182 Williams — Petrography of Fernando de Nor onha. 



fresh, although it is often surrounded by a rusty border proba- 

 bly the result of infiltration. The hornblende is dark brown 

 with almost complete absorption || c. It is surrounded by a 

 border of magnetite and augite grains frequently associated 

 with sphene crystals. The yellow spots macroscopically visible 

 mark the position of some mineral rich in iron which has now 

 wholly disappeared. They appear as a large yellow stain in 

 which irregular bits of some opaque iron oxide or a group of 

 sphene crystals are occasionally clustered. The whole ground- 

 mass of the rock is also everywhere spotted with similar, but 

 much smaller yellow stains, which have resulted from the rust- 

 ing of minute magnetite crystals. The groundmass itself con- 

 sists of a felt-like network of minute slender feldspar micro- 

 liths, with a large amount of non-polarizing interstitial matter 

 (colorless glass). The same small brightly polarizing needles 

 with high extinction angle, noticed in the last specimen, are 

 also present in this. Strong chlorhydric acid attacks this rock 

 to a certain extent, but this is due only to the presence of the 

 glassy base. No indication was discovered of the presence of 

 nepheline. It was determined by Mr. Coates to contain 55*6 

 per cent, of Si0 2 . 



No. 19. Hyalotrachyte, Bahia do Sueste. — This specimen and 

 No. 20, which was not examined microscopically, present a soft 

 chalky mass, of a white or cream color, and contain no por- 

 phyritic crystals. Under the microsco]3e only microliths of 

 sanidine are visible which show in their arrangement a decided 

 flow-structure and are imbedded in a very large proportion of 

 a somewhat globulitic glass. The structure of this rock is 

 typically trachytic and quite porous. 



No. 131. Hornblende andesite, loose pebbles from the 

 beach. — This contains abundant large crystals of plagioclase 

 and a few of brown hornblende scattered throu -h a reddish, 

 somewhat vesicular groundmass. No other porphyritic con- 

 stituents are present. The groundmass is made up of plagio- 

 clase microliths showing flow structure, and occasional minute 

 green augite grains, imbedded in a glassy matrix which is 

 itself apparently colorless, but rendered nearly opaque by the 

 amount of magnetite dust present in it. 



II. Phonolite. 



The eleven specimens of typical phonolite in Professor 

 Branner's collection all come from the eastern half of the main 

 island and were collected either along the north shore as far 

 west as the Peak, or on the line joining this eminence with the 

 one on the south shore called Atalaia Grande. All are repre- 

 sentative nephelinoid phonolites, gelatinizing readily with acid 



