124 GEOLOGY OF THE COMSTOCK LODE. 



Slide 375. Outcrop at junction of Sutro and Quarry roads. 



Micaceous hornblende-andesite. Thls is a SOUiewliat tracliy tic-looking I'Ock, With 



very white feldspars embedded in a rough gray groundmass. Mica is also 

 visible, though not prominent. Under the microscope it is plainly only a 

 micaceous variety of the suri'ounding hornblende-andesite. The feldspar is 

 wholly triclinic, and the large crj'stals give the angles of extinction of 

 labradorite. They are much decomposed, but contain recognizable glass 

 inclusions. There are also numerous secondary fluid inclusions. The mica 

 is decomposed, largely to chlorite and epidote. There are also hornblendes, 

 or rather their outlines. Much of the groundmass is devoid of microlites, 

 shows a feeble aggregate polarization, and is probably a partiallj^ devitri- 

 fied glass. I could find nothing which could be interpreted as augite. 



Slide 326. Sutro Tunnel^ 17,100 feet from entrance. 



Specimen showing stages of decomposition. — This is a greeuisli-gray rock, with por- 

 phyritical crystals of feldspar and hornblende. It also shows some pyrites, 

 and has evidently undergone considerable decomposition. Under the micro- 

 scope the slide shows much brown hornblende, some augite, triclinic feld- 

 spars, and an andesitic groundmass. The hornblendes are peculiarl)^ inter- 

 esting because they exhibit the process of decomposition in all its stages. 

 The hornblende is brown, much of it is twinned, and none of it shows black 

 borders. The first step in the degeneration is the formation of chlorite, which, 

 of course, largely follows the cleavages. In some cases narrow, even bands 

 penetrate a crystal nearly from one end to the other like twin lamellae, 

 while in other instances irregular patches of chlorite occur in the hornblende. 

 In some such patches, and still better in others which are distributed through 

 the groundmass, and may or may not represent former crystals of horn- 

 blende, the formation of epidote may be followed; its j^rismatic microlites 

 are to be seen invading the chlorite, just as in the McKihhen Tunnel diorites. 

 Other patches of chlorite are in process of conversion into, or substitution 

 by, calcite. Where this change goes on in a partially decomposed crystal 

 of hornblende, the central portion of the area is generally occupied by 

 the fresh mineral and chlorite; whereas the calcite, sometimes accompanied 

 by a little quartz, occupies the border of the pseudomorpli. When the 



