104 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
altered  to  quartz,  the  main  wall  fractures  have  been  the  most  favor- 
able, so  thai  sometimes  a  hanging- wall  streak  of  quartz  and  a  foot- 
wall  streak  are  found  witli  only  altered  andesite  between  (fig.  4). 
Sometimes,  also,  either  the  hanging-wall  or  the  foot-wall  streak  may 
be  wanting.  Next,  streaks  of  quartz  parallel  with  the  walls  may  be 
found,  or  the  quartz  may  form  a  network  in  the  andesite.  Thus  the 
process  may  be  traced  to  the  stage  where  the  whole  of  the  andesite  is 
replaced  by  quartz,  forming  a  solid  vein  several  feet  in  width.     As  a 
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II 
Fig.  4.— Detailed  vertical  section  of  one  of  minor  Valley  View  veins  at  surface. 
rule,  however,  there  is  more  or  less  decomposed  andesite  forming  part 
of  the  vein. 
As  exceptions  there  are  found  streaks  of  quartz,  usually  small, 
within  the  vein,  which  show  crustification  and  comb  structure,  and 
thus  bear  evidence  of  having  been  formed  in  cavities.  These  cavi- 
ties, however,  were  of  irregular  shape  and  were  not  fissures,  properly 
speaking,  but  spaces  of  dissolution,  and  were  the  effect  of  the  miner- 
alizing waters  themselves. 
The  largest  example  of  a  crustified  vein  is  found  in  certain  parts  M 
the  Montana  Tonopah  workings,  where  the  cavities  were  sometimes  2 
or  3  feet  in  diameter  and  gave  rise  to  well-banded  ores. 
