GEOLOGY 
GEOLOGICAL OBSERVATIONS ON THE NEIGHBOURHOOD OP RIO DE JANEIRO, 
BY MR. COLLIE. 
The high, peaked, and abrupt form of the 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 lustre. The quartz is grey and transparent. With 
these are mixed, in greater or less quantity, minute and irregularly crystallized garnets. 
The rock 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. 
At the most elevated part of the road leading to the Cascata of Tejuca, green-stone 
prevails for some way ; and in another part of the same road there is a bed of greyish 
opaque and 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 oppo- 
site a few rocks near the shore, which are uncovered at low water, a regular dyke of 
basalt cuts perpendicularly through the granite. The basalt breaks into long and rather 
angular fragments, the 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 at one place into the adjoining rock, and in another place a 
small oval patch of basalt seems completely separated from the vein : 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 slip- 
pery 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 CONCEPCION, ON THE COAST OF CHILI. 
Observations by 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, notonly 
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