54 GEOLOCIY OF TOE COMSTOCK LODE. 



PisTKiCT. The jirc'valeiit viiriet}* contains nnu'h auglto; sometimes even 

 more aug-ite than hornblende, bnt no mica. There are also micaceons oc- 

 cinTence>=, and these are nearly or quite free from augite. 



Hornblende. — Tlic hombleude is always brown in the fresh rocks, occa- 

 sionally with a reddish, and often with a greenish, tinge. Of course it is 

 highly dichroitic, and the angles of extinction appear' in some cases to 

 exceed 20°. The crystal form is the ordinary combination of prism and 

 clinopinacoid ; terminal faces too, though rarer than in augite, sometimes 

 oci'ur. The cleavages are usually developed, though in the freshest crystals 

 thev are marked bv such narrow lines that under a low power they seem 

 absent. In one case a clinopinacoidal cleavage was observed. Twins are 

 very conunon. Glass inclusions occur, generally as negative crystals, and 

 apatites are often inclosed. ^ ery rarely indeed a slide shows a particle or 

 fragment of hornblende inclosed in another mineral, but as a rule all the 

 hornblende is concentrated in jiorphyrltical crystals, and does not enter into 

 the groundmass. I discovered only a single very small area in which the 

 rock shows a large amount of hornblende distributed through the groundmass 

 in minute particles; and even in this case the diil'erence seems to be one of 

 degree rather than of kind; for the minute hornblendes are in laige part 

 well developed and appear to be "crystals of first consolidation." The black 

 border accompanies all the hornblendes in most of the andesites. Often it is 

 very heavy, and sometimes so encroaches on the crystal that little or none of 

 the mineral appears in the center. I have noticed no Instances in which black- 

 bordered hornblendes accompany crystals of the same mineral without 

 black borders. In several cases a double black border is visible, the inner 

 one concentric with the outer, leaving a zone of hornblende between. 

 Such a case is described under slide 450, and shown in Fig. 17, Plate III. 

 I venture to offer some speculations on this phenomenon elsewhere. The 

 black border is readily soluble in chlorhvdric acid, even where the slide con- 

 tains ilmenite A very few slides show hornblendes without black borders. 

 One such exception is from the Sat.o Tittuiel in a region of intense sol- 

 fataric activity. Here the hornblendes are in part very fresh, while tlie 



' I say appear, because it is seldom possible to make absolutely sure that a crystal is cut exactly 

 in either of the three principal zones, and a very small obliquity often greatly alters the angle of extinc- 

 tion. 



