110  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225. 
waters  which  produced  the  early  andesite  veins  were  unusually  active, 
and  a  large  amount  of  vein  formation  is  indicated.  It  is  beyond 
question,  therefore,  that  the  veins  thus  far  found  are  only  a  fraction 
of  those  which  exist  in  the  vicinity.  These  veins  will  be  chiefly 
unoxidized,  since  they  are  protected  from  oxidation  by  capping  rocks. 
In  them  undoubtedly  large  portions — probably  much  the  larger  j)or- 
tions — will  be  relatively  barren  quartz,  in  which  shoots,  chimneys,  or 
irregular  bunches  of  rich  ore  may  be  expected  to  a  considerable  depth. 
These  quartz  veins,  carrying  silver  and  gold  and  occurring  in  Ter- 
tiary andesite,  belong  to  a  larger  group  of  veins  occurring  in  similar 
andesite  and  found  along  a  north-south  zone.  These  veins  are  exem- 
plified by  the  Comstock  in  Nevada  and  by  the  important  district  of 
Pachuca  in  Mexico.  Probably  Pachuca  is  the  nearest  analogue,  and 
its  description,  as  given  by  the  Mexican  geologist,  Ordonez,  may  be 
of  value  in  considering  the  characters  of  the  Tonopah  veins.  Judging 
from  these  districts  it  is  possible  that  in  depth  a  rather  larger  propor- 
tion of  the  baser  metals,  such  as  copper,  and  possibly  lead  and  zinc 
(although  no  traces  of  the  last  two  have  as  yet  been  found  in  the 
veins),  may  come  in,  and  that  the  values  in  gold  and  silver  may 
decrease  somewhat.  This,  however,  if  it  happens,  should  do  so  at  a 
considerable  depth. 
