122 GEOLOGY OF THE COMSTOCK LODE. 



remarkably few augites, while in this I was unable to find a single one, in 

 which respect it and slide 375 form the only exceptions among the earlier 

 andesites of the Distkict. The hornblende is of the same color and general 

 character as that in slide 311, but the crystals are fewer in number and 

 larger in size. The decomposition of the hornblende in this specimen is 

 peculiar and interesting. The first change appears to have been to chlorite 

 masses, of which a few are still surrounded by fresh hornblende. Some spots 

 of this chlorite contain bunches of epidote, evidently formed from it, but 

 much of the chlorite has been converted into, or has given place to, quartz. 

 Decomposition has set in along cracks or cleavages of the hornblende crys- 

 tals, producing little veins of chlorite, and the substitution of quartz for 

 chlorite has subsequently taken place from the hornblende walls of the veinlet 

 towards the central line, but has sometimes left a narrow seam of chlorite 

 along the middle of the vein. This is shown in Fig. 3, Plate II. A question 

 might be raised as to whether the quartz had not first partially filled the veins, 

 the chlorite representing a subsequent infiltration; but the thoroughly fresh 

 condition of the hornblende walls seems to forbid such a supposition. In some 

 of the smaller crystals a fresh kernel of hornblende is seen surrounded by 

 a zone of quartz, and this again by a narrow border of epidote. Taking the 

 appearance just described into account, it appears to me probable that these 

 hornblendes were in process of conversion into chlorite from the edges, and 

 that an alteration to epidote had begun on the periphery, when the silicify- 

 ing action set in, leaving the hornblende and the epidote unaffected. The 

 hornblendes carry small bubble-bearing glass inclusions. The slide also 

 contains much decomposed mica. That mineral has been replaced by chlo- 

 rite and this, again, is full of patches of epidote, evidently parasitic on the 

 chlorite. A portion of this chlorite, as well as that derived from horn- 

 blende, appears to have been converted into quartz. The feldspars are 

 much decomposed, but are evidently triclinic. 



Slide 454. Cedar Hill Canon, 1,500 feet due west of Water Tunnel. 



Highly augitic variety. — This is a dark bluish rock, which shows a considerable 

 number of macroscopical hornblendes. Under the microscope the augite is 

 seen to predominate over the hornblende, but as it occurs in a typical horn- 



