186 Williams — Petrography of Fernando de Noronha. 



that another phonolite specimen of this collection (No. 5) does 

 undoubtedly contain such coarse-grained secretions composed 

 of large crystals of nepheline and sanidine. The appearance 

 of these is however undeniably different from that of the 

 apparent inclusion ; and, in the absence of more abundant 

 material, the writer would refrain from expressing a decided 

 opinion. 



III. Basaltic Rocks. 



No. 31. Nepheline-basanite, Sao Jose Island. — This is a very 

 compact black rock in which minute crystals of olivine and 

 augite are macroscopically visible. Under the microscope 

 these two constituents are seen to be present in abundant well- 

 formed crystals possessing their usual characters. The ground- 

 mass in which they are imbedded is a fine holocrystalline 

 aggregate of augite and plagioclase microliths and magnetite 

 grains. Some interstitial nepheline is also present, but this is 

 of so small amount as to ally this rock to the feldspar basalts. 



No. 34 is a fragment of a coarsely granular inclusion or secre- 

 tion in the last described specimen. It is composed of per- 

 fectly fresh olivine and enstatite with hardly a trace of any 

 other mineral, and is analogous to the so-called " olivine bombs," 

 common in many basaltic rocks but whose origin is still a mat- 

 ter of discussion. 



No. 2. Nephelinite-dolerite. — A dark gray, coarse grained 

 rock collected in the !N".E. part of the main island, but nowhere 

 found in situ. Long black crystals of augite and small grains 

 of olivine are macroscopically visible. The microscopic char- 

 acter of this rock agrees very exactly with Rosenbusch's de- 

 scription of the doleritic type of nephelinite ;* and, in spite of 

 its comparatively large amount of olivine, it is here assigned 

 to this class on account of its close resemblance to the classic 

 occurrences at Meiches and Lobau. 



The structure of this rock is holocrystalline and granular 

 (hypidiomorjih-kdmig in the sense of ftosenbuschf) like that 

 of a plutonic mass. Its most prominent constituent is augite. 

 whose jet black crystals are often a centimeter or more in 

 length and impart to the hand-specimen a porphyritic appear- 

 ance. Under the microscope these crystals have a brownish 

 red color, a decided pleochroism and a distinct zonal structure. 

 The colors of the different rays are c and b reddish brown ; a 

 greenish yellow; absorption c>b / y a. The outer zones are 

 invariably more intensely colored (i. e. richer in ferric iron) 

 than the inner and are hence more pleochroic. The hour-glass 

 structure is also frequent as a form of zone growth. The ex- 



* Die massigen G-esteine, 2d ed., p. 791. 

 fib., p. 11. 



