Feb. 1832.
absorbed or evaporated before it reached the ground. I
will not at present attempt to describe the gaudy scenery of
this noble bay, because, in our homeward voyage, we called
here a second time, and I shall then have occasion to remark
on it.
The geology of the surrounding country possesses little
interest. Throughout the coast of Brazil, and certainly for
a considerable space inland, from the Rio Plata to Cape
St. Roque, lat. 5° S., a distance of more than 2 0 0 0 geographical
miles, wherever solid rock occurs, it belongs
to a granitic formation. The circumstance of this enormous
area being thus constituted of materials, which almost
every geologist believes to have been crystallized by the
action of heat 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.* 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
tlie 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.” The coating is here of a rich brown instead of
a black colour, and seems to be composed of ferrugineous
matter alone. Hand specimens fail to give a just idea of
these brown, burnished, stones which glitter in the sun’s rays.
* P e r s . N a r r ., v o l.v ., p t . i., p . 18.
They occur only within the limits of tidal action; and as the
rivulet slowly trickles down, the surf must supply the polishing
power of the cataracts in the great rivers. In the same
manner, the rise and fall of the tide probably answers to
the periodical inundations; and thus the same causes are
present under apparently very different circumstances. The
real 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 constant.
One day I was amused by watching the habits of a Diodon,
which was caught swimming near the shore. This fish 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 was absorbed by the mouth,
and perhaps likewise by the branchial apertures. 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, I observed, entered in a stream
through the mouth, which was wide open and motionless:
this latter action must, therefore, depend on suction. The
skin about the abdomen is much looser than that of 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 likewise 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
were out of water; but a stream drawn in by the mouth,
constantly flowed through them.
The fish, having remained in this distended state for a