vanhise.]       ISOLATED    AREAS    OF    THE    MISSISSIPPI    VALLEY.  259 
time  between  which  there  was  erosion  and  uietamorphism  of  the  lower 
series.  The  granite  is  coarsely  crystalline.  It  is  concluded  that,  be- 
cause of  the  great  amount  of  feldspar  in  the  granite,  because  pieces  of 
schist  are  inclosed  in  it  without  any  transition,  because  the  granite 
masses  in  the  schist  have  a  long  lenticular  shape,  because  of  the  coarse- 
ness 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  east- 
ern slate  division  bearing  the  lean  ores  are  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  characteristic  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,6  in  1885,  on  account  of  the  presence  of  staurolite  in  the 
Black  hills  schists,  places  these  formations  as  the  probable  equivalent 
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,7  in  1888,  finds  that  the  two  groups  of  Archean  as  mapped 
by  Newton  are  quite  sharply  defined  from  each  other.     It  is  said  that 
in  the  eastern  series  of  slates  are  pebbles  which  have  been  quite  cer- 
tainly derived  from  the  harder  rocks  of  the  western  series.     The  strike 
of  the  schists  is  found  to  curve  around  the  granitic  and  gneissic  area 
ind  the  normal  dip  of  the  strata  is  away  from  this  nucleal  granitic 
mass.    A  conglomerate  is  associated  with  the  quartzite  of  the  eastern 
ljate  area,  the  pebbles  of  which  have  suffered  extensive  deformation  by 
jfroinpression.     The  granitic  rocks  of  the  schist  area  do  not  penetrate 
he  slates  of  the  newer  series,  although  this  is  not  devoid  of  eruptive 
ocks.    The  newer  series  of  slates  is  correlated  with  the  Taconian  of 
estern  New  England.    The  conclusion  is  reached  that  the  granite, 
stead  of  being  of  eruptive  origin,  is  peginatitic. 
Carpenter,8  in  1888,  states  that  the  unconformity  supposed  to  exist 
Newton  between  the  eastern  slate  and  western  schist  series  is  sup- 
rted  by  an  observation  upon  Spring  creek  east  of  Hill  city.    A  huge 
ike  of  igneous  rock  a  thousand  feet  broad  in  places  is  described  as 
issing  through  the  entire  length  of  the  eastern  series. 
Van  Hise,9  in  1890,  finds  that  the  prominent  structures  of  the  Black 
lis,  which    have  heretofore  been  taken  as  bedding,  are  secondary 
ructures.     As  evidence  of  this  is  the  fact  that  alternating  bands  of 
. 
