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GEOLOGY.
GEOLOGICAL OBSERVATIONS ON T H E NE IGHBOURHOOD OF R IO DE JA N E IR O ,
BY M R . C O L L IE .
T h e high, peaked, and abrupt form of tbe mountains around Rio de Janeiro
denote to the approaching navigator a country of primitive formation. The hills are
formed almost entirely of granite and gneiss, passing gradually from one to the other.
In some places the mica is so abundant as to change the hue from grey nearly to black.
The felspar, in various sized crystals, is generally greyish, glassy, and lamellar, not
unfrequently exhibiting a pearly Instre. The quartz is grey and transparent. With
these are mixed, in greater or less quantity, minute and irregularly crystallized garnets.
The rook also contains veins of garnet, mica, felspar, and quartz distinctly crystallized.
The mica in the veins is usually less abundant than in the granite, and in large white
plates ; the felspar, on the contrary, predominates, forming large and regular crystals
of pearly lustre, and tinged with red. These generally contain smaller irregular
crystals of grey quartz, and are thus converted into graphic granite. Fibrous tremolite
and dodecahedral quartz crystals occur also in these veins.
A t the most elevated part of the road leading to the Cascata of Tejuca, green-stone
prevails for some way ; and in another part o fth e same road there is a bed of greyish
opaque aud coarse horn-stone, which decomposes into a granular and friable earth.
On the south side of Three Fathom Bay, nearly half-way to the bottom, and opposite
a few rocks near the shore, which are uncovered a t low water, a regular dyke of
basalt cuts perpendicularly through the granite. The basalt breaks into long and rather
angular fragments, tbe longest natural joints being across the vein. The breadth of
this dyke is one foot five inches ; its direction is north and south. A sharp-pointed
portion of basalt projects a t one place into the adjoining rock, and in another place a
small oval patch of basalt seems completely separated from the v e in : there are also
two slight shifts of the dyke, both sides continuing parallel, and deviating together.
The rock around the dyke is particularly free from fissures and veins.
Among the rocks of this primitive formation is a great quantity of red clay, which
not only forms many of the cliffs, but renders the mountain roads in wet weather slippery
and disagreeable. This clay results from the decomposition of the red felspar of
the granite.
Respecting earthquakes at Rio de Janeiro, I could get no further information than
that they are rare— C.
BAY OF CONCEPC ION, ON T H E COAST OP C H IL I .
Observations hy the Editor.
The geological map and sections of the Bay of Concepcion have been supplied by
Lieutenant Belcher; the description is partly from his notes, and partly from those of
Mr. Collie. These gentlemen appear to have included under the term alluvial, not only
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