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 I
will 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.
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 unfathomable depth; whereas
on the east. a shallow sea, nowhere exceeding 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. Then the intervening land sunk down, 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 small amount of
depression the land need have undergone to produce it.
But the fact of the Aru Islands having once been connected with
New Guinea does not rest on this evidence alone. There is such a
striking resemblance between the productions of the two countries
as only- exists between portions of a common territory. I
collected one hundred species of land-birds in the Aru Islands,
and about eighty of them, have been found on the mainland of New
Guinea.
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