ANUESIT1C PEARLITE AND DACITE. 



The rock in question is a pearlite with a variously modified glassy grouudinass 

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

 quart/. 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 orthoclase. In only a very few 

 individuals was the nature of the feldspar in doubt and its orthotomic 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; crystal, a carlsbad twin in hand 

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

 brilliant unstriated basal cleavage, proved optically to be sanidiue. The crystal form 

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

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

 in many instances the stria? are scarcely perceptible, and in a few cases they are 

 altogether wanting. The angles of extinction range from a few degrees to between 

 25 and 31. The feldspars are therefore labradorite in part, though a large propor- 

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

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

 inclusions of glass and of the grouudmass are the same as those in the feldspar of 

 the pyroxene-andesite and hornblende-mica-audesite. 



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- 

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

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

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

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

 position to almost completely altered hypersthene. Fig. 5, PI. in. The hyi>ersthen' 

 MON XX 24 



