728  PRE-CAMBRIAN    GEOLOGY   OP   NORTH   AMERICA. 
Newton,41  in  1880,  gives  a  systematic  account  of  the  Black  Hills  of 
Dakota.  The  Black  Hills  are  a  geological  area  which  is  admirably 
circumscribed.  They  consist  of  a  nucleal  area  of  metamorphic  slates 
and  schists  containing  masses  of  granite,  about  which  is  an  inward- 
facing  unconformable  escarpment  of  Potsdam  sandstone  and  Car- 
boniferous limestone  which  dip  away  on  all  sides  from  the  axis  of  the 
hills.  The  Archean  rocks  as  a  whole  occupy  an  area  of  about  850 
square  miles,  being  about  60  miles  long  and  25  miles  wide  at  its  maxi- 
mum. At  numerous  points  within  the  hills  are  centers  of  volcanic 
eruption  which  was  probably  contemporaneous  with  the  elevation  of 
the  mountains.  It  is  impossible  to  estimate  with  any  degree  of  ac- 
curacy the  thickness  of  the  Archean  schists,  as  they  are  highly  in- 
clined and  distorted,  and  in  their  present  metamorphosed  and  de- 
nuded condition  it  can  not  be  determined  whether  they  are  the  rem- 
nants of  several  great  folds  or  whether  they  are  the  broken  strata  of 
one  vast  fold,  though  the  latter  seems  to  be  the  more  probable  struc- 
ture ;  in  Avhich  case  the  total  thickness  of  the  Archean  strata  must  be 
more  than  100,000  feet,  about  25  miles.  The  examination  showed  no 
evidence  of  the  duplication  of  any  parts  of  the  Archean  strata,  and  it 
is  presumed,  if  a  repeated  folding  has  taken  place,  that  it  did  not 
occur  within  the  area  exposed  in  the  hills. 
The  Archean  sedimentaries  are  divided  into  two  groups,  schists  and 
slates.  The  schists  include  quartzose,  garnetiferous,  ferruginous, 
and  micaceous  varieties,  together  with  some  gneiss,  chloritic,  talcose, 
and  hornblendic  schists,  and  quartzite.  The  schists  are  occasionally 
staurolitic.  The  whole  series  is  coarse  in  texture,  highly  crystalline, 
and  contains  seams  or  veins  of  quartz  conformable  with  the  stratifica- 
tion and  having  a  lenticular  form.  The  slates  are  distinguished  from 
the  schists  mainly  by  their  fine  and  compact  texture,  although,  as 
shown  by  Caswell,  their  ultimate  mineral  composition  is  similar. 
They  are  mainly  micaceous  clay  slate,  siliceous  slate,  and  quartzite, 
which  are  sometimes  associated  with  specular  oxide  of  iron.  On  Box 
Elder  Creek  is  a  ridge  400  feet  in  height,  which  is  a  vast  deposit  of 
siliceous  hematite  resembling  the  siliceous  hematites  of  the  Lake  Supe- 
rior region.  The  quartzites  of  the  two  classes  are  similar.  The  mica 
schist  passes  into  chlorite  schist,  siliceous  schist,  and  quartzite.  The 
schists  of  the  southern  part  of  the  hills  are  associated  with  an  area  of 
highly  feldspathic  granite  which  culminates  in  the  region  of  Harney 
Peak.  On  the  outskirts  of  this  district  are  many  smaller  masses  of 
granite.  So  far  as  the  structure  was  made  out,  each  of  the  bodies 
has  a  lenticular  shape  and  is  intercalated  among  the  strata  of  the 
schists.  No  granite  is  found  associated  with  the  slate.  The  general 
strike  of  the  rocks  is  northwest-southeast.  The  topography  shows 
that  there  is  a  series  of  ridges  in  this  direction  which  mark  the  posi- 
tion of  the  particularly  hard  layers,  such  as  quartzites.     The  schists 
