184 Williams — Petrography of Fernando cle Noronha. 



The sanidine is abundant in all specimens both as porphvritic 

 crystals and in the groundmass. The former have a flat tabu- 

 lar habit parallel to their plane of symmetry, which, as a rule, 

 lies parallel to the cleavage of the rock. Carlsbad twins are 

 rather the rule than the exception. This mineral is younger 

 than the hauyne but of about the same age or a little younger 

 than the porphvritic nepheline. In some cases (No. 9) a 

 continuation of the growth of an intratelluric crystal may be 

 seen to have taken place during the final period of consolida- 

 tion. The sanidine of the groundmass consists of lath-shaped 

 microliths, which often exhibit in their arrangement a well 

 marked flow structure. (No. 51.) Brown hornblende occurs 

 only in sparsely disseminated porphyritic crystals; never in 

 the groundmass. It is not present in all the thin sections, but 

 this may be accidental as the hand specimens show no marked 

 difference in this respect. Is is always surrounded by the mag- 

 netite corrosion-rim showing that it must have been wholly a 

 product of intratelluric crystallization. Within this rim sphene 

 is of frequent occurrence. Around the outside of the magne- 

 tite rim is a fringe of minute crystals of green pyroxene 

 (segirine). The hornblende is, in some cases at least, younger 

 than the hauyne. Its pleochroism is as usual, but particularly 

 strong. 



Hauyne is abundant in many of the specimens. Its well de- 

 fined crystal outline shows it to belong to the earliest secretions 

 of the magma, as does also its inclusion in all the other constitu- 

 ents. It is in every instance colorless but filled with its char- 

 acteristic opaque inclusions which in one section (No. 9) are so 

 numerous as to make the mineral appear nearly black. Altera- 

 tion to zeolites and to calcite are common ; the latter being the 

 ground for referring the mineral to hauyne rather than to 

 nosean. 



Nepheline is very frequent in porphyritic crystals which 

 often possess a diameter of a millimeter or more. In this 

 form it belongs to the oldest crystallizations of the magma. 

 As a constituent of the groundmass it is sometimes very dis- 

 tinct in short stout crystals showing well defined hexagonal or 

 rectangular outlines. In other cases it is poorly individualized 

 and indistinct. The green bisilicate constituent of the Fer- 

 nando phonolites is without doubt an alkaline pyroxene 

 (segirine), although its optical properties are so like those of 

 hornblende that in minute individuals devoid of crystalline form, 

 it is not strange that it should be confused with this mineral. 

 As a porphyritic constituent it is rare and unimportant, the 

 crystals being always comparatively small. These generally 

 exhibit a zonal structure, with a gray or violet-colored center 

 and a weak pleochroism. In the groundmass the segirine 

 occurs either as short stout crystalloids or in more or less radi- 



