BY C. A. SUSSMILCH AND II. I. JENSEN. 187 



No.X.639. Trachyte-Porphyry. Loc. : east side of The Old 

 Man. 



Handspeciraen : a reddish rock containing large, black, 

 iridescent, irregularly cracked phenocrysts, and felspar plieno- 

 crysts, some of which are vesicular, others quite pumiceous. 



Texture : holocrystalline, porphyritic, with even-grained 

 hypidiomorphic, granular base. 



Composition : the base consists essentially of untwinned ne])hi- 

 linitoid and prismatic felspar, decomposing to kaolin. The 

 felspar phenocrysts are of three kinds : (1) a clear glassy sani- 

 dine: (2) a moiree microperthite, with patchy extinction; and 

 (3) a basic labradorite. All these are broken, and surrounded 

 with a corrosion-rim, round which a zone of orthoclase of the 

 second generation appears. 



The dark mineral is haematite, which has often a nucleus of 

 magnetite, and a rim of limonite. Leucoxene occurs as a decom- 

 position-product, indicating that the magnetite is titaniferoas. 



Name : the magma from which this is derived is a mixture of 

 the typical trachytic and typical andesitic, The vesicular pheno- 

 crysts and pumiceous aggregates indicate that, after the 

 formation of the phenocrysts, magmatic vapours, charged with 

 acids, penetrated and leached the phenocrysts, perhaps at the 

 same time altering magnetite to limonite. 



c. The Ayidesites. The dominant type of andesite is that met 

 with on The Old Man. 



No.X.634. Black, vesicular, porphyritic Andesite. Loc. : Top 

 of The Old Man. 



Texture : holocrystalline; porphyritic, hiatal; hyalopilitic base. 



Composition : the constituents are felspar, augite, ilmenite, 

 magnetite, and glass. The phaneric felspars are twinned on the 

 Carlsbad, Albite, and Pericline laws ; zoning is frequently 

 observed; refractive index higher than that of canada balsam; 

 and extinction-angle 15'^ to 20*^. The microlites of the base liave 

 apparently the same composition. This constituent is, therefore, 

 an andesine or acid labradorite. The augite occurs in small, 



