ANDESITIU I'KAIJLITK AM) D.VCITE. 



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The rock in question i.s a pearlite with a variously luodirted jrhis.sy }>:rouii(liiias.s 

 and abundant phenocrysts of feldspar, hornblende, hypcrsthene, augite, biotite and 

 (jnart/.. The character of these minerals is constant throughout and similar in most 

 respects to those found in the more crystalline andesites. They may be here described 

 then for the whole series. The feldspar is triclinic, eight-tenths of the individuals 

 being striated and the remainder giving angles of extinction for twins in the zone at 

 right angles to the composition plane too great for ortlioclase. In only a very few 

 individuals was the nature of the feldspar in doubt and its orthotoinic nature possi- 

 ble, but even here the evidence was entirely negative. It is probable that orthoclase 

 is present to a very limited extent — one macroscopic crysta*!, a carlsbad twin in hand 

 specimen 51, a hornbleude-mica-andesite from Sierra Canyon like 52, having a 

 brilliant unstriated basal cleavage, i)roved optically to be sanidiue. The crystal form 

 of the plagiodase is like that already described, but a greater number are in angular 

 fragments. The zonal structure is marked and the polysyuthetic twinning irregulai-; 

 in many instances the stria- are scarcely iierceptible, and in a few ca.ses they are 

 altogether wauting. The angles ol' extinction range from a few degrees to between 

 25° and 31°. The feldsi)ars are therefore labradorite in part, though a large jjropor- 

 tion are probably andesine. In several thin .sections they corresjtoud to anorthite, 

 which will be more fully noticed under the description of the different varieties. The 

 inclusions of glass and of the gioundmass are the same as those in the feldsjjar of 

 the pyroxenc-audesite and horublende-micaandesite. 



The hornblende is without a dark border of any kind. It is in all other 

 respects like that found in the pyroxene-andesite and the fresher hornblende-mica- 

 ande.site of Sierra Canyon (52). It occurs in well developed crystals of a greenish 

 brown colcu- with very strong pleochroism. There are no characteristic inclusions, 

 it being for the most part quite five from them. It appears to have withstood decom- 

 position better than the hypersthem?, as it shows no signs of alteration even in juxta- 

 position to almost completely altered liy|)ersthene. Fig. 5, PI. in. The iiypersthene 

 MON XX 24 



