NEPHELINE-ROCKS IN BRAZIL. 



473 



Mar and Mantiqueira in the valley of the Parahiba ; but these are 

 certainly newer than the eruptions of similar material of Itatiaia. 

 Unfortunately the other localities mentioned in this paper afford no 

 prospect of throwing light on the geological age of the nepheline- 

 rocks, as there are no sedimentary beds in their vicinity interme- 

 diate between the gneiss and the recent deposits. They promise, 

 however, to give important results on the relations of various types 

 of eruptive rocks, and it is hoped that an opportunity will soon be 

 afforded for examining and describing them more fully. 



Discussion. 



The President said it was seldom that a paper containing such 

 important facts was presented to the Society. It was reserved to 

 Mr. Derby to have proved that plutonic rocks containing nepheline 

 (foyaite) passed into volcanic masses which were true phonolites. 

 This Mr. Derby had clearly established by observations in the field. 

 He had also shown that leucite existed in rocks of palaeozoic age, 

 thus rendering untenable the last stronghold of those who insisted 

 on making geological age a primary factor in petrographical classi- 

 fication. He alluded also to the value of the independent determi- 

 nations of Prof. Bosenbusch. 



Mr. Batjerman had been over portions of the ground with the 

 Author, and was glad to add his testimony to the value of the 

 paper. He spoke of the importance, in a geological sense, of these 

 generalizations. It was remarkable how highly crystalline masses 

 of rock pass over into a sort of phonolite. These were associated 

 with palaeozoic masses, which were pre-Permian or at least pre- 

 Triassic. He alluded to the difficulty of investigating Eernando 

 Noronha, and also to the difficulties attendant upon the investigation 

 of rocks in Brazil, which were subject to such an enormous amount 

 of local alteration. 



Prof. Bonnet also expressed his sense of the value of the paper. 

 He alluded to the comparative rarity of nepheline- and leucite-rocks, 

 and to the confusion in the nomenclature. He was reminded of the 

 nepheline-rocks near Montreal, where dolerite was broken through 

 by nepheline- syenite, associated with tephrites and phonolites. 

 Although there might be a doubt here, these rocks were most pro- 

 bably of Silurian age ; but the evidence in Brazil was still clearer 

 as to the palaeozoic age, and he believed that, in the case of some 

 other masses, the evidence had satisfied the Canadian geologists. 

 He alluded also to the nepheline-rocks in the Katzen-Buckel, where 

 there was a similar passage from coarse-grained to fine-grained. 



Dr. Hatch said that in this case leucite was clearly shown to be of 

 palaeozoic age, and he regarded the paper as a step towards the better 

 classification of this group of rocks. 



Prof. Seeley asked for evidence as to the identification of the 

 leucite. 



The President thought there was no possibility of mistake in 

 this respect. As regards the rocks of the Katzen-Buckel, none 

 were truly hoi o crystalline, and hence they could not be compared 

 with foyaite or elaeolite-syenite. 



