260 



MR. O. A. PrRHY ON NEPHELTNE-ROCKS IN BRAZIL. 



difference in texture, to which the unequal action of the atmosphere 

 fan he attrihuterl, can be detected. The same feature is equally 

 marked in many of the rocks of the Serra do Itatiaia, but has not 

 been observed in those of Poqos de Caldas and Cabo Frio, where, if it 

 occurs, it is not sufficiently prominent to have attracted attention. 



Fig. 2. 







Fluted weathering of foyaite. From a photograph. 



Included frao;ments of other rocks, or of different types of the 

 same rock, are rare. The only one observed is a large anguLar 

 piece of augite-syenite, of medium grain and rich in magnetite, 

 which affects a linear arrangement, giving a gneissoid aspect to 

 the rock*; but this lias not been found in independent masses. 

 Segregations of various kinds are extremely common. The most 

 abundant and characteristic are confined to the porphyritic type of 

 foyaite, and present the aspect of crystals or groups of crystals. 

 An unusually perfect example, now preserved in the Xational Mu- 

 seum, is represented in fig. 3 on the next page. These are evidently 

 of the same nature as the smaller polyhedral inclusions in the true 

 phonolite, which have been discussed by Graeff and Hussak f, and de- 

 termined by the latter as pseudo-crystals in the form of leucite. The 

 accompanying figure (fig. 4), from a photograph, shows the mode of 



* This tendency to enrielnnent in magnetite is interesting, as at another 

 locaUty (Ipanenia, Sao Paulo) workable ore-bodies occur as segregations in 

 dykes of a rock of this type. 



t Neues Jahrb. 1SS7, vol. ii. p. 255, and 1890, vol. i. p. IGfi. The first-named 

 author, who gives exfellent detailed figure.*', considers them as incInsior..s of the 

 foyaite,which is cut by the dyke and with which they are identical in structure and 

 composition; the last named, having the advantage of much more favourable 

 miterial, shows that they have the form of leucite, though iciihouf a rccoynhablc 

 trace of that mhieral. The material placed in the hands of Dr. Hussak was 

 obtained, after a diligent search for specimens, in just the proper stage of decay 

 to cleave freely around the granular mass, so as to expose the outer face. Only 

 one such could be found, which afforded several sliarply-cut leucitohedrons of 

 the form 202 (21 1) and angles of 131° and 140°. 



