320 0. A. Derby— Magnetite Ore Districts in Brazil. 



enclosed in the foyaite of Tingua (the only specimen of laur- 

 vikite found at this locality) in which pyroxene and magnetite 

 are linearly arranged.* A small loose block found associated 

 with foyaite at Jacupiranga has large idiomorphic phenocrysts 

 of dark violet pyroxene, full of inclusions and with beautiful 

 zonal structure, embedded in a holocrystalline ground-mass of 

 orthoclase with abundant bisilicates (green pyroxene and 

 biotite) and occasional grains of plagioclase. This rock, which 

 illustrates beautifully the tendency towards a basic type, pre- 

 sents some features that suggest a comparison with certain 

 phases of jacupirangite. The frequent occurrence of olivine 

 as an accessory in rocks of the laurvikite type also points in 

 the same direction and suggests a possible hypothesis to ac- 

 count for the presence of enstatite in some of the Ipanema 

 rocks, r 



In this connection it is interesting to note the same tendency 

 to a basic phase in the granitoid, or foyaite, type of nepheline- 

 bearing rocks. At several points in the Jacupiranga district, 

 considerable masses (dykes '() appear of a plagioclase-pyroxene- 

 biotite rock that has been referred to diabase or gabbro by 

 several eminent petrographers to whom they have been sub- 

 mitted. At two points the blocks of this type are mingled 

 with those of foyaite in a manner to suggest a geological con- 

 nection, while occasionally, as at Cabo Frio, also particulariza- 

 tions of the same type have been observed in the latter rock. 

 Moreover, a diligent search made by Dr. Hussak reveals an oc- 

 casional grain of nepheline in the plagioclase rock. A rock of 

 similar character with idiomorphic violet pyroxene and an 

 abundance of olivine also contains rare nepheline grains and is 

 referred to the same type;}: as is also a plagioclase-analeime- 

 pyroxene rock identical in all essential particulars with the 

 teschenite of Elgoth, Silesia, that occurs near the river Bibeira 

 at Jaguary. These plagioclase rocks, which may be compared 

 with the theralithes of Montana and the teschenites of various 

 localities, simulate closely the diabases and gabbros, but differ 

 in their genetic relations, from the ordinary types of that 

 family. Various other peculiar types of eruptive rocks abound 

 in the Jacupiranga district, but so far as known they have no 



* The quantity of magnetite in this rock would be large for a rock of purely 

 basic type and is extraordinary for one in which orthoclase is the predominant 

 element. It shows the tendency to basic segregations that culminates in the 

 Ipanema on bodies. 



f If the enstatite is primary it is extraordinary that a mineral so subject to al- 

 teration should be perfectly fresh while all the other elements, except magnetite 

 and apatite, are altered beyond recognition. If secondary it might have been 

 formed from original olivine as suggested above, or possibly from the magnesia of 

 the decomposed pyroxene. 



% This, with more abundant nepheline. would correspond very closely with the 

 theralithe of Crazy Mts.. Montaua. 



