Williams — Petrography of Fernando de NoronTia. 1 85 



ating " ocellar " tufts crowded with small nepheline crystals. 

 Its lack of well developed form indicates that it was a late 

 product of crystallization. The late Prof. C. E Wright deter- 

 mined the inclination of the axis of greatest elasticity (a) to the 



vertical axis (c) as between 7°'4: l, 2 / and 16°; double refraction 

 negative. Among the more compact crystalloids in the ground- 

 mass of No. 41 a few cross-sections were found which showed 

 the prismatic development. These sections were hexagonal 

 bounded by (110) and (100), while (010) is wanting. The pleo- 



chroism, which is often quite strong, is : b (b) green ; c (a nearly) 

 green; d (c) yellow; absorption. a> o^> c. All of these 

 determinations agree closely with those given by Rosenbusch.* 



The other minerals observed were sphene, magnetite and 

 very sparingly apatite, all of which present their character usual 

 in the phonolites. Noticeable for the sphene is its frequent 

 occurrence in the black corrosion rims around the hornblende. 



A point of considerable interest with regard to the Fer- 

 nando phonolites is the possible occurrence in them of inclu- 

 sions of eleolite-syenite as mentioned by Prof. Rosenbusch.f 

 The material in hand is not however sufficient to definitely 

 decide this question. In a communication from Prof. Orville 

 Derby of Rio de Janeiro, in whose hands the entire collection 

 made by Professor Brauner formerly was and who still has 

 charge of the greater part of it, he says that in this large 

 amount of material only a single specimen showed any sign of 

 such an inclusion. This was a pale gray and rather coarse 

 grained phonolite from the Peak (No. 50) which contained a 

 sharply defined inclusion, apparently of a typical eleolite-syen- 

 ite about 3 cm square. Of this inclusion and a portion of the 

 surrounding phonolite one fragment was sent by Prof. Derby 

 to Prof. Rosenbusch and another to myself. Prof. Derby has 

 also kindly furnished me with a photograph of this specimen. 

 If the inclusion is really an eleolite-syenite it belongs to an 

 exceptionally porphyritic type, inasmuch as more than one-half 

 of its small area (3 X 3 cm ) is occupied by a single crystal of a 

 blue iridescent orthoclase (2xli cm ). Both Profs. Derby and 

 Branner agree that inclusions of this sort must be of rare 

 occurrence in the Fernando phonolites,;}; and in no way com- 

 parable to those so abundant in the tinguaites of the Rio de 

 Janeiro neighborhood. Prof. Derby is inclined to regard this 

 inclusion as an early and intratelluric secretion of the phonolite 

 magma. This view§ may derive some support from the fact 



* Die massigen Gesteine, 2d ed., p. 616. 

 f Die massigen Gesteine, 2d ed., pp. 91, 628 and 821. 

 % Quart. Jour. Geol. Soc, xliii. p. 459, 1887. 



§ Opinion expressed in a letter to the writer (cf. reference just cited and Neues 

 Jahrbuch fur Min., etc., 1887, II, p. 258.) 



