DETAILED DESCRIPTION OF SLIDES. 117 



the microscope <a large amount of augite and some apatite and iron ore make 

 their appearance. 



The feldspars appear to be, without exception, triclinic. The large 

 crystals give labradorite angles, while the raicrolites appear to be for the 

 most part referable to oligoclase. The large feldspars in these andesites 

 very commonly show both albite and periclinic twinnings, and polysyn- 

 thetic individuals are frequently combined as Carlsbad twins. The feldspars, 

 which strangely enough, considering the fresh condition of the bisilicates, 

 are largely converted into calcite and quartz, contain some glass inclusions. 



Hornblende is present only in large masses of somewhat irregular out- 

 line, surrounded by a deep black border. The substance of the hornblende 

 is for the most part quite fresh, and of a deep greenish-brown. It contains 

 minute opaque inclosures, which are probably of the same nature as those 

 described under slide 421. The augites are very numerous, but small. 

 The percentage of the two silicates cannot differ greatly, but the hornblendes 

 give the character to the rock. The augites are very perceptibly dichroitic, 

 and are often crystallographically well developed. Many of them are twinned 

 and some are decomposed to fibrous chlorite, which polarizes in dark bluish 

 colors. There is much of this mineral in the slide which has evidently 

 been transported, and has settled in patches in which there is a strong tend- 

 ency to spherolitic arrangement. The patches of chlorite are accompanied 

 by quartz, which usually occuj^ies the periphery. In some cases particles 

 of epidote may be seen in the chlorite. 



The groundmass is made up of microlites of oligoclase, with a con- 

 siderable amount of augite, magnetite, and apatite. The last is almost all 

 of a deep brown color, and in consequence markedly dichroitic. There is 

 scarcely a trace of fluidal structure in this slide. 



Slide 228. KuoU uorthwest of Combination shaft. 



A second typical specimen. — Tlils is a purplish-gray rock, and in that respect 

 peculiar. Under the microscope it is very similar to that last described. 

 It shows a decided fluidal structure, but no glass base. The feldspars con- 

 tain good glass inclusions. Some of the hornblendes are twinned. In spite 

 of its purplish color this hornblende-andesite is microscopically typical of 

 the Washoe occurrences, and is illustrated in Fig. 30, Plate V. 



