180 Williams — Petrography of Fernando de Noronha. 



III. Basalt in Hocks. 



Nepheline basanite _ 31,34. 



N eplielinite-dolerite 2. 



Nepheline basalt 45,72,27. 



Augitite 115. 



Limburgite 14,65. 



Basalt bombs 3, 48, 58. 



Basalt tuffs 54,62. 



I. Trachyte. 



Darwin observed at the base of Fernando de Noronha beds 

 of whitish tuff cut by dykes of trachyte,* and Professor Bran- 

 nerf states that these soft light-colored rocks have a yery con- 

 siderable development at low levels and at the base of certain 

 of the eminences, notably Atalaia Grande and Morro Francez. 



No. 121. Amphibole-Trachyte. East base of Morro Francez. — 

 This rock is a very light greenish gray, homogeneous and com- 

 paratively compact mass, in which porphyritic crystals of 

 sanidine and black hornblende are only rarely discernible. 

 Under the microscope the thin section shows the same poverty 

 in porphyritic crystals as the hand-specimen. Two or three 

 sharply defined Carlsbad twins of sanidine and a single crystal 

 of dark brown, intensely pleochroic hornblende are all that are 

 present. The main mass of the rock is composed of a rather 

 coarse-grained aggregate of feldspar crystals in two forms. 

 The most abundant are short rectangular sections with well- 

 defined outlines and a zonal structure, often more altered 

 internally than at the periphery (" orthophyric feldspar" of 

 liosenbusch). The other feldspar is in narrow acicular or lath- 

 shaped microliths ("trachytic feldspar" of Rosenbusch)4 In 

 this aggregate are further observable irregular patches of a. 

 brown globulitic glass ; abundant sharp octahedrons of magnetite; 

 minute brightly polarizing needles too small to be positively 

 determined but doubtless pyroxene microliths with a very high 

 extinction angle ; small diamond-shaped crystals of titanite ; 

 and finally occasional minute, but very sharp dodecahedral 

 (rarely octahedral) crystals which are transparent with a violet 

 color and which may be perofskite, although their amount is 

 too small in this specimen to allow of their certain identifica- 

 tion. § The general structure of this rock is such as is included 

 by Rosenbusch under the Drachenfels-type. The powder of 

 the rock is not materially attacked by strong chlorhydric acid 



* Geological observations on Volcanic Islands, 1844, p. 23. 

 f Yid. ante, p. 9. 



\ Die massigen G-esteine, 2d ed., pp. 594-5. 



§ In a letter from Professor Renard dated Jan. 30th, 188? he announces to the 

 writer his identification of perofskite and sodalite in the Fernando phonolite. 



