MR. O. A. DERBY ON NEPHELINE-ROCKS IN BRAZIL. 265 



pauicd by varyinpj proportions of amphibole and biotitc, and is thus 

 a typical aupjitite. Olivine is almost always present, and, par- 

 ticularly at Tingua, often becomes so prominent as to produce a 

 limburgite. The glass basis tends to become individualized as pla- 

 gioclaso and nejjheline, giving a tephrite-like form, or as jjlagioclase 

 alone — so that, in the extreme forms with almost complete disappear- 

 ance of glass, a type is produced that by many would be called a 

 felspar-basalt. This tendency to fclspathic types is more pronounced 

 at Tingua than at the other localities. All of these forms occur 

 associated in such a way as to appear to be contemporaneous and 

 modifications of the same original magma. The three dykes above 

 mentioned, for example, all cut the tufaceous phonolite at intervals 

 of a few metres onlj-, and, while one is a typical augitite, another 

 might be called a limburgite, and the third a felspar-basalt. 



Discussiox. 



The Chairman (Mr. Hudleston) supposed, from the evidence, that 

 the Tingua mass was a volcano, and that therefore it was not of very 

 great age, as it had not been entirely denuded. 



Mr. Batjerman was pleased to see the excellent way in which the 

 Author was carefully working out the geology of the district he had 

 described. He himself had visited the district, and was struck with 

 the many features of great geological interest displayed therein. 



Mr. HuLKE was reminded of the rock of the Hirschenberg, in the 

 Eifel, by the Author's descriptions. The Hirschenberg rock contains 

 leucites, which were with difficulty recognizable, but in the cracks 

 of the rocks were little groups of crystals with the crystalline faces 

 preserved. 



Prof. Green asked whether the foyaite masses might possibly be 

 intrusive sheets. 



The Author, in reply, stated that there was nothing between 

 gneiss and Quaternary in the region. The only place where there 

 was hope of getting a clue was at Po§os de Caldas, described in his 

 previous paper, where sedimentary rocks occur, containing eruptive 

 rocks of the same nature as the supposed volcanic rocks, and cut by 

 dykes of the same nature. Not very far away are Carboniferous 

 fossils ; the rocks containing them continue to near the foot of the 

 mountain and agree with the Carboniferous rocks in character ; 

 moreover, no other sedimentary rocks are known in the whole region. 

 All the nephcline-bearing rocks examined by the Author present 

 the same characters, and look as though they might be con- 

 temporaneous. Cavities in rocks in the district have been found 

 lined with analcime, but no leucite has been yet found in such 

 cavities, though he had no doubt it would ultimately be found in the 

 rocks. He had seen leucites from Arkansas having the same cha- 

 racters as those presented by the masses he had described, and noted 

 that Dr. Hussak had referred to a similar structure from Monte 

 Somma. He suggested in the paper that the foyaite-sheets were 

 not absolutely superficial, but forced through the incoherent upper 

 masses of a volcanic cone. 



Q. J. G. S. No. 187. XT 



