428 La Plata : part n. 



with geros of various kinds, often in a fractured state, 

 owing, as some have supposed, to the collapse of geodes, 

 and that they contain gold and diamonds. At Rio, it 

 appeared to me that the gneiss had been softened before 

 the excavation (no doubt by the sea) of the existing, 

 broad, flat-bottomed valleys ; for the depth of decom- 

 position did not appear at all conformable with the 

 present undulations of the surface. The porphyritic 

 gneiss, where now exposed to the air, seems to with- 

 stand decomposition remarkably well ; and I could see 

 no signs of any tendency to the production of argilla- 

 ceous masses like those here described. I was also 

 struck with the fact, that where a bare surface of this 

 rock sloped into one of the quiet bays, there were no 

 marks of erosion at the level of the water, and the 

 parts both beneath and above it preserved a uniform 

 curve. At Bahia, the gneiss rocks are similarly decom- 

 posed, with the upper parts insensibly losing their 

 foliation, and passing, without any distinct line of 

 separation, into a bright red argillaceous earth, includ- 

 ing partially rounded fragments of quartz and granite. 

 From this circumstance, and from the rocks appearing 

 to have suffered decomposition before the excavation of 

 the valleys. I suspect that here, as at Rio, the decom- 

 position took place under the sea. The subject 

 appeared to me a curious one, and would probably well 

 repay careful examination by an able mineralogist. 



The Northern Provinces of La Plata. — According 

 to some observations communicated to me by Mr. Fox, 

 the coast from Rio de Janeiro to the mouth of the 

 Plata seems everywhere to be granitic, with a few 

 trappean dikes. At Port Alegre. near the boundary of 

 Brazil, there are porphyries and diorites. 1 At the 

 mouth of the Plata, I examined the country for twenty- 

 1 IT. Isabella, ' Voyage a Buenos Avres.' p. 479. 



