FAYAITE-IJOLITE SERIES OF MAGNET COVE 62 1 



While I and II are alike in a general way, yet there are 

 marked differences in SiOg, FeO, MgO and S. The specimen 

 analyzed by Williams carries considerable pyrite, while mine 

 onl}^ showed a few specks of it. The differences in the other 

 constituents named may be attributed to alteration, especially in 

 view of Williams' statement that the specimen analyzed by him 

 was not fresh/ A.nalysis II calculates out readily as follows : 



TTa 



Nephelite - - - - 4 



Diopside - - - - 64 



Augite - - - - 15 



Biotite 5 



Magnetite - - - - 8.7 



Pyrite - - - - - 0.7 



Calcite - - - - 0.6 



100. o 

 In my former paper this pyroxenite was referred somewhat 

 doubtfully to the jacupirangite of Derby. Through the kind- 

 ness of this gentleman, to whom I would express here mv deep 

 acknowledgments, I have lately received numerous specimens 

 of the Brazilian types. A comparison of these with the Magnet 

 Cove specimens makes it evident that the two occurrences differ 

 chiefly in size of grain, the Arkansas rock being very coarse, 

 while those from Brazil are much finer grained. In all other 

 essential respects the two are closely alike. 



From the microscopical examination of the specimens which 

 Professor Derby sent me, it is evident that the " Jacupirangites" 

 of Brazil vary from rocks rich in nephelite, and which are true 

 ijolites, closely analogous to those of Magnet Cove and Finland, 

 through rocks composed predominantly of pyroxene, with small 

 and varying amounts of magnetite and nephelite, to types 

 extremely rich in magnetite and with no nephelite or only traces 

 of this mineral. Accepting then th^ name of Jacupirangite for 

 the medium type, the application of this name to the Magnet 

 Cove rock is abundantly justified, since the only difference is the 

 comparatively unimportant one of size of grain, both being 

 holocrystalline. 



'Williams, op. cit., p. 227. 



