ilH. 0. A. DERBY ON NEPHELINE-R0CK8 IN BRAZIL. 251 



16. On Nepheline-Rocks in Brazil. — Part II. The Tingua Mass. 

 By Orville A. Derby, Esq., P.G.S., &c. (Read December 17, 

 1890.) 



In a former paper under the above general title (Quart. Journ. 

 Geol. Soc. vol. xliii. 1887, pp. 457 et seq.), the distribution, so far as it 

 is at present known, of the nepheline-bearing rocks in Brazil was 

 given, together with a somewhat detailed description of the Pocos de 

 Caldas locality. Since that time three important petrographical 

 papers relating to this group of rocks have appeared *, one of which 

 is accompanied by a detailed geological description of the insular, 

 and presumably later, eruptive mass of Fernando de Noronha. 



In this and subsequent papers, it is proposed to treat of the other 

 continental localities, with special reference to the geological rela- 

 tions and mode of occurrence of this interesting group. One of the 

 localities enumerated in that paper — the Serra de Bocaina — should 

 be eliminated from the list, as a recent examination shows it to be 

 composed exclusively of gneiss and granite. The specimens reported 

 to have come from this locality are probably from the neighbouring 

 mass of the Serra de Itatiaia. 



The peak of Tingua is one of the two prominent masses that 

 break the somewhat monotonous, approximately even-topped out- 

 line of the Serra do Mar, as seen from Rio de Janeiro ; the other 

 being the well-known Serra dos Orgoes. Of the two, the former, 

 although of less elevation (1600 as compared with 2232 metres), is, 

 to the geological eye at least, the most striking, since its topo- 

 graphical features, and more particularly its conical form and appa- 

 rent distinctness from the range to which it belongs, are more 

 suggestive of a difference in geological structure and origin. The 

 Serra dos Orgoes, consisting of gneiss and granite, which are every- 

 where the characteristic components of the range, forms an integral 

 part of it ; the eruption of the immense boss of granite (granitite) 

 that constitutes the highest points probably dating from or near the 

 epoch of upheaval that gave the range, as a whole, its present form 

 and character. Tingua, on the other hand, is a. parasitic or super^ 

 imposed mountain, according to the nomenclature of Yon Richthofen, 

 the product of a purely local eruption of very different character 

 and of much later age t- 



Although overtopping by some 500-600 metres the mean eleva- 

 tion of the Serra do Mar in its vicinity, the Tingua peak is not even 



* Graeff, ' Mineralogische-petrographische Untersuchung von ElaeoHth- 

 syeniten von der Serra de Tingua,' ]^eues Jahrb. vol. ii. (1887). 



Jordano Machado, ' Beitrag zur Petrographie der siidwestlicben Grenze 

 zwischen Minas Geraes und S. Paulo ' [P090S de Caldas region], Tsciiermak's 

 Mittheil. vol. ix. (1887). 



Branner and Williams, ' Geology of Fernando de Noronha,' Amer. Journ. 

 Science, vol. xxxviii, (1889). 



t The very similar eruptive mass of P090S de Caldas is of late Carboniferous 

 or early Secondary age. 



