ALTERATION OF THE LATER ANDESITE. 239 



groundiuass shows fresh crystals of feldspar and augite largely altered to 

 serpentine. 



Under the microscope the groundmass is seen to be densely packed with 

 microlites of feldspar and augite partly altered in the same characteristic way as 

 the phenocrysts, which are to be next described. Magnetite is plentiful. Siderite 

 in small specks is scattered throughout in characteristic cloudy, semitransparent 

 white aggregates. Sometimes this mineral forms a rim around the magnetite, 

 showing derivation from it. In some cases there may be discerned characteristic 

 rhombic cleavage and even rhombic crystal outlines. 



The phenocrysts vary in size from the microlites up to occasionally moderately 

 large crystals. They are of feldspar and colorless augite. 



The feldspar is in general remarkably fresh. It is usually striated, and is 

 sometimes in complex forms. Two optical determinations by the Fouque method 

 showed, in one case andesine, in another labradorite. It is seamed and cracked, 

 and the cracks are filled with calcite and serpentine, evidently infiltration products. 

 In places the feldspathic substance is attacked and replaced by these minerals. 



Idiomorphic colorless augite is abundant. Alteration to calcite and serpentine 

 is present in all stages, so that while some augite crystals are unattacked others are 

 completely transformed. Chlorite was not identified. Small apatite crystals were 

 noted as inclusions in the augitc. 



2. Nearly fresh later andesite (SJfl) from Halifax shaft, 275 feet down. 

 Greenish rock, showing phenocrysts of glassy feldspar (altered along the outside), 

 greenish augite, and biotite. 



Under the microscope the groundmass is glassy, with fine microlites of fresh 

 feldspar and augite, magnetite, micaceous hematite, and considerable cloudy kaolin. 

 Quartz (secondary?) is common. 



The phenocrysts are relatively few. The feldspar is fresh, and one crystal 

 was determined as andesine. Sometimes it is altered to a cloudy white aggregate 

 of kaolin along its margin, and in one case a small crystal was completely altered to 

 calcite, kaolin, and quartz, the clear quartz forming an envelope for the rest of the 

 crystal. The fresh feldspar is cracked and infiltrated with micaceous hematite. 

 The augite is pale green; no alteration of it was noted. 



Fresh brown biotite crystals sometimes have a border of magnetite. 



3. Entirely altered later aiidesite (331) from North Star shaft, 305 feet down. 

 This has a general gray color, with dull-white altered feldspar phenocrysts; it 

 contains many small specks and seams of pyrite. Under the microscope it is seen 

 to be entirely altered. In the fine groundmass can be distinguished fine secondary 

 quartz and chalcedony, calcite, pyrite, siderite, and some zeolite needles. 



The phenocrysts are also entirely altered. Pseudomorphs after biotite were 

 distinguished, consisting mainly of quartz and siderite. Numberless tiny crystals 



