ANDESITES. 33 



LATER AXDESITE ( BIOTITE-AUGITE-AXDE8ITE). 



Appearance. The later andesite is much like the earlier andesite, but is 

 slightly less siliceous. It is often found nearly fresh, and is in other places 

 profoundly decomposed, but the general process of decomposition is usually 

 different from that of the earlier andesite. Typically it is a rock of medium 

 dark color, mottled with crystals of feldspar and biotite, and sometimes with 

 pyroxene. It has generally been more or less altered and has turned dark green. 

 Near the surface the red of the oxidized iron combines with these colors to form 

 a characteristic rich purple. In some places the rock has been thoroughly altered 

 to calotte, chlorite, serpentine, quartz, siderite, and pyrite, and other secondary 

 minerals, and in other places has been so thoroughly leached as to be soft and 

 white. 



Composition and alteration, The porphyritic crystals or phenocrysts are 

 larger than in the earlier andesite, and are also much more abundant. There is 

 usually a graded crystallization, the crystals varying from very large size by 

 easy transitions down to tiny ones, which pass into the microlitic groundmass. 

 These crystals consist chiefly of feldspar, biotite. augite. and hornblende. 



The feldspar occurs as stout crystals, which have an irregular form caused 

 by complex twinning or intergrowth. When fresh enough the species may 

 be determined to be predominantly between andesine and labradorite, although 

 there are more calcic and more sodic varieties, varying between oligoclase and 

 bytownite. The feldspar is therefore more calcic than in the earlier andesite, 

 where it is predominatingly oligoclase-andesine. It is usually altered more or 

 less completely to calcite, chlorite, and quartz. Any one or two of these 

 alteration products may be scant or absent, and chlorite, kaolin, and zeolites 

 may be present. 



Biotite, which occurs in good-sized crystals, is usually bleached to a light- 

 green or transparent color, or is partly or wholly recrystallized to muscovite, 

 pyrite, calcite, and siderite, and occasionally a chloride aggregate. Triangular 

 skeletons of rutile (sagenite webs) are included in the biotite, and are left free 

 by its decomposition. The siderite, evidently derived from the breaking up of 

 the iron silicate in the biotite, general!}' occurs intimately throughout the crystal, 

 along cleavage lines, etc., while the pyrite is usually conlined to the outside or 

 the outer edges of the crystals. 



The augite is pale green and is usually altered. The alteration products 

 vary considerably, but are generally serpentine, chlorite, siderite, pyrite, calcite, 

 and quartz. Kaolin and the zeolites also sometimes occur. 



The hornblende is not abundant, and is almost always entirely altered. The 

 decomposition products are very similar to those of the augite, and include 

 16843 No. 4205- 3 



