spencer.]  THE    JUNEAU    GOLD    BELT,    ALASKA.  39 
origin,  from  the  facts  of  their  occurrence,  though  the  actual  source 
of  their  gold  has  not  been  discovered.  The  country  rocks  are  mainly 
black  slate,  containing  some  limestone,  in  which  Carboniferous  fossils 
occur,  but  there  are  no  massive  flows  of  greenstone  like  those  of  the 
Juneau  belt,  and  no  sufficiently  detailed  studies  have  been  made  to 
warrant  correlating  these  rocks  with  the  slates  in  the  southern  districts. 
The  region  lies  several  miles  from  the  main  contact  of  the  dioritic 
rocks  which  form  a  continuation  of  the  Coast  Kange  intrusives,  and 
as  these  invading  rocks  are  crosscutting  toward  the  west  in  the  vicinity 
of  Berners  Bay,  it  is  probable  that  the  rocks  of  the  Porcupine  field 
belong  to  the  formations  which  carry  mineral  veins  on  Admiralty 
Island  and  on  the  west  side  of  Lynn  Canal,  at  James  Bay,  and  on  Endi- 
cott  River. 
Treadwell  mines. — The  main  feature  of  interest  on  Douglas  Island  is 
the  Treadwell  group  of  mines,  consisting  of  the  Alaska-Treadwell, 
700-foot,  Alaska-Mexican,  and  Ready  Bullion  properties.  The  mines 
are  advantageously  located  on  the  inner  or  mainland  side  of  the  island, 
close  to  tide  water.  The  ore,  which  is  of  low  grade,  averaging  about 
$2  in  value,  occurs  in  a  series  of  ore  bodies  lying  between  a  mass 
of  greenstone  on  the  hanging  wall  and  black  slate  on  the  foot  wall. 
The  strike  of  the  deposits  and  of  the  country  rock  is  about  N.  45°  W. , 
slightly  diagonal  to  the  shore  of  Gastineau  Channel,  which  trends  about 
N.  40°  W.,  and  the  average  dip  toward  the  northeast  is  about  50°. 
The  ore  bodies  are  brecciated  masses  of  intrusive  syenite,  filled  with 
a  network  of  quartz  and  calcite  veinlets,  and  impregnated  with  pyrite, 
which  occurs  both  in  the  veinlets  and  in  the  rock  itself.  The  gold 
occurs  both  in  association  with  the  pyrite  and  native,  and  a  large, 
though  variable,  proportion  of  the  values  are  saved  by  amalgamation. 
Visible  specks  of  the  metal  are  sometimes,  though  rarely,  found. 
The  following  associated  minerals  have  been  observed:  Pyrrhotite 
and  magnetite  are  always  present,  and  molybdenite  is  of  common 
occurrence;  native  arsenic,  realgar,  and  orpiment  have  been  noted, 
and  arsenic  is  commonly  found  by  the  assayer,  probably  indicating 
the  presence  of  arsenopyrite;  stibnite  occurs  in  small  amounts  with 
the  quartz.     Bullion  assays  indicate  only  small  amounts  of  silver. 
The  syenite  bodies  which  have  been  worked  occur  as  somewhat 
irregular  dikes.  In  the  Alaska-Treadwell  property  two  of  these  dikes 
are  mined,  each  having  a  width  of  about  200  feet.  Between  them 
there  is  a  band  of  barren  slate  perhaps  50  feet  across,  and  there  are 
some  minor  masses  of  the  country  rock  included  in  the  ore  bodies. 
The  south  or  foot- wall  dike  has  a  known  length  along  the  strike  of  800 
feet,  and  from  the  lenticular  outline  of  its  horizontal  cross  section  the 
entire  extent  can  not  be  much  in  excess  of  this  figure.  The  hanging- 
wall  dike  has  been  mined  continuously  for  a  distance  of  nearly  2,000 
feet  through  the  Treadwell  and  700-foot  workings,  but  beyond  this 
