ISOLATED   AREAS   IN    MISSISSIPPI  VALLEY.  729 
are  found  in  the  western  and  southwestern  parts  of  the  area  and  the 
slates  in  the  eastern  and  northeastern. 
Between  these  two  groups  Jenney  noted  a  distinct  discordance  of 
dip  on  Castle  Creek,  but  in  the  absence  of  corroborative  observations 
the  unconformity  of  the  two  series  can  not  be  insisted  upon.  The 
division  of  the  system  into  two  series  is,  then,  based  on  lithological 
differences  purely  and  is  warranted  on  this  ground.  The  lithological 
difference  is,  however,  more  a  mineral  ogical  than  a  chemical  one, 
being  probably  due  to  difference  in  metamorphism.  The  apparent 
discordance  discovered  by  Jenney  and  this  lithological  difference  give 
strong  support  to  the  view  that  the  slate  and  schist  periods  were  sepa- 
rated by  an  interval  of  time  during  which  there  was  erosion  and 
metamorphism  of  the  lower  series.  The  granite  is  coarsely  crystalline. 
It  is  concluded  that,  because  of  the  great  amount  of  feldspar  in  the 
granite,  because  pieces  of  schist  are  inclosed  in  it  without  any  transi- 
tion, because  the  granite  masses  in  the  schist  have  a  long  lenticular 
shape,  because  of  the  coarseness  and  evenly  granular  character  of  the 
granite,  and  because  there  is  never  any  transition  between  the  schist 
and  granite,  the  latter  is  an  eruptive  rock  in  the  schists.  That  the 
Archean  rocks  were  upturned  and  metamorphosed  before  Potsdam 
time  is  shown  by  the  fact  that  the  basement  conglomerate  of  the 
Cambrian  contains  fragments  of  slate,  schist,  and  granite  precisely 
like  the  underlying  rocks. 
Since  the  lithological  character  of  the  Black  Hills  Archean  is  the 
only  means  of  judging  their  affinities  it  should  have  some  weight. 
The  eastern  slate  division  bearing  the  lean  ores  is  very  similar  to  the 
Huronian  rocks  of  the  south  shore  of  Lake  Superior  and  Canada. 
The  western  schist  series  containing  granite  masses  differs  from  the 
Huronian  and  from  the  Laurentian  in  that  gneiss,  the  most  character- 
istic rock  of  the  latter,  is  nearly  lacking,  so  that  no  correlations  are 
made  further  than  to  call  the  slate  series  newer  Archean  and  the 
schist  series  older  Archean. 
Blake,42  in  1885,  on  account  of  the  presence  of  staurolite  in  the 
Black  Hills  schists,  places  these  formations  as  the  probable  equiva- 
lent of  the  Coos  group  of  Hitchcock  in  New  Hampshire,  and  it  is 
said  that  there  is  sufficient  breadth  of  formation  to  include  all  the 
rocks  from  the  Huronian  to  the  Coos. 
Crosby,43  in  1888,  finds  that  the  two  groups  of  Archean  as  mapped 
by  Newton  are  rather  sharply  defined  from  each  other.  It  is  said 
that  in  the  eastern  series  of  slates  are  pebbles  which  have  been  almost 
certainly  derived  from  the  harder  rocks  of  the  western  series.  The 
strike  of  the  schists  is  found  to  curve  around  the  granitic  and  gneissic 
area,  and  the  normal  dip  of  the  strata  is  away  from  this  nucleal 
granitic  mass.    A  conglomerate  is  associated  with  the  quartzitc  of  the 
