ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY.  731 
The  mica  slates,  mica  schists,  and  mica  gneisses  are  found  to  be 
clastic  rocks,  the  processes  of  change  from  their  original  clastic  con- 
dition to  their  present  crystalline  one  being  traced  out.  Associated 
with  the  clastic  rocks  are  other  green  crystalline  schists,  which  are 
metamorphosed  basic  eruptives  that  were  probably  intruded  before 
the  earlier  folding  of  the  slates.  Corroborating  Newton's  conclusion. 
it  is  said  that  the  Black  Hills  rocks  exhibit  a  remarkable  lithological 
analogy  to  certain  of  the  iron-bearing  series  of  the  Lake  Superior 
region,  which  in  the  past  have  been  included  under  the  term  Huro- 
nian. While  this  correlation  is  not  beyond  doubt,  there  is  no  question 
that  these  series  in  common  belong  to  the  Algonkian. 
Todd,40  in  1895,  maps  and  gives  a  general  description  of  the  geol- 
ogy of  South  Dakota.  Archean  a  rocks  are  present  in  the  Black  Hills, 
near  Sioux  Falls,  and  near  Bigstone  Lake  in  the  eastern  part  of  the 
State. 
In  the  Black  Hills  the  Archean  rocks  are  slates  and  schists,  in- 
truded by  granite.  The  metamorphic  effects  in  the  slates  and 
schists  become  more  pronounced  as  the  contact  with  the  granite  is 
approached.  Following  Van  Hise,  it  is  believed  that  their  metamor- 
phism  was  largely  brought  about  by  the  intrusion  of  the  granite. 
The  thickness  of  the  slates  and  schists  is  from  10,000  to  100,000  feet. 
In  age  they  are  believed  to  correspond  to  the  Lower  Huronian  of  the 
Lake  Superior  region.  The  granites,  while  younger  than  the  slates 
and  schists,  are  still  pre-Cambrian. 
The  Sioux  quartzite  is  similar  to  the  quartzite  of  Baraboo  and  the 
Chippewa  Valley,  of  Wisconsin,  and,  following  Irving  and  Van  Hise, 
it  is  believed  to  be  of  Upper  Huronian  age. 
Near  Bigstone  Lake  are  exposures  of  granites,  probably  of  Lauren- 
tian  age. 
All  the  Archean  rocks  are  overlain  unconformable  by  Cambrian 
rocks,  which  in  general  dip  away  from  the  Archean  exposures. 
Todd,47  in  1898,  reports  on  a  section  across  the  Black  Hills  from 
Rapid  City  westward.  The  alternating  slate  and  quartzite  beds  of 
the  Algonkian  were  found  to  be  folded  in  a  most  intricate  fashion.  A 
number  of  the  folds  were  Avorked  out.  In  most  cases  the  lamination 
and  stratification  seem  to  correspond  in  direction. 
Frazer,48  in  1898,  sketches  the  geology  of  an  area  in  the  vicinity  of 
Galena,  in  the  northern  Black  Hills  of  South  Dakota.  Mica  schists, 
thought  to  be  upper  members  of  the  Archean,  are  found  striking 
northeast-southwest  and  dipping  at  angles  of  38°  to  85°.  They  arc 
generally  micaceous  and  coarse  grained.  I > 1 1 1  vary  greatly,  sometimes 
passing  into  nacrite  schist  or  hydromica  schist,  and  sometimes, 
though  more  rarely,  assuming  a  heavily  bedded  character  reminding 
one  of  gneiss. 
"Archean  is  used  i<>  designate  the  pre-Cambrian. 
