126  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
ORE  DEPOSITS. 
If  the  placer  workings,  which  are  distributed  widely  over  the  entire 
hills,  be  excluded  the  productive  mining  region  of  the  northern  Black 
Hills  comprises  only  a  limited  area  of  about  100  square  miles.  It 
extends  from  the  town  of  Perry,  on  Elk  Creek,  where  the  Clover 
Leaf  mine  is  situated,  northwestward  to  the  town  of  Carbonate,  on  the 
east  brand  of  Spearfish  Canyon,  while  its  widest  as  well  as  most  pro- 
ductive portion  lies  between  Terry  Peak  on  the  southwest  and  Garden 
City  on  the  northeast. 
Within  this  rather  restricted  region  are  closely  grouped  together  as 
many  as  nine  distinct  types  of  ore  deposits.  They  occur  in  each  case 
in  a  particular  geological  series,  and  are,  with  one  exception,  not 
found  in  the  rocks  belonging  to  any  other  formation.  Classifying 
them  according  to  the  rocks  in  which  they  occur,  the  following  five 
divisions  of  ore  deposits  may  be  distinguished:  (1)  In  Algonkian  rocks] 
(2)  in  Cambrian  rocks;  (3)  in  Carboniferous  rocks;  (1)  in  eruptive 
rocks;  (5)  in  rocks  of  recent  formation. 
In  the  crystalline  schists  and  metamorphic  rocks  of  Algonkian  age 
are  found  the  free-milling  gold  ores,  some  small  deposits  of  tin,  and  a 
few  trifling  prospects  of  copper,  which  have  not  }^et  assumed  any 
great  importance.  In  addition  to  these  there  are  certain  deposits  of 
graphite  which  have  lately  attracted  some  interest. 
In  the  Cambrian  rocks  there  are  gold-bearing  gravels  which  lie  at 
the  base  of  the  system,  the  refractory  siliceous  ores  which  have  of 
late  years  become  of  great  importance,  the  lead-silver  ores  of  Galena 
and  vicinity,  and  some  deposits  of  wolframite  which  have  from  time 
to  time  produced  considerable  quantities  of  this  mineral. 
In  the  heavy,  gray  limestones  of  the  Carboniferous  there  have  been 
found  in  the  vicinity  of  Ragged  Top  Mountain  high-grade  siliceous 
ores,  and  at  the  town  of  Carbonate  the  same  rocks  have  }delded  large 
amounts  of  lead-silver  ores  closety  resembling  those  of  Leadville, 
Colo.     A  few  deposits  also  occur  in  eruptive  rocks. 
In  the  latest  rocks  of  all — the  gravels  which  till  the  beds  of  modern 
streams — have  been  found  the  placer  deposits,  and  while  they  are  now 
chiefly  of  historic  interest  as  representing  the  earlier  development  of 
mining  in  the  hills,  they  have  in  past  years  produced  heavily. 
ORE   DEPOSITS  IN   METAMORPHIC   ALGONKIAN    SCHISTS. 
Since  the  attention  of  miners  has  been  diverted  from  the  gold- 
bearing  gravels,  so  frequently  the  first  producers  of  a  mining  region, 
the  free-milling  ores  which  occur  in  the  Algonkian  schists  have 
assumed  an  increasing  importance  in  this  region,  until  now  they  arel 
the  heaviest  producers  and  constitute  by  far  the  most  prominent  factor 
in  the  gold  production  of  the  region. 
