130 GEOLOGY OF THE COMSTOCK LODE. 



glass when fresh. Among the many pseudomorphs of chlorite after augite 

 which it contains, one is especially beautiful, and is illustrated in Fig. 4, 

 Plate II. Decomposition has evidently started from the cross-fractures, 

 and also from the centers of the fragments isolated by the cracks, but these 

 two varieties of decomposition have j)roceeded somewhat diiferently. The 

 chlorite around the exteiior of the crystal, and along the cracks, betrays 

 structure only by a very slight dichroism; between crossed Nicols it is not 

 perceptibly luminous. The chlorite which has developed from the cen- 

 ters of the fragments is brownish-green, radially fibrous, strongly dichroitic, 

 and polarizes in dull-brownish colors. 



LATER HOENBLENDE-ANDESITE. 

 Slides 472 and 473. Quarry 2,000 feet northeast of Sutro Tunnel Shaft III. 



Trachytic-iooking variety. — This is a vcry coarsc-graiucd soft rock, with large 

 porphyritical feldspars and visible mica and hornblende. Its ground mass is 

 purplish-gray. This rock is that commonly employed on the Comstock for 

 engine foundations and the like. 



Under the microscope it is seen to consist of plagioelase, hornblende, 

 mica, and magnetite, with a few Carlsbad twins and apparently simple 

 crystals which might be sanidin. Some of these last show minute stripes 

 under close examination, and others can be shown to be plagioelase by 

 their angles of extinction. The highest angles of extinction of the properly 

 oriented feldspars indicate labradorite ; but many of the larger crystals show 

 a strongly marked zonal structure. Inferences as to their composition 

 have been drawn on page 68. The feldspathic microlites appear to 

 be oligoclase. The mica is bi'own and intensely dichroitic. Cleavage- 

 scales give an hyperbolic interference figure, whicli could not for a moment 

 be confounded with a cross, and it is either a biotite in which the angles of 

 extinction- are uncommonl}- large or another species The hornblendes 

 are brown and well crystallized, and are all black-bordered; but while some 

 have comparatively narrow borders, others are almost wholly converted to 

 magnetite, leaving only a particle of the fresh mineral near the center. The 



