ALTERATION OF THE EARLIER ANDESITE. 209 



characters prove the truth of the change demonstrated in other cases by observed 

 transitions. Sometimes the alterations to adularia and to sericite go on side by 

 side, the original feldspar altering in part to one and in part to the other and 

 the two minerals sometimes forming an interlocking aggregate. 



ALTERATION OF THE GROUNDMASS. 



The microlitic, nearly glassy groundmass has been very largely decomposed 

 to or replaced by fine granular quartz, with fine muscovite (sericite), etc. The 

 quartz in the more highly silicitied specimens shows grains of larger growth and 

 is often segregated in bunches or veinlets. Pyrite and siderite are very commonly 

 disseminated throughout. Original zircon is frequently present. Sometimes 

 adularia can be made out as a portion of the fine secondary aggregate. Tiny 

 veinlets of adularia and others of quartz also seam the rock. 



Apatite, usually brownish or yellowish and slightly pleochroic, is relatively 

 abundant, and not being easily attacked by the agents which have brought about 

 the alteration of the rest of the rock is very characteristic in the considerably 

 silicified phases. 



ADVANCED STAGE OF ALTERATION. 



In the advanced stages of alteration nearly all the iron has disappeared; the 

 similar alteration products of the feldspars, the ferromagnesian minerals, and the 

 groundmass merge to form a quartz-sericite aggregate. The quartz varies in grain 

 from microcrystalline or nearly cryptocrystalline to moderately coarse, a charac- 

 teristic applying also to the quartz of the mineral-bearing veins, which are 

 mostly the extreme alteration product of the andesite, as is shown by both field 

 and microscopic study. In these extreme phases the quantity of sericite becomes 

 less and that of the quartz more. 



OCCURRENCE OF KAOLIN. 



While kaolin is not an ordinary alteration product in the siliceous alteration 

 of the earlier andesite, it is frequently present. Specimens in which it has been 

 detected have usually been taken from near a fault or fracture, or other water 

 course connecting with the surface. Therefore the hypothesis has been formulated 

 that while the sericite is manifestly the work of the vein-forming solutions the kaolin 

 is the work of descending surface waters, and is probably of later origin, the 

 kaolinization attacking the unsericitized residual feldspar. Kaolin and sericite 

 are frequently found together in varying proportion. 

 16843 No. 4205 14 



