THE ROCKS OP THE WASHOE DISTRICT. 55 



remainder of the rock is not. Gases occur on tlie surface in which it is 

 evident that the bhxck border lias been attacked before tlie hornldende, and 

 tliis shde may represent such an instance. 



Augite. — The augites are essentially similar to those of the au<;itc-ande.sites, 

 but it may be mentioned that in one case a pinacoidal cleavage was observed 

 which I have never noticed in the augite rock. In a slide from an area 

 which I have classed as hornblende-andesite, the augite also shows heavy 

 black borders like those of the hornblende. Augite is frequently present 

 in the groundmass in crippled crystals and irregular grains, which appear 

 to me referable to "secondary consolidation." The proportion of augite to 

 hornblende is always large e.xcept in the micaceous andesites, and, according 

 to Professor Rosenbusch, this is common elsewhere ; while in the augite- 

 andesites of the Washoe disthict there must be more than one hundred 

 times as nuich augite as hornblende. I have not always seen my way, 

 however, to determining slides containing a decided excess of augite other- 

 wise than as hornblende-andesite, for such rocks occur in areas which appear 

 characteristically hornblendic. While in such cases, which are exceptional, 

 the endeavor has been made to take all the circumstances into considera- 

 tion, it must be confessed that where very augitic hornblende-andesites and 

 very hornblendic augite-andesites come together, the lines of contact laid 

 down may be somewhat inaccurate, though the error cannot be great; aiul 

 as these conditions appear to i)revail only along Cedar Hill Cailon it is of 

 small importance. 



The mica of the andesites gives the interference figure of biotite. It 

 is frequently black-bordered, and the border is usually deeper than that 

 around the accompanying hornblende. 



Feldspar. — Thc feldspars of the hgrnblende-andesites are nearly without 

 exception tricliiiic, and of course they can be divided into porphyritical 

 crystals of fir.st consolidation and nu'crolites of second con.solidation. As for 

 the species, the porphyiitical cry.stals are either labradorite or anorthite, and 

 the microlites either oli'goclase or labradorite. Crystals giving anorthite 

 angles of extinction have been found in only a few cases, and in these I 

 suspect a mixture of anorthite and labradorite, because while many crystals 

 seemed so placed that had they jjeen anorthite they must have given ano-les 



