rising here and there into abrupt hillocks, or cut into steep and narrow ravines.
Except the patches of swamp which are found at the mouths of most of the small rivers, there is no absolutely level ground, although the greatest elevation is probably not more than two hundred feet. The rock which everywhere appears in the ravines and brooks is a coralline limestone, in some places soft and pliable, in others so hard and crystalline as to resemble our mountain limestone.
The small islands which surround the central mass are very numerous; but most of them are on the east side, where they form a fringe, often extending ten or fifteen miles from the main islands. On the west there are very few, Wamma and Palo Pabi being the chief, with Ougia, and Wassia at the north-west extremity. On the east side the sea is everywhere shallow, and full of coral; and it is here that the pearl-shells are found which form one of the chief staples of Aru trade. All the islands are covered with a dense and very lofty forest.
The physical features here described are of peculiar interest, and, as far as I am aware, are to some extent unique; for I have been unable to find any other record of an island of the size of Aru crossed by channels which exactly resemble true rivers. How these channels originated were a complete puzzle to me, till, after a long consideration of the whole of the natural phenomena presented by these islands, I arrived at a conclusion which Iwill now endeavour to explain. There are three ways in which we may conceive islands which are not volcanic to have been formed, or to have been reduced to their present condition, by elevation, by subsidence, or by separation from a continent or larger island. The existence of coral rock, or of raised beaches far inland, indicates recent elevation; lagoon coral-islands, and such as have barrier or encircling reefs, have suffered subsidence; while our own islands, whose productions are entirely those of the adjacent continent, have been separated from it. Now the Aru Islands are all coral rock, and the adjacent sea is shallow and full of coral, it is therefore evident that they have been elevated from beneath the ocean at a not very distant epoch.
But if we suppose that elevation to be the first and only cause of their present condition, we shall find ourselves quite unable to explain the curious river-channels which divide them. Fissures during upheaval would not produce the regular width, the regular depth, or the winding curves which characterise them; and the action of tides and currents during their elevation might form straits of irregular width and depth, but not the river-like channels which actually exist. If, again, we suppose the last movement to have been one of subsidence, reducing the size of the islands, these channels are quite as inexplicable; for subsidence would necessarily lead to the flooding of all low tracts on the banks of the old rivers, and thus obliterate their courses;whereas these remain perfect, and of nearly uniform width from end to end.