lw24  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull.  225. 
complicated  connections  with  the  surrounding  country  that  make  most 
geological  questions  so  difficult  to  comprehend. 
In  his  classic  work  on  this  region  Henry  Newtona  has  described  the 
Black  Hills  as  an  elevated  area,  roughly  elliptical  in  outline,  compris- 
ing a  central  core  of  metamorphic  crystalline  rocks  about  which  are 
grouped  in  rudely  concentric  belts  strata  of  later  geological  age,  clip- 
ping away  in  all  directions  from  the  elevatory  axis  or  region  of  the 
hills. 
Were  the  strata  which  originally  covered  the  core  of  schists  which 
forms  the  center  of  the  hills  still  present,  we  would  have  an  elevated 
dome  of  very  great  height,  rising  far  above  the  level  of  the  surrounding 
country.  The  gradual  erosion  of  these  uplifted  rocks,  however,  has 
gone  on  together  with  their  upheaval,  so  that  there  remains  a  region 
only  slightly  higher  than  the  surrounding  plains.  In  the  center  is 
the  uncovered  area  of  schists,  and  at  the  sides  the  stratified  rocks  dip 
outward  beneath  the  flat  prairie  land  beyond.  This  central  core  of 
old  crystalline  rocks  has  in  general  a  due  north-south  direction,  but 
at  its  northern  extremity  it  turns  abruptly  toward  the  northwest, 
forming  a  sort  of  geological  cul-de-sac,  shut  in  on  three  sides  by  up- 
turned strata,  but  separated  from  the  main  portion  of  the  core  to 
the  south  by  a  narrow  belt  of  Cambrian  rocks  and  their  included  masses 
of  porphyry. 
Throughout  this  northern  area  erosion  has  not  cut  so  deeply  into  the 
crystalline  schists  as  farther  south,  so  that  besides  the  rude  belt  of 
inclosing  strata,  isolated  patches  of  the  old  sedimentary  covering  lie 
on  the  higher  hills  within  the  area  of  schists. 
In  the  northern  hills  four  systems  of  rocks  can  readily  be  distin- 
guished from  one  another.  The  lowest  series  is  composed  of  meta- 
morphic schists.  It  consists  of  a  series  of  crystalline  mica-schists, 
mica-slates  or  phyllites,  chlorite-schists,  and  laminated  quartzites. 
Together  with  these  are  found,  in  the  southern  portion  of  the  Black 
Hills  and  in  the  region  known  as  Nigger  Hill,  large  intruded  masses 
of  granite,  very  coarse  in  its  texture  and  sometimes  containing 
deposits  of  tin.  In  the  northeastern  portion  of  the  hills  there  is  no 
granite  present,  but  its  place  is  taken  by  numerous  dikes  and  great 
irregular  patches  of  a  dark-greenish  hornblende  rock,  termed  amphib- 
olite.  Bodies  of  this  rock  are  particularly  noticeable  in  the  vicinity 
of  Lead,  and  extend  as  far  south  as  Custer  Peak.  It  is  possible  that 
they  may  have  had  some  connection  with  the  occurrence  of  gold 
in  the  Homestake  mine,  but  there  is  no  definite  evidence  in  favor 
of  this  theory.  These  rocks  are  strongly  laminated  and  are  every- 
where tilted  at  a  high  angle.     The  lamination  often  crosses  the  planes 
n Newton,  Henry,  Geology  of  the  Black  Hills:  U.  S.  Geol.  and  Geog.  Surv.  Rocky  Mt.  Region,  1880, 
pp.  1-222. 
