Williams — Petrography of Fernando de JYoronha. 183 



and possessing the compact structure and somewhat oily luster 

 characteristic of that rock. The specimens vary considerably 

 in the relative abundance of their porphyrinic crystals, range 

 in color from dark green to pale grey, and differ in their micro- 

 scopic composition and structure. JSTo relation could, however, 

 be discovered between these differences and the areal distribu- 

 tion of the rocks. The St. Michael's Mount (Sella a Gineta) 

 phonolite described by Renard is not represented in this collec- 

 tion. 



The distribution of the phonolite specimens (cf. map) is as 

 follows : JSTo. 137, N.E. corner of island, between the village 

 and Bahia de Sto. Antonio. Professor Branner's notes say, 

 " This is the rock most common east of the village along the 

 north shore of the island." Nos. 1 and 35, Pedra da Conceicao. 

 ]STos. 51, 52 and 88, Peak. ISTos. 41 and 43 north of Atalaia 

 Grande from loose blocks. ISTos. 5, 9 and 40 Atalaia Grande. 



The phonolites of Fernando de JSToronha are remarkable for 

 having a columnar parting more prominently developed than 

 the separation into plates parallel to the cooling surface, so 

 common in this rock. 



With reference to their microscopic characters the specimens 

 of phonolite at hand may be arranged as follows : 



I. Nepheline of the ground-mass in distinct crystals • green hisili- 



cate in compact stout crystalloids. 



Grain fine, little brown hornblende, no hauyne No. 41 



Grain fine, black hauyne abundant, no hornblende 9 



Grain very fine, hauyne and sphene, but no hornblende 40 



Grain very fine, like last without hauyne 35 



II. JVepheline of the groundmass moderately distinct; green 



bisilicate in more or less radiating ocellar sheaves. 



Ocellar sheaves very distinct 43 



Sheaves less distinct ; nepheline crystals plainer 5 



III. JVepheline of the groundmass indistinct / green bisilicate not 



prominent. 



Porphyritic nepheline, hauyne, sphene, flow structure 51 



Porphyritic nepheline, hauyne, eutaxitic structure S8 



Porphyritic nepheline, hauyne, brown hornblende, dull ground- 

 mass 137 



Groundmass dull and opaque, hauyne, brown hornblende 1 



Like last 52 



The constituent minerals agree very closely with the diag- 

 nosis given by Professor Rosenbusch for the components of 

 typical phonolite. 



Am. Jour. Sci— Third Series, Vol. XXXVII, No. 219.— March, 1889. 

 12 



