LAKE    SUPERIOR   REGION.  123 
of  uncertain  age.  No  attempt  is  here  made  to  describe  or  classify 
them. 
The  diorites,  dioritic  schists,  and  related  rocks  range  in  structure 
from  very  fine  grained  or  compact  (almost  aphanitic)  to  coarsely 
granular  and  crystalline,  being  sometimes  porphyritic  in  character. 
The  rock  passes  on  the  one  hand  into  a  hornblende  rock  and  on  the 
other  into  a  rock  resembling  a  diorite.  It  is  eminently  schistose  in 
character,  splitting  easily,  and  appearing  more  like  chloritic  schist 
than  any  other  rock.  At  several  points  dioritic  schists,  semi-amygda- 
loidal  in  character,  were  observed,  and  in  one  instance  the  rock  had  a 
strong  resemblance  to  conglomerate.  The  bedding  of  the  rocks  is 
generally  obscure,  and  sometimes  entirely  wanting.  It  is  only  by  a 
full  study  of  the  rock  in  mass  and  its  relations  to  the  adjacent  beds 
that  one  becomes  convinced,  whatever  its  origin,  that  it  presents  in 
mass  precisely  the  same  phenomenon  as  regards  stratification  as  do 
the  accompanying  schists  and  quartzites.  Chloritic  magnesian  schists 
are  associated  with  the  pure  and  mixed  ores.  Oftentimes  these  mag- 
nesian schists  several  feet  in  width  cut  across  the  stratification,  and 
are  called  slate  dikes.  It  is  difficult  or  impossible  to  draw  the  line  be- 
tween these  magnesian  schists  and  the  dioritic  schists.  It  is  suggested 
that  on  the  New  England-KSaginaw  range  and  at  the  Lake  Superior 
mine  tepid  alkaline  waters  have  penetrated  the  formation  and  have 
dissolved  out  the  greater  portion  of  the  siliceous  matter,  leaving  the 
iron  oxide  in  a  hydrated  earthy  condition. 
At  the  S.  C.  Smith  mine  and  along  Plumbago  Brook  is  found  car- 
bonaceous matter.  These  carbonaceous' shales  burn  white  before  the 
blowpipe  and  mark  paper  like  a  piece  of  charcoal. 
Above  the  Cascade  ore  is  a  bed  of  coarse  conglomerate.  The  upper 
quartzite  of  Kepublic  Mountain,  near  its  base,  is  a  conglomerate  con- 
taining large  and  small  fragments  of  flaggy  ore.  At  the  New  Eng- 
land mine,  between  the  ore  and  the  quartzite  is  a  mass  of  specular 
conglomerate  similar  to  that  at  Republic  Mountain. 
The  iron-bearing  series  is  unconformably  above  the  older  Lauren- 
tian  rocks.  The  contact  is  observed  in  Plumbago  Brook,  where  a 
talcy  red  rock,  unmistakably  belonging  to  the  Huronian,  dipping  at 
a  low  angle  to  the  northwest,  is  in  contact  with  the  Laurentian  chlo- 
ritic gneiss,  which  dips  at  an  angle  of  about  35°  SSW.  The  same 
phenomena  can  be  seen  near  Republic  Mountain,  where  the  Pluronian 
schists  strike  nearly  at  right  angles  to  the  Laurentian  gneiss,  only  50 
feet  distant;  both  series  dipping  at  high  angles,  the  Laurentian  east 
of  north,  and  the  Huronian  about  45°  west  of  north.  The  non- 
conformability  is  further  shown  by  the  fact  that  the  Laurent ian 
generally  abounds  in  dikes  of  granite  and  diorite,  which  are  almost 
entirely  absent  from  the  Huronian. 
