ruRR.]  ORES    OF    GOLDFIELD,    NEV.  133 
v\hich  is  sometimes  of  granitic  structure,  and  sometimes  very  fine 
grained,  even  resembling  quartzite.  White  mica  or  muscovite  is 
ometimes  present,  and  pure  quartz  veins  or  dikes  of  similar  origin 
Iso  occur.  These  alaskitic  rocks  are  intrusive  into  a  dark  siliceous 
ock  (jasperoid),  which  is  probably  the  result  of  the  silicification  of 
.n  original  limestone.  We  may  believe  this  limestone  to  have  been 
Eleozoic,  and  the  alaskite  is  certainly  pre-Tertiary.°  On  the  north 
nd  of  the  mountain  the  rock  is  a  very  much  altered  rhyolite,  in 
feich  are  broad  masses  of  white  to  purplish  and  reddish  cherty 
[uartz,  extending  irregularly  in  a  northerly  direction.  This  quartz 
s  simply  a  highly  silicified  rhyolite.  The  silicified  areas  have  ill- 
lefined  walls,  and  the  highly  mineralized  portions  which  they  in- 
lose  are  very  irregular. 
Product} re  area. — The  area  of  known  ore  bodies  has,  since  last  year, 
pread  far  out  from  Columbia  Mountain,  so  that  now  it  may  be  esti- 
aated  at  about  G  miles  square.  The  most  productive  area  is  inclosed 
n  a  square,  2  or  2-|  miles  in  either  direction.  The  chief  mines  at 
he  present  time  are  the  Jumbo,  the  Combination,  the  Florence,  and 
he  January,  all  grouped  together  about  a  mile  south  of  the  south- 
ast  end  of  Columbia  Mountain.  About  5  miles  southeast  of  Co- 
imbia  Mountain  is  the  Diamondfield  group,  including  the  Vernal, 
le  Quartzite,  and  the  Black  Butte.  Shipments  have  also  been 
lade  from  this  group.  Other  ore  deposits  have  been  developed  in 
arious  parts  of  the  field. 
Nature  of  rocks. — The  rocks  in  that  part  of  the  field  examined  dur- 
H  this  last  trip  (the  region  of  Columbia  Mountain,  Diamondfield, 
^id  the  Jumbo  group  of  mines),  were  found  to  be  almost  entirely 
plcanic,  consisting  of  rhyolite,  rhyolite  tuffs,  andesite,  and   basalt, 
11  probably  of  Tertiary  age.  The  alaskite  and  jasperoid  of  Columbia 
ountain  are  hardly  represented  in  the  surrounding  district.''  The 
|redominant  rocks  are  abundant  rhyolites  and  andesites,  while 
i salt  is  rare.  One  andesite  from  near  the  Tonopah  Club,  examined 
Ijicroscopically,  is  a  hornblende-andesite,  resembling  the  earlier  an- 
ssite  at  Tonopah.  A  patch  of  basalt  from  near  the  Florence  is  an 
bgite  basalt,  like  the  basalt  of  Siebert  Mountain  at  Tonopah.  The 
lyolite  resembles  the  rhyolite  of  the  Gold  Mountain  district,  which 
bs  about  4  miles  south  of  Tonopah,  on  the  road  between  Tonopah 
jid  Goldfield,  and  this  Gold  Mountain  rhyolite  again  resembles 
•josely  some  of  the  phases  of  the  earlier  (dacitic)  rhyolite  at  Tonopah. 
fie  relative  ages  of  the  rocks  at  Goldfield  have  not  been  determined, 
|t  it  may  be  that  they  are  like  similar  rocks  at  Tonopah. 
'  The  writer  regards  its  age  as  post-Jurassic. 
''  In  a  press  bulletin,  given  out  earlier,  the  rock  constituting  the  ore  at  the  Tonopah 
<'ib  mine  was  characterized  as  probably  jasperoid  (silicified  shaly  limestone).  The 
S'cimeu  collected,  however,  proves  upon  microscopic  examination  to  be  a  somewhat  (lis 
iegrated  glassy  volcanic  rock. 
