SOUTHWESTERN    NEVADA    AND    EASTERN    CALIFORNIA.  65 
GENERAL  GEOLOGY. 
The  barest,  outline  of  the  geology  will  be  sufficient  for  present  needs.  Sedimentation; 
with  few  important  breaks,  extended  from  the  Cambrian  period  to  the  end  of  the  Carbon- 
iferous. The  rocks  appear  to  have  been  predominantly  deposited  in  rather  shallow  seas, 
and  include  limestones,  usually  dark  in  color,  quartzites  of  medium  grain,  and  shales,  often 
olive-green  in  color.  In  probably  post-Jurassic  time  these  rocks  were  injected  by  granites, 
now  widely  distributed  throughout  the  area,  and  diorites,  which  are  largely  confined  to  the 
western  portion.  The  age  difference  between  these  holocrystalline  rocks  is  not  great,  and 
there  is  evidence  which  points  to  their  possible  origin  by  differential  ion  from  a  common 
magma.  Contemporaneously  with  the  intrusion  mountains  were  formed  or  possibly  pre- 
existing mountains  were  elevated.  The  intrusion  was  accompanied  by  buckling  of  the 
strata,  reversed  faulting,  and  considerable  metamorphism  of  the  sedimentary  rocks  near 
the  granite  contact. 
During  the  latter  part  of  Mesozoic  time  the  mountains  were  subjected  to  long-continued 
erosion,  and  at  the  end  of  the  period  surface  irregularities  were  undoubt  edly  less  accentuated 
than  now. 
In  Tertiary  time  a  long  period  of  volcanism  was  inaugurated.  Rhyolite  and  andesite 
in  flows,  sheets,  and  dikes  followed  one  another  repeatedly.  Tuffaceous  fades  are  rather  com- 
mon; Possibly  in  Miocene  time  considerable  portions  of  the  lower  regions  were  covered 
by  lakes  which  were  contemporaneous  with  basalt  flows.  Then  rhyolitic  and  andesitic 
eruptions  followed.  The  Tertiary  eruptions  were  accompanied  by  subaerial  erosion, 
normal  faulting,  tilting,  and  flexing. 
In  recent  time  erosion  has  continued  and  great  thicknesses  of  gravel  and  sand  have  been 
washed  into  the  valleys  as  a  result  of  cloudbursts,  filling  the  inclosed  basins  as  we  now  see 
them. 
The  ore  deposits  so  far  as  known  occur  in  the  Paleozoic  sedimentary  rocks,  the  granite 
and  diorite,  and  the  older  Tertiary  eruptives. 
HISTORY   OF   MINING    DEVELOPMENT. 
Two  waves  of  activity  in  prospecting  have  passed  over  the  portions  of  Nevada  and  Cali- 
fornia under  consideration.  The  first  started  in  the  late  sixties,  reached  its  maximum 
height  in  the  seventies,  and  died  out  before  1890.  The  hardy  prospector  and  miner  in 
these  years  confined  his  attention  to  the  limestone  and  granite.  Lida,  Old  Camp,  and 
Montezuma  were  flourishing  mining  centers. 
Mining  in  the  areas  studied  was  dormant  in  the  nineties,  but  with  the  discovery  of  Tono- 
pah's  phenomenal  ore  bodies  in  1900  the  desert  region  was  attacked  with  new  ardor.  The 
Tertiary  eruptives  have  been  more  especially  the  chosen  field  of  the  prospector,  although 
the  limestones  and  granites  have  been  by  no  means  neglected.  Goldfield  and  Bullfrog, 
the  most  important  new  camps,  are  now  being  studied  in  detail  by  Mr.  F.  L.  Ransome  and 
his  assistants,  Messrs.  Emmons  and  Garrey,  and  will  be  the  subjects  of  future  Survey 
publications.     Notes  on  the  outlying  districts  only  are  embodied  herein. 
GENERAL  CHARACTER  OF  THE  ORE  DEPOSITS. 
The  ore  deposits  herein  described  are  apparently  the  work  of  two  distinct  main  periods 
of  mineralization — one  post-Jurassic  and  probably  pre-Tertiary,  the  other  Tertiary.  The 
veins  at  Chloride  Cliff  may,  indeed,  be  even  pre-Jurassic,  but  their  resemblance  to  the  post- 
Jurassic  veins  renders  this  questionable. 
POST-JURASSIC    AND   PRE-TERTIARY    DEPOSITS. 
Deposits  in  granite. — (1)  Pegmatitic  dikes:  Dikes  of  pegmatite  at  Oak  Springs  are 
reported  to  carry  gold  and  silver  values. 
