40  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull. 225. 
toward  the  southeast  there  are  several  separate  ore  bodies  of  lenticular 
cross  section,  which  occupy  the  same  position  under  the  greenstone, 
as  is  shown  in  the  Alaska-Mexican  and  Ready  Bullion  mines.  Toward 
the  northwest  only  narrow  dikes  are  present  where  the  base  of  the 
greenstone  has  been  prospected. 
The  distribution  of  values  is  irregular,  as  in  nearly  all  gold  deposits, 
but  there  is  no  suggestion  of  impoverishment  in  the  deepest  workings, 
which  are  nearly  1,000  feet  below  the  sea. 
While  only  the  hanging-wall  bodies  of  the  syenite  have  thus  far 
been  found  productive,  many  other  dikes  of  the  rock  occur  for  a  dis- 
tance of  about  3,000  feet  across  the  strike  of  the  black  slates,  toward 
the  southwest.  They  are  almost  entirely  confined  to  this  side  of  the 
greenstone,  though  a  few  are  found  between  it  and  the  channel,  and  a 
large  mass  forms  an  island  about  one-fourth  mile  offshore.  Along  the 
strike  the  dikes  extend  for  a  distance  of  3  miles,  mainly  northwestward 
from  the  vicinity  of  the  mines,  and  the  greenstone  may  be  traced  for 
a  much  longer  distance  in  this  direction. 
OTHER    DEPOSITS   ON    DOUGLAS   ISLAND. 
Aside  from  the  deposits  of  altered  syenite,  the  black  slates  are 
locally  found  to  contain  systems  of  quartz  stringers  or  sometimes 
veins  of  fair  size.  One  of  these  systems  of  stringers  forms  an  exten- 
sion of  the  Treadwell  deposits,  and  is  traceable  for  several  miles  along 
the  mainland  side  of  Douglas  Island,  and  though  it  has  been  pros- 
pected to  a  certain  extent  no  mines  have  been  discovered.  :■ 
Several  other  areas  of  strong  mineralization  are  found  on  Douglas 
Island,  two  of  which  occur  in  the  massive  basaltic  greenstone  which 
forms  the  outer  two- thirds  of  the  island  and  may  prove  of  future 
value.  One  of  these  is  situated  on  Nevada  Creek,  about  3i  miles 
southwest  of  the  Alaska-Treadwell  mine,  where  an  area  of  bleached 
rock  impregnated  with  small  cubes  of  iron  pyrites  extends  about 
1  mile  along  the  creek  below  the  main  divide  of  the  island  and  for  a 
distance  of  about  1$  miles  along  the  strike  of  the  rocks.  Parts  of 
the  altered  and  mineralized  greenstone  give  fairly  good  assay,  but  no 
well-defined  ore  bodies  have  been  discovered.  Locally  there  are  nar- 
row, irregular  veinlets  which  carry  galena  and  sphalerite,  and  these 
are  richer  both  in  gold  and  silver  than  the  main  mass  of  the  rock.  A 
few  irregular  quartz  veins  occur,  but  as  a  rule  the  sulphides  are  not 
accompanied  by  this  mineral. 
The  second  case  of  mineralization  mentioned  above  is  a  zone  in  the 
greenstone  occurring  about  1  mile  west  of  the  Treadwell  deposit. 
The  mineralized  band  follows  the  structure  of  the  rocks  and  varies  in  i 
width  from  perhaps  100  to  300  feet.  It  is  traceable  for  more  than  a, 
mile  by  the  altered  and  bleached  condition  of  the  greenstone,  through! 
which  pyrite  is  distributed  in  the  form  of  small  cubes  similar  to  those 
