Along The Whole Coast Of Brazil, For A Length Of At Least
2000 Miles, And Certainly For A Considerable Space
Inland,
wherever solid rock occurs, it belongs to a granitic formation.
The circumstance of this enormous area being constituted of
Materials which most geologists believe to have
been crystallized when heated under pressure, gives rise to
many curious reflections. Was this effect produced beneath
the depths of a profound ocean? or did a covering of strata
formerly extend over it, which has since been removed?
Can we believe that any power, acting for a time short of
infinity, could have denuded the granite over so many thousand
square leagues?
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.
The fish, having remained in this distended state for a
short time, generally expelled the air and water with
considerable force from the branchial apertures and mouth. It
could emit, at will, a certain portion of the water, and it
appears, therefore, probable that this fluid is taken in partly
for the sake of regulating its specific gravity. This Diodon
possessed several means of defence. It could give a severe
bite, and could eject water from its mouth to some distance,
at the same time making a curious noise by the movement
of its jaws. By the inflation of its body, the papillae, with
which the skin is covered, become erect and pointed. But
the most curious circumstance is, that it secretes from the
skin of its belly, when handled, a most beautiful carmine-red
fibrous matter, which stains ivory and paper in so permanent
a manner that the tint is retained with all its brightness
to the present day: I am quite ignorant of the nature
and use of this secretion. I have heard from Dr. Allan of
Forres, that he has frequently found a Diodon, floating alive
and distended, in the stomach of the shark, and that on
several occasions he has known it eat its way, not only
through the coats of the stomach, but through the sides of
the monster, which has thus been killed. Who would ever
have imagined that a little soft fish could have destroyed
the great and savage shark?
March 18th. - We sailed from Bahia. A few days afterwards,
when not far distant from the Abrolhos Islets, my;
attention was called to a reddish-brown appearance in the
sea. The whole surface of the water, as it appeared under a
weak lens, seemed as if covered by chopped bits of hay, with
their ends jagged. These are minute cylindrical confervae,
in bundles or rafts of from twenty to sixty in each. Mr.
Berkeley informs me that they are the same species
(Trichodesmium erythraeum) with that found over large spaces
in the Red Sea, and whence its name of Red Sea is derived. [8]
Their numbers must be infinite: the ship passed through
several bands of them, one of which was about ten yards
wide, and, judging from the mud-like colour of the water,
at least two and a half miles long.
Enter page number
PreviousNext
Page 6 of 205
Words from 5038 to 6043
of 208183
