On A Point Not Far From The City, Where A Rivulet Entered
The Sea, I Observed A Fact Connected With A Subject Discussed
By Humboldt.
[7] At the cataracts of the great rivers
Orinoco, Nile, and Congo, the syenitic rocks are coated by
a black substance, appearing as if they had been polished
with plumbago.
The layer is of extreme thinness; and on
analysis by Berzelius it was found to consist of the oxides
of manganese and iron. In the Orinoco it occurs on the
rocks periodically washed by the floods, and in those parts
alone where the stream is rapid; or, as the Indians say, "the
rocks are black where the waters are white." Here the coating
is of a rich brown instead of a black colour, and seems
to be composed of ferruginous matter alone. Hand specimens
fail to give a just idea of these brown burnished stones
which glitter in the sun's rays. They occur only within the
limits of the tidal waves; and as the rivulet slowly trickles
down, the surf must supply the polishing power of the cataracts
in the great rivers. In like manner, the rise and fall
of the tide probably answer to the periodical inundations;
and thus the same effects are produced under apparently different
but really similar circumstances. The origin, however, of
these coatings of metallic oxides, which seem as if
cemented to the rocks, is not understood; and no reason, I
believe, can be assigned for their thickness remaining the
same.
One day I was amused by watching the habits of the
Diodon antennatus, which was caught swimming near the
shore. This fish, with its flabby skin, is well known to possess
the singular power of distending itself into a nearly
spherical form. After having been taken out of water for
a short time, and then again immersed in it, a considerable
quantity both of water and air is absorbed by the mouth,
and perhaps likewise by the branchial orifices. This process
is effected by two methods: the air is swallowed, and is then
forced into the cavity of the body, its return being prevented
by a muscular contraction which is externally visible: but
the water enters in a gentle stream through the mouth,
which is kept wide open and motionless; this latter action
must, therefore, depend on suction. The skin about the
abdomen is much looser than that on the back; hence, during
the inflation, the lower surface becomes far more distended
than the upper; and the fish, in consequence, floats
with its back downwards. Cuvier doubts whether the Diodon
in this position is able to swim; but not only can it thus
move forward in a straight line, but it can turn round to
either side. This latter movement is effected solely by the
aid of the pectoral fins; the tail being collapsed, and not
used. From the body being buoyed up with so much air, the
branchial openings are out of water, but a stream drawn in
by the mouth constantly flows through them.
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