spencer.]  THE    JUNEAU    GOLD    BELT,   ALASKA,  31 
stones,  and  more  or  less  metamorphosed;  (2)  highly  metamorphic 
schists;  (3)  intrusive  dioritic  rocks,  forming  the  main  mass  or  core  of 
the  Coast  Range. 
The  average  width  of  the  sedimentary  belt  on  the  mainland  is  about 
6  miles,  while  the  diorite  zone  is  from  50  to  80  miles  across,  and  there- 
fore extends  into  Canadian  territory. 
Black- slate- greenstone  series. — The  series  composed  mainly  of  black 
slates  and  greenstones,  with  some  beds  of  limestone,  occurs  all  along 
the  shores  of  Stephens  Passage  and  Lynn  Canal  up  to  the  crossing  of 
the  great  mass  of  dioritic  rocks  about  30  miles  below  Skagway.  It 
occurs  also  on  many  of  the  adjacent  islands  of  the  Alexander  Archi- 
pelago, but  its  westward  extent  has  not  been  determined.  The  slates 
have  been  derived  from  line-grained  carbonaceous  shales,  their  slaty 
cleavage  being  a  secondary  structure  produced  by  pressure  metamor- 
phism  which  accompanied  folding  and  upturning  of  the  rocks  of  the 
region.  Carbonaceous  matter,  which  occurred  in  the  original  shales, 
has  been  converted  almost  entirely  into  graphite,  which  is  disseminated 
throughout  the  slates  and  is  encountered  in  large  amounts  in  some  of 
the  mines.  The  limestones  sometimes  contain  graphite,  but  as  a  rule 
their  carbonaceous  matter  has  not  been  crystallized  by  metamorphic 
action,  and  freshly  broken  fragments  usually  give  a  strong  odor  resem- 
bling that  of  petroleum,  which  indicates  the  presence  of  hydrocarbons. 
Fossils  of  Paleozoic  age  have  been  found  in  the  limestones  belonging 
to  the  series  at  Taku  Harbor,  20  miles  south  of  Juneau,  and  lower 
Carboniferous  forms  occur  in  the  Porcupine  district,  120  miles  to  the 
northwest,  but  near  Juneau  organic  remains  have  not  been  found. 
The  greenstones  interbedded  with  the  slates  are  mostly  volcanic 
rocks,  which  flowed  out  over  the  surface  at  different  periods  during 
the  deposition  of  the  sedimentary  strata  in  which  they  occur. 
Schist  series. — East  of  the  slate-greenstone  series,  and  sharply  defined 
from  it,  there  is  a  series  of  highly  metamorphic  schists,  characterized 
by  hornblende,  mica,  and  garnet.  A  few  bands  of  quartzite  and  lime- 
stone are  intercalated  in  the  schists,  which,  in  connection  with  exist- 
ing strata-like  alternations  in  the  composition  of  the  schists,  prove  the 
sedimentary  origin  of  the  series.  The  schists  occupy  a  belt  about  3 
miles  wide  opposite  Juneau,  but  their  area  of  outcrop  narrows  in  both 
directions  along  the  strike,  and  they  are  not  found  beyond  30  miles 
northwest  of  Juneau,  being  gradually  cut  out  by  Coast  Range  diorites. 
Coast  Range  intrusives. — Beyond  the  schists,  when  these  are  pres- 
ent, but  elsewhere  occurring  next  to  the  slate-greenstone  series,  coarse 
granular  rocks  having  the  appearance  of  granite  occur.  These  form 
the  great  mass  of  the  Coast  Range,  not  only  opposite  Stephens  Pas- 
sage and  Lynn  Canal,  but  throughout  the  corresponding-  mountains 
which  border  Pacific  Ocean  southward  nearly  to  the  boundary  between 
