spurr.]       GEOLOGY    OF    THE    GOLDFIELDS    DISTRICT,   NEVADA.  119 
suggested;  and  since  museovite  is,  like  orthoclase,  essentially  a  sili- 
cate of  aluminum  and  potassium,  but,  unlike  it,  usually  contains  a 
weighable  amount  of  fluorine,  the  action  of  this  gas  is  probable.  In 
one  of  the  thin  sections  a  crystal  of  probable  fluorite  was  detected. 
Another  variation  is  pure  quartz,  which  occurs  in  the  alaskite  in  small 
blotches,  lenses,  and  even  in  masses  2  to  1  feet  in  diameter.  All  these 
are  intrusive  into  a  dark  siliceous  rock  (jasperoid),  which  is  probably 
the  result  of  the  silicification  of  an  original  limestone.  By  analogy 
with  the  similar  geologic  conditions  in  other  districts  in  the  same  gen- 
eral region — as  at  Silver  Peak  and  Belmont — we  may  suspect  that  the 
limestone  is  of  Cambrian  or  Silurian  age,  and  that  the  alaskite  may  be 
correlated  with  the  granitic  rock  intrusive  into  the  limestones  in  those 
districts. 
The  relation  between  the  rhyolite  and  the  alaskite  is  uncertain. 
Some  phases  of  the  alaskite  are  not  distinguishable  in  the  hand  speci- 
men from  the  rhyolite,  and  since  they  have  about  the  same  composi- 
tion one  is  tempted  to  consider  the  hypothesis  that  they  are  differently 
crystallized  portions  of  the  same  magma;  but  the  strong  flow  structure 
of  the  rhyolite  and  its  usually  glassy  groundmass  indicate  that,  even 
if  this  is  so,  they  were  formed  under  different  conditions  and  at  differ- 
ent times,  and  that  the  rhyolite  was  formed  near  the  surface,  the 
alaskite  at  a  considerable  depth. 
Auriferous  veins. — The  chief  veins  or  ore  bodies  being  prospected 
at  the  time  of  the  writer's  visit  are  broad  masses  of  white  to  purplish 
and  reddish,  iron-stained,  cherty  quartz,  extending  irregularly  in  a 
northerly  direction  (N.  10°  to  15°  W.).  There  are  usually  no  well- 
defined  walls  to  these,  and  the  width  of  the  mineralized  zones  varies 
from  many  feet  in  one  place  to  nothing  in  another.  As  reported,  the 
assays  made  up  to  that  time  are  very  irregular,  some  average  samples 
giving  $25  to  $50,  others  only  a  trace.  The  values  of  these  are  all  in 
gold,  very  little  or  no  silver  being  present. 
When  examined  under  the  microscope,  it  was  found  that  the  ore  is 
only  a  highly  silicified  rhyolite,  and  the  rhyolite  country  rock,  wher- 
ever examined,  is  itself  much  silicified.  There  are  many  of  these 
silicified  reefs,  usually  forming  the  crests  or  combs  of  ridges  on  account 
of  their  greater  hardness. 
Near  the  southern  end  of  the  principal  ridge  described,  an  average 
sample  of  the  alaskite  from  many  different  places  was  taken.  This 
contains  a  trace  of  gold  and  0.11  ounce  .silver.  In  the  same  place  a 
sample  was  taken  of  a  lens  of  feldspathic  quartz,  segregated  in  the 
alaskite.  This  gave  a  trace  of  gold  and  0.05  ounce  silver/'  This 
quartz  and  other  similar  lenses,  however,  are  of  a  different  type  from 
the  reefs  of  silicified  rhyolite  which  form  the  principal  ledges. 
a  Assayed  by  a  special,  careful  method  by  R.  H.  Officer  &  Co.,  Salt  Lake  City. 
