234 GEOLOGY OF THE EUEEKA DISTRICT. 



rock .is of a light reddish-purple color, for the most part holocrystalline in 

 structure, with well developed secretions of the two principal minerals, 

 accompanied by varying amounts of biotite. The black bordered horn- 

 blende is almost exclusively confined to macroscopic individuals which are 

 for the most part decomposed into chloritic material, frequently giving the 

 altered rocks a green color. Magnetite, though not sufficiently abundant 

 to be regarded as an essential ingredient, appears to be evenly disseminated 

 throughout the groundmass, and quartz, wanting in most varieties, serves as 

 an accessory mineral in certain localities, especially in the more acid types. 

 The mica gradually comes in as the rock becomes more and more acidic 

 and as the acidic variety of this group predominates over the basic the 

 mica occurs as a frequent if not constant constituent. Hornblende-mica- 

 andesite covers much larger areas than normal horublende-andesite. A 

 characteristic and important feature of the hornbleude-andesite of this dis- 

 trict is the absence of all pyroxene. Very little of the horublende-andesite 

 is perfectly fresh and most of it has undergone a considerable decomposi- 

 tion, changing the color of the rock to light shades of red and yellow, 

 while those portions which are most altered appear nearly white. Opal 

 and chalcedony as secondary products are by no means uncommon in the 

 more altered varieties. 



Andesitic Peariiteu. Nearly every occurrence of hornblende-andesite is 

 accompanied by more or less extensive outflows of andesitic pearlites, 

 which so far as their mineral composition is concerned are quite similar to and 

 in many instances identical with the crystalline types. They are in general 

 more acidic in composition than the hornblende-mica-andesites, carry fewer 

 well developed crystals, and in place of the holocrystalline structure are 

 rich in glass base with microcrystalline secretions disseminated through it. 

 It is the almost infinite varieties of this glass base which give to these 

 pearlitic rocks their varying physical habit, color, luster, and density. 

 Owing to their more acidic character, quartz becomes more frequent, but is 

 shown rather in macroscopic secretions than in minute grains scattered 

 through the groundmass. Sanidine, wanting in the normal varieties, 

 may occur, although as a nonessential mineral, while biotite comes promi- 

 nently to the front and to the eye appears as the most abundant macro- 



