Physical Geography. 489 



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 isl- 

 ands, whose productions are entirely those of the adjacent 

 continent, have been sej^arated 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. Fis- 

 sures during uj^heaval would not produce the regular width, 

 the regular depth, or the winding curves which characterize 

 them ; and the action of tides and currents during their ele- 

 vation 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 subsi- 

 dence, 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. 



Now if these channels have ever been rivers they must 

 have flowed from some higher regions, and this must have 

 been to the east, because on the north and west the sea-bottom 

 sinks down at a short distance from the shore to an unfathom- 

 able depth ; whereas on the east a shallow sea, nowhere ex- 

 ceeding fifty fathoms, extends quite across to New Guinea, a 

 distance of about a hundred and fifty miles. An elevation 

 of only three hundred feet would convert the whole of this 

 sea into moderately high land, and make the Aru Islands 

 a portion of New Guinea ; and the rivers which have their 

 mouths at Utanata and "Wamuka, might then have flowed 

 on across Aru, in the channels which are now occupied by 

 salt water. When the intervening land sunk doAvn, we must 

 suppose the land that now constitutes Aru to have remained 

 nearly stationary, a not very improbable supposition, when 

 we consider the great extent of the shallow sea, and the very 



