Williams — Petrography of Fernando de Noronha. 187 



tinction angle is very high and twinning lamellae intercalated 

 parallel to the orthopinacoid (Oil) of common occurrence. To 

 all appearances this augite is identical with that of the Kaiser- 

 stnhl basalts which has been studied by Knop and found by 

 him to be titaniferous.* 



The olivine of this rock is in some slides quite abundant ; in 

 others less so. It is very fresh and of a pale yellow color. It 

 occurs commonly in small grains, but more rarely in sharp 

 crystals. 



The colorless component is for the most part nepheline in 

 large crystals, easily recognized by negative uniaxial character, 

 its parallel extinction, lack of cleavage and the ease with which 

 it is attacked by acids. Its specific gravity is from 2*61 to 

 2-59. 



An unstriated feldspar, probably sanidine, is present in small 

 quantity,f and also occasionally particles of a striated feld-spar. 



Sodalite in irregular patches is present in almost every sec- 

 tion, its isotropic substance being penetrated in every direction 

 by brightly polarizing needles of some zeolite which has re- 

 sulted from its alteration. 



Apatite is abundant in its characteristic forms. The iron 

 ore is octahedral — probably a titaniferous magnetite. A small 

 amount of the peculiar copper-colored and but slightly pleo- 

 chroic mica, so common in nepheline rocks, is also present. 



Nepheline-basalt is represented by three specimens in Prof. 

 Branner's collection. One of these (No. 45) from Morro 

 Francez agrees almost exactly with the figure and description 

 given by Renard of a similar rock from Rat Island;}: (Una 

 Rapta.) Its only porphyritic constituents is olivine in sharp 

 crystals or irregular grains. These are surrounded by a yellow 

 border of iron hydroxide and more or less opaque iron oxide 

 resulting from decomposition, and appear in the compact 

 black hand specimen as rusty yellow spots. The interior of 

 these olivines is however shown by the microscope to be quite 

 fresh and colorless. The groundmass is a fine aggregate of 

 idiomorphic augite crystals, octahedrons of magnetite and 

 nepheline, without any unindividualized base. 



No. 72, from Uha Rapta, differs from the last described speci- 

 men in having the porphyritic olivines nearly devoid of the 

 yellow border and in possessing a slightly coarser groundmass. 

 Within the latter may also be seen occasional flakes of a brown 

 mica and transparent octahedral crystals which are without 

 doubt perofskite. 



* Zeitschrift fur Krystallographie, x, p. 58, 1885. 



f Knop has shown that a barium orthoclase is present in the closely allied 

 nephelinite from Meiches in the Vogelsgebirge. Neues Jahrbuch fur Min., etc., 

 1865, p. 687. 



J Bull. d. Acad. roy. de Beige (3), III, No. 4, 1882. 



