CHAPTER   V. 
ISOLATED  AREAS  OF  THE  MISSISSIPPI  VALLEY. 
SECTION  I.   THE   BLACK  HILLS. 
LITERATURE. 
Hayden,1  in  1862,  states  that  the  nucleus  of  the  Black  hills  consists 
of  red  feldspathic  granites,  with'stratified  metamorphic  Azoic  slates  and 
schists,  upon  which  rests  unconformably,  forming  a  zone  around  the 
ellipsoidal  nucleus,  a  series  of  reddish  ferruginous  sandstones,  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,2  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,  quartzose  and  calcareous  rocks. 
Hayden,3  in  1872,  describes  the  Black  hills  as  being  the  most  com- 
plete illustration  of  an  anticline  not  complicated  by  any  other  influences 
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  portion  of  the  West 
from  the  Potsdam  sandstones  to  the  top  of  the  Tertiary  lignites. 
Winchell,4  in  1875,  describes  in  the  Black  hills,  below  the  Primor- 
dial sandstones  and  quartzites,  a  series  of  mica-slates  and  mica- *<•! lists 
which  contain  intercalated  beds  of  quartz.     These  rocks  often  stand 
nearly  vertical.     In  the  neighborhood  of  the  granite  areas  they  arjB 
interstratined  with  beds  of  true  granite,  and  with  this  granite  is  found 
tourmaline.    The  granite  area  is  near  the  southern  part  of  the  hills  and 
f  this  Harney  peak  may  be  taken  as  a  center. 
Newton,5  in  1880,  gives  a  systematic  account  of  the  Black  hills  of 
)akota.    The  Black  hills  is  a  geological  area  which  is  admirably  cir- 
aimscribed.    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  Carboniferous 
imestone  which  dip  away  on  all  sides  from  the  axis  of  the  hills.     The 
Irchean  rocks  as  a  whole  occupy  an  area  of  about  850  square  miles, 
eing  about  60  miles  long  and  25  miles  wide  at  its  maximum.     At  mi- 
erous  points  within  the  hills  are  centers  of  volcanic  eruption  of  an  age 
Bull.  86—17 
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