VII. — On the Geology of Rio de Janeiro. 

 By Alexander CALDCLEUGH, Esq. M.G.S. 



[Read 6th June, 1823.] 



IHE specimens which I have now the honour of presenting to the Geological 

 Society were collected in the neighbourhood of Rio de Janeiro^ a spot so long 

 celebrated for the beauty of its surrounding scenery. The Bay running in 

 the direction of north and south, and extending nearly thirty miles into the 

 interior, stands perhaps unrivalled, whether viewed as a resort for shipping, 

 or as merely affording a fit subject for the painter. It is surrounded by conical 

 hills, which in one instance only (the Corcovado) exceed nine hundred or 

 a thousand feet in height, and preserve the rounded character of gneiss, which 

 is peculiar to this rock even in Europe, where the agents of destruction are 

 feeble compared with those of a tropical country. Many of these hills are 

 perfectly bare, while others are clothed to their summits with the most luxu- 

 riant vegetation. In some spots they rise abruptly from the sea, in others a 

 small intermediate space is left. Wherever any soil is deposited by the heavy 

 periodical rains, a thick plica of vegetation springs up, which in several in- 

 stances prevents an examination of the subjacent strata. 



The alluvial soil is mostly of a yellow or red colour ; the latter variety is 

 frequently washed by the children of the place for the minute portion of gold 

 which it is known to contain. In making any excavation through the loose 

 soil on the surface, regular layers of crystals of feldspar and pieces of quartz 

 are met with, and with no appearance of having been transported thither. I 

 am inclined to suppose that much of the superficial soil proceeded from a de- 

 composition in situ, rather than from any washing down from the hills. On 

 the shore the sand is of a brilliant white colour, and I think there are appear- 

 ances to justify the idea that the sea has retired from this part of the coast. 



The district which has fallen under my examination is confined to the im- 

 mediate neighbourhood of the city of Rio de Janeiro, situate on the left or 

 west side of the harbour. 



The substance of which the mass of the mountains is composed, is gneiss ; 

 granite in no instance constitutes a distinct formation. The numerous quar- 



