wright.]      THE    PORCUPINE    PLACER    MINING    DISTRICT,    ALASKA.        61 
GENERAL  GEOLOGY. 
A  few  miles  north  of  Klehini  River  is  the  edge  of  a  belt  of  intrusive 
rocks  80  miles  or  more  in  width,  striking  northwest,  and  composed 
principally  of  diorite.  Adjacent  to  this  on  the  south  is  a  zone  8  miles 
in  width  occupied  mainly  by  folded  and  metamorphosed  black  shales 
and  limestone,  which  trend  parallel  with  the  general  northwest-south- 
east course  of  the  diorite  contact.  Fossils  of  lower  Carboniferous  age 
were  found  in  a  stratum  of  limestone  on  Porcupine  Creek.  The  upper 
part  of  the  Porcupine  cuts  into  a  belt  of  quartz-diorite  2  to  4  miles  in 
width,  also  striking  northwest-southeast,  parallel  with  the  Klehini  Val- 
ley, be}Tond  which  toward  the  southwest  no  observations  were  made. 
This  intrusive  mass  extends  eastward  to  Cottonwood  Creek  on  Salmon 
River,  where  it  disappears.  Several  greenstone  dikes  were  observed 
crosscutting  the  slate  country  rock,  but  the  relative  age  of  these  and 
the  diorite  could  not  be  determined,  as  the  two  were  not  found  near 
the  contact.  An  interrupted  zone  of  mineralization  carrying  large 
amounts  of  iron  sulphide  and  intersected  by  narrow  veins  of  quartz 
and  calcite,  also  mineralized,  occurs  in  the  sedimentary  series,  and 
from  this  zone  the  placer  gold  has  probably  been  derived. 
SOURCE  OF  THE  PLACER  GOLD. 
It  has  been  generally  believed  by  the  discoverers  and  operators  of 
Porcupine  and  Nugget  creeks  that  the  gold  contained  in  their  gravels 
!has  been  transported  from  some  region  outside  the  district.  The  sup- 
posed necessity  of  this  view  is  urged  from  the  coarseness  and  worn 
condition  of  the  nuggets  and  from  the  failure  to  find  coarse  gold  or 
more  than  small  amounts  of  the  free  metal  in  any  of  the  local  veins. 
The  distribution  of  the  placers  is,  however,  distinct^  against  this 
dew  and  favorable  to  a  local  origin.  If  the  occurrence  of  gold  in  the 
gravels  is  due  to  glacial  or  water  transportation  there  is  no  reason 
ipparent  for  its  concentration  in  one  stream  and  its  absence  from  adja- 
ent  gulches.  This  localization  and  the  fact  that  pay  gravels  are  not 
found  in  the  upper  portions  of  the  gold-bearing  creeks,  beyond  the 
irea  of  mineralization  or  where  they  enter  the  diorite  intrusive  belt, 
strongly  suggests  that  the  gold  has  been  derived  from  the  surround- 
ng  country  rock. 
Besides  the  creek  gravels,  there  are  important  auriferous  bench 
leposits  tilling  abandoned  channels  a  few  hundred  feet  above  the 
)resent  creek  bottoms.  These  not  only  furnish  workable  placers,  but 
lave  also  contributed  some  of  their  nuggets  to  the  gulch  gravels  through 
he  tributary  streams  which  have  cut  into  the  deposits  and  effected  a 
econcentration  of  their  gold.  This  suggests  an  explanation  for  the 
►ccurrence  of  rich  deposits  in  potholes  and  the  irregular  distribution 
