188 Williams — Petrography of Fernando de Noronha. 



No. 27 from a point on the south side of Fernando agrees 

 closely with the last specimen except that it contains no perof- 

 skite. 



No. 65 from Portao, at the west end of Fernando is a typ- 

 ical limburgite. In the hand-specimen it is of a dark gray 

 color and filled with vesicular cavities of all sizes which are 

 either coated or completely filled with zeolites. Small rusty 

 yellow spots indicate the position of abundant olivine crystals 

 which are the only constituent macroscopically visible. Under 

 the microscope well-formed olivines surrounded by a yellow 

 border are seen imbedded in a groundmass of augite micro- 

 liths, magnetite octahedra and a colorless glass. Thus the rock 

 is a limburgite of the second class in the sense of Bucking, 

 which, as we might expect from the present association, is 

 more nearly allied to the nepheline, than to the feldspar basalts. 



No. 14, from Atalaia Grande, may also be classed as a lim- 

 burgite, although from its poverty of olivine it is closely related 

 to the augitites. The hand-specimen of this rock presents a 

 striking contrast to the last, it being compact and black like 

 the nepheline-basalts. Small, sharply-formed augite crystals 

 and a few olivines are the only components visible. Under 

 the microscope this specimen is seen to differ greatly from all 

 the others. The olivines are few but very fresh. The augites 

 are distinguished by their pronounced zonal structure, their 

 interior being of a brilliant green and pleochroic, while the 

 outer zone is reddish gray. Their extinction is very high. 

 Smaller crystals of brown basaltic hornblende are also abund- 

 ant. The groundmass consists of a brown glass somewhat 

 devitrified with globulitic dust and occasional arborescent 

 growth forms. This contains augite microliths, magnetite in 

 octahedra and occasional crystals of blue hauyne. As a sec- 

 ondary product analcite occurs, either after the hauyne or in 

 minute cavities. 



No. 115, from Morro Francez, is a more typical augitite in 

 being wholly free from olivine, although it appi-oaches the 

 nephelinites in containing nepheline, mostly in the form of 

 porphyritic crystals. The hand-specimen shows abundant 

 black porphyritic crystals which are in part hornblende and in 

 part augite. The groundmass is mostly composed of a color- 

 less glass containing augite microliths and magnetite. A little 

 nepheline is also present in it. This mineral is, however, more 

 abundantly present in crystals or grains of medium size im- 

 bedded in the groundmass. 



Three specimens, No. 48 r from Morro Francez, No. 3 from 

 the S.W., and No. 58 from the W. corner (Japato) of Fer- 

 nando, represent very scoriaceous ejected fragments (volcanic 

 bombs) of basaltic rocks. The first is a mottled black and red 



