ISOLATED   ABEAS   IN    MISSISSIPPI   VALLEY. 
similar  to  the  quartz  slate  of  the  Penokee-Gogebic  district.  The 
supposed  fossil  remains  reported  by  N.  H.  Winchell  have  been  exam- 
ined by  Walcott  and  thought  not  to  be  organic.  The  series  is 
surrounded  by  Cretaceous  rocks  and  is  unconformably  overlain  by 
them.  The  structural  relations  indicate  only  that  the  quartzite  is 
pre-Cretaceous,  but  because  of  its  lithology  and  its  proximity  to  the 
Lake  Superior  region  it  has  uniformly  been  referred  to  the  Huronian. 
Previously  the  rocks  have  been  referred  to  the  upper  Huronian 
because  of  the  existence  in  conglomeratic  phases  of  chert  and  jasper 
fragments  presumably  derived  from  an  underlying  Huronian  group. 
It  is  now  known  that  these  fragments  may  have  been  equally  well 
derived  from  the  Archean,  and  thus  their  presence  gives  no  evidence 
of  the  age  of  the  rocks. 
SECTION  3.     BLACK  HILLS  OF  SOUTH  DAKOTA. 
SUMMARY  OF  LITERATURE. 
Hayden,37  in  18G2,  states  that  the  nucleus  of  the  Black  Hills  con- 
sists of  red  feldspathic  granites,  with  stratified  metamorphic  Azoic 
slates  ami  schists,  upon  which  rests  unconformably,  forming  a  zone 
around  the  ellipsoidal  nucleus,  a  series  of  reddish  ferruginous  sand- 
stones, which  by  their  organic  remains  are  shown  to  belong  to  the 
Potsdam.  In  the  Potsdam  are  found  as  pebbles  the  different  varieties 
of  the  changed  rocks  beneath. 
Hayden,38  in  1863,  describes  the  Black  Hills  as  an  outlier  of  the 
Rocky  Mountains.  They  are  formed  of  a  granite  nucleus  surrounded 
by  a  series  of  Azoic  highly  metamorphosed  strata  standing  vertical, 
and  comprise  slates,  gneiss,  syenite,  and  quartzose  and  calcareous 
rocks. 
Hayden,39  in  1872,  describes  the  Black  Hills  as  being  the  most  com- 
plete illustration  of  an  anticline  not  complicated  by  any  other  in- 
fluences that  he  has  found  in  the  West.  The  nucleus  is  a  massive 
feldspathic  granite  with  a  series  of  gneissic  beds  outside  of  it,  which 
incline  in  every  direction  from  this  nucleus  in  a  sort  of  narrow  oval 
quaquaversal,  and  include  all  the  unchanged  beds  known  in  this  por- 
tion of  the  West  from  the  Potsdam  sandstones  to  the  top  of  the  Ter- 
tiary lignites. 
Winchell,40  in  1875,  in  a  report  on  the  Black  Hills,  describes  a 
series  of  mica  slates  and  mica  schists  with  intercalated  beds  of  quartz, 
below  the  Primordial  sandstones  and  quartzites.  These  rocks  often 
stand  nearly  vertical.  In  the  neighborhood  of  the  granite  areas  they 
are  interstratified  with  beds  of  true  granite,  and  with  this  granite  is 
found  tourmaline.  The  granite  area  is  near  the  southern  part  of  the 
hills,  and  of  this  Harney  Peak  may  be  taken  as  a  center. 
