108 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 22b. 
siderable.  The  whole  later  andesite  region  is  worthy  of  careful 
yet  cautious  prospecting.  The  localities  where  the  later  andesite  is 
intruded  by  the  rhyolite  ought,  as  a  rule,  to  be  avoided  in  this  pros- 
pecting, for  such  places  are  not  so  favorable  as  those  where  the  ande- 
site is  undisturbed.  Such  places,  however,  are  likely  to  be  pre- 
ferred by  many  mining  men,  since  they  are  apt  to  show  some  of  the 
smaller  and  poorer  veins  of  the  later  rhyolitic  mineralization.  Again, 
the  region  of  faults  should  be  avoided,  for  these  faults  have  affected 
the  ore-bearing  veins,  and  if  veins  are  found  near  them  they  are  diffi- 
cult, and  sometimes  almost  impossible,  to  follow.  The  rocks  along 
fault  zones  are  often  decomposed  and  the  decomposed  material  may 
Hypothetical     Hypothetical   Early  andesite   Later  andesite  Dacite  breccia     Brecciated         Lake  beds  Faults  Later  dacite  Early  andesite  veins 
deep-seated        deep-seated  earlier  rbyolites  and  rhyolite  0e33er  veins  belong- 
granite  limestone  intrusions       ing  to  other  periods 
O0C  represented) 
Fig.  5  .—Ideal  cross  section  of  Tonopah  rocks.  (This  section  does  not  represent  any  particular 
place,  and  is  simply  intended  to  illustrate  the  geological  conditions,  as  described  in  the  text.) 
have  the  appearance  of  what  is  known  as  "  vein  matter."  From  this 
material  real  or  fictitious  assays  may  be  obtained,  and  such  a  locality 
may  appear  to  the  miner  as  more  hopeful  than  less  altered  rock  which 
does  not  look  like  a  vein  and  does  not  show  any  assays.  Some  of  the 
principal  faults  are  shown  on  the  accompanying  map,  but  certainly 
not  all  of  them  have  been  detected,  and  underground  developments 
will  probably  reveal  many  more. 
Concerning  the  dacites  and  rhyolites  of  the  later  intrusions,  the 
volcanic  necks  and  the  dikes  should,  as  a  rule,  be  avoided  by  pros- 
pecting operations.  The  greater  parts  of  the  principal  low  mountains 
and  hills  in  the  vicinity,  such  as  Butler,  Brougher,  Siebert,  most  of 
Golden,  etc.,  are  regarded  as  offering  very  little  hope  for  prospecting, 
and  even  near  the  rhyolite  hills,  like  Rushton,  Oddie,  and  Ararat, 
there  has  been  a  displacement  of  the  veins,  for  the  hills  are  essen- 
tially intrusive.  The  writer  is  aware  that  the  proximity  of  the  rich 
veins  of  Mizpah  Hill  to  Oddie  Mountain  has  caused  this  mountain  to  be l 
