Derby.] -58 [May i 6f 



The diorite can scarcely be supposed to have furnished the gems, not 

 alone on account of the nature of the rock, but because in the upper part 

 of the valley, where diamonds are not uncommon, diorite is extremely rare, 

 if it occurs at all, and because the pebbles which always accompany the 

 gems, certainly do not come from the diorite. This last has most probably 

 furnished, by decomposition, the red clay, above the gravel at Tibagy. 

 The only other rocks which, as far as I observed, could have given such a 

 clay, are those about Castro ; but it would be difficult to account for its 

 transportation from there to Tibagy, while large dykes of diorite are com- 

 mon near the latter place. 



The secondary origin of the gravel is not far to seek. The sandstone is 

 everywhere full of pebbles, and on every slope where this rock is exposed 

 the surface is strewn with gravel, set free by disintegration. The primary 

 origin of the pebbles is equally clear; they, in common with all the ma- 

 terial of the Devonian beds, are derived from the raetamorphic series. 

 That the diamonds have the same primary origin can hardly be doubted, 

 as they cannot be supposed to have been produced in the sandstone, which 

 does not show the slightest sign of metamorphism or of crystallization of 

 any kind. That the diamond must have originated in some series rich in 

 crystals is evident from the fact, that it is always accompanied by a 

 variety of crystals, called by the miners informations. I have not had an 

 opportunity of determining those of Tibagy, which do not differ materially 

 from those already described from Bahia and Minas Geraes. 



It may then be regarded as extremely probable, if not absolutely certain, 

 that the diamonds originated in the metamorphic series ; that, during the 

 Devonian age, they were washed out and redeposited in the sandstone, 

 from which they have been again extracted, to find their third resting 

 place in the sand banks and pot-holes of the river, and in the gravel deposits 

 of the campos. May we not suppose that the rare patches of gravel, with 

 blue cement, are nests formed in the sandstone and laid bare by the forma- 

 tion of pot-holes? 



As to what portion of the extensive metamorphic series constituted the 

 original matrix of the diamond, I could obtain no data in Parana. The 

 evidence on the subject, which is being slowly accumulated, tends appa- 

 rently to the confirmation of the old idea, that it belongs to some part of 

 the itacolumite series. 



After my studies in Parana, it seems to me probable that the extensive 

 high sandstone plateaus of Central Brazil, which we have been accustomed 

 to consider of Tertiary age, are in reality much older, and probably Palaeo- 

 zoic. It is yet too early to form a decided opinion respecting them, but if 

 my suspicion regarding their age proves correct, we can explain the course 

 of geological events in Brazil much more satisfactorily than at present. 



The lower plateaus, of almost precisely similar topographical and litho- 

 logical character, along the coast and on the Amazonas, are certainly later 

 than the Cretaceous ; but none of those, whose age can be positively deter- 

 mined, rise much above 1000 metres, and the higher plateaus of the inte- 

 rior have been referred to the Tertiary, solely on resemblances in Iithologi- 

 cal characters which, in Brazil, are peculiarly deceptive and untrustworthy. 



