98  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
Indeed,  there  is  every  evidence  that  the  veins  were  formed  by  ascend- 
ing hot  waters  succeeding  and  connected  with  the  early  andesite  intru- 
sion, and  that  these  waters  had  apparently  become  inactive  by  the 
time  of  the  later  andesites. 
The  mineralization  at  this  period  was  extraordinarily  active,  as  the 
profound  alteration  of  the  early  andesite  testifies.  Among  the  known 
veins  formed  at  this  period  those  of  the  Valley  View,  Mizpah,  and 
Montana  Tonopah  groups  are  the  most  important,  though  certainly 
there  are  others  which  have  not  yet  been  discovered.  These  veins 
carry  gold  and  silver,  in  the  proportion  of  about  1  part  of  gold  to 
100  of  silver,  by  weight.  They  are  unusually  free  from  base  metals — 
no  lead,  arsenic,  etc.,  has  been  detected.  In  some  places  there  is  a 
very  little  copper,  in  others  none.  The  gangue  is  quartz,  with  fre- 
quently a  mineral  which  is  a  variety  of  orthoclase  feldspar  (valen- 
cianite).  The  sulphide  ores,  so  far  as  developed,  show  primary  step- 
hanite,  with  probably  some  polybasite,  scant  pyrite,  and  comparatively 
rare  chalcopyrite.  Secondary  sulphides  coating  the  cracks  in  these 
ores  are  rub}'  silver,  argentite,  and  probably  pyrite  and  chalcopyrite. 
The  oxidized  ores  show  abundant  silver  chloride,  with  occasional 
bromides,  etc.,  and  sometimes  free  gold. 
Mineralization  subsequent  to  the  early  rhyolite  intrusion. — Along 
the  borders  of  the  gray  or  red  glassy  rhyolite  intrusions,  especially  in 
the  northern  portion  of  the  district,  there  has  been,  as  before  noted, 
considerable  alteration  and  mineralization,  which  must  be  attributed 
to  a  cause  similar  to  that  which  produced  the  veins  in  the  early  ande- 
site— namely,  the  action  of  hot  ascending  waters  immediately  suc- 
ceeding and  genetically  connected  with  the  rhyolite  intrusion.  This 
alteration  is  in  the  form  of  silicification  and  the  formation  of  pyrite,  | 
and  has  acted  on  the  rhyolite  as  much  as  the  intruded  rock.  Quartz 
veins  have  been  formed,  in  the  majority  of  cases  relatively  small. 
These  veins  contain  precious  metals,  but  usually  very  irregularly 
distributed.  High  assays,  especially  on  the  surface,  may  even  be 
obtained,  but  it  is  likely  that  some  of  these  are  the  result  of  the  well- 
known  process  of  increase  of  values  at  the  surface  during  oxidation. 
Specimens  of  the  veins  have  a  general  resemblance  to  those  of  veins 
of  the  early  andesite,  and  show  pyrite,  often  finely  disseminated  in  a 
quartz  gangue.  Some  of  these  veins  have  been  found  to  be  of  consid- 
erable size;  therefore  it  may  often  be  difficult  to  distinguish  them 
from  the  andesitic  veins  without  study  of  the  rock  in  which  they  lie. 
Chemically  they  appear  to  be  characterized,  so  far  as  yet  developed, 
by  very  low  average  values  and  by  the  frequent  but  not  regular 
preponderance  of  the  gold  values  over  those  of  silver. 
Mineralization  subsequent  to  the  later  rhyolite  intrusion. — It  has 
been  described  how  the  white  rhyolite  of  Oddie,  Rushton,  and  Ararat 
hills  is  probably  later  than  the  dense,  usually  gray  rhyolites  referred 
to  above.     For  example,  the  top  of  Ararat  seems  to  be  a  plug  or  neck 
