spurr.]  ORE    DEPOSITS    OF   TONOPAH,   NEVADA.  91 
The  values  in  the  ores  are  entirely  gold  and  silver.  The  propor- 
tion of  gold  to  silver  is  usually  remarkably  uniform,  and  is  the  same 
in  both  the  oxidized  and  the  sulphide  ores,  namely:  The  gold  is  to 
the  silver,  by  weight,  as  1 :  90  or  1 :  100,  making  the  values  about  two- 
sevenths  gold  and  five-sevenths  silver.  In  some  of  the  excessively 
rich  ores  the  proportion  of  gold  becomes  greater,  occasionally  making 
up  half  of  the  total  value. 
TOPOGRAPHY. 
That  the  mountain  range  in  which  Tonopah  is  situated  is  of  volcanic 
origin  is  shown  by  its  topography.  It  consists  of  a  series  of  eroded 
mesas  and  detached  or  connected  hills.  These  mesas  and  hills  are 
irregularly  distributed,  with  no  definite  valley  systems  between.  They 
are  separated  by  patches  of  sloping  or  rolling  country,  largely  covered 
with  "wash,"  or  debris  worn  from  the  hills.  The  mesas  are  the  rem- 
nants of  volcanic  flows;  the  isolated  or  connected  irregular  hills  are 
denuded  volcanic  necks. 
An  excellent  topographic  map  of  the  country  immediately  around 
Tonopah,  on  the  scale  of  800  feet  to  an  inch,  has  been  made  by  Mr. 
W.  J.  Peters,  of  the  United  States  Geological  Survey.  On  this  it  is 
seen  that  the  general  elevation  of  the  slight  valley-like  depression  in 
which  the  town  of  Tonopah  lies  is  about  6,000  feet  above  sea  level, 
while  the  top  of  Butler  Mountain,  which  is  the  highest  mountain  near 
the  town,  is  7,160  feet. 
GENERAL  GEOLOGICAL  HISTORY. 
Pre- Tertiary  formations. — In  the  immediate  vicinity  of  Tonopah  the 
rocks  are  all  Tertiary  volcanics  or  sedimentary  tuffs  belonging  to  the 
volcanic  period,  and  composed  partly  of  ash  and  partly  of  silt  derived 
from  the  erosion  of  the  lavas.  Eight  or  nine  miles  south  of  the  camp, 
however,  there  is  limestone,  very  likely  of  Cambrian  or  Silurian  age,  and 
in  this  granitic  rock  is  intrusive.  Limestones  and  granites  occur  also 
several  miles  north  of  Tonopah,  and  at  intervals  between  there  and 
Belmont.  At  Belmont  the  limestone,  into  which  granite  is  intrusive, 
is  known  to  be  Silurian. 
At  Tonopah  itself  occasional  limestone  fragments  and  more  abun- 
dant fragments  of  a  fine,  even-grained  pegmatite  occur  in  the  volcanic 
breccias.  They  are  blocks  which  were  hurled  out  from  the  volcanoes 
at  the  time  of  their  eruption.  Thus  it  is  shown  that  at  an  uncertain 
depth  below  the  present  surface  the  ascending  lavas  broke  through 
rocks  of  this  character. 
Early  andesite  eruption. — The  oldest  rock  outcropping  in  the  imme- 
diate vicinity  of  the  camp  is  a  hornblende-andesite,  or,  rather,  was 
such,  for  no  fresh  portions  have  been  found  and  the  rock  has  been 
altered  so  as  to  acquire  an  aspect  in  general  very  different  from  its 
original  one. 
