Geology. — “On the Alkalirocks of the Serra do Gericino to the 
northwest of Rio de Janeiro and the Resemblance between 
the Eruptive Rocks of Brazil and those of South-Africa.” 
By Prof. H. A. Brouwer. (Communicated by Prof. G. A. F. 
MOLENGRAAFF). 
(Communicated at the meeting of January 29, 1921). 
On the boundary between the State of Rio de Janeiro and the 
Districto Federal lies near the station of Maxambomba of the E. F. 
Central do Brazil, the Serra do Gericino'), extending in W.S.W.- 
N.N.E. direction over a length of about twenty k.m. and a breadth 
of about eight k.m. It is chiefly composed of nephelinesyenites like 
the Tingua eruptive province which lies about 30 km. farther to 
the North and of which the alkali-rocks have been described by 
Graurr®) and Dursy*). As I could not personally visit the Serra do 
Gericino during my stay in Brazil, several samples for further 
investigation were sent me by GonzaGa DE Campos, Director of the 
“Servico geologico e mineralogico do Brazil”. 
Geological Survey. 
The alkali-rocks constitute an eruptive centre amidst the old 
gneisses of the mountain-range Serra do Mar, which runs parallel 
to the Brazilian Coast. (Fig. 1). We only know that the gneisses 
are intruded by alkali-rocks which consequently must be younger 
than the gneisses. No data were obtained concerning the exact 
geological age. 
Coarse-grained, as well as dyke-, and effusive rocks occur, just 
as in other Brazilian occurrences of alkali-rocks. Although nearly 
the whole region and especially the highest parts consist of coarse- 
grained rocks, the occurrence of effusive rocks allied to them, still 
1) MarHias G. DE OLIVEIRA Roxo. Resumé of the preliminar note on the 
Gericinó eruptive centre. Empresa Brasil Editora 1920. 
*) Fr. GraEFF, Mineralogisch-petrographische Untersuchung von Eleolithsyeniten 
von der Serra de Tingua. Neues Jahrb. f. Min., Geol. u. Pal. 1887. II, blz. 222 e.v. 
3) O. E. DerBy. On nepheline rocks in Basil Quart. Journ. Geol. Soc. Vol. 
XLIII. 1887, blz. 457; Vol. XLVII, 1891, blz. 251. 
80 
Proceedings Roval Acad. Amsterdam. Vol. XXIII. 
