irving.]        ORE    DEPOSITS    OF    THE    NORTHERN    BLACK    HILLS.  131 
exposed  a  sheet  of  mica-diorite-porphyry  about  40  feet  thick,  with  beds 
of  Cambrian  rock  both  above  and  below.  Through  these  rocks  run  a 
series  of  vertical  fractures  striking  about  N.  85°  E.,  along-  which  silic- 
ification  has  occurred  and  from  which  telluride  of  gold  has  been 
introduced  into  the  adjacent  rock,  often  to  considerable  distances  from 
a  fracture.  Some  of  the  crystals  of  telluride — presumably  sylvanite — 
are  rather  large.  The  deposition  has  occurred  chiefly  in  the  diorite- 
porphyry,  but  also  to  a  minor  degree  in  the  Cambrian  rocks.  At  the 
surface  where  the  rocks  are  highly  oxidized,  gold  may  be  seen  along 
the  fractures  in  a  free  condition.  There  are  other  places  in  which  ore 
has  been  found  in  eruptives,  either  as  fillings  of  fissures  or  as  impreg- 
nations, but  they  are  not  of  any  economic  value.  The  eruptive  rocks 
as  a  whole  have  not  been  the  loci  of  considerable  deposits. 
ORE    DEPOSITS    IN    CAMBRIAN    ROCKS. 
As  a  producer  of  gold  the  Cambrian  is  second  in  importance  only  to 
the  Algonkian  system.  In  the  rocks  of  Cambrian  age,  or  those  which 
lie  immediately  above  the  metamorphic  schists,  there  are  four  varie- 
ties of  ore:  (1)  the  gold-bearing  conglomerates  or  gravels,  generally 
known  as  the  cement  deposits;  (2)  the  refractory  siliceous  ores;  (3)  the 
pyritic  ores;  and  (4)  the  lead-silver  ores. 
The  gold-hearing  conglomerates. — At  the  base  of  the  series  of  Cam- 
brian strata  immediately  above  the  upturned  schists  there  is  generally 
a  bed  of  gravel.  It  varies  in  thickness  from  a  few  inches  to  more 
than  30  feet.  Throughout  a  large  number  of  areas  where  the  Cam- 
brian strata  yet  remain  uneroded  this  conglomerate  is  generally  about 
3  or  4  feet  thick,  and  passes  upward  into  a  hard,  dense  quartzite,  which 
has  a  vertical  range  of  from  15  to  30  feet.  This  quartzite  is  almost 
invariably  present  at  the  base  of  the  Cambrian  series.  The  gravel  is 
generally  thin,  but  attains  a  notable  thickness  in  a  few  localities.  One 
of  these,  in  the  vicinity  of  Lead,  is  of  unusual  economic  importance,  as 
here  the  gravel  is  gold  bearing,  and  has  produced  very  heavily  in  the 
past.  The  productive  areas  of  this  gold-bearing  gravel  are  closely 
grouped  about  the  Homestake  belt.  They  are  five  in  number.  One, 
comprising  the  Durango  and  Harrison  mines,  is  west  of  the  Homestake 
lode,  near  the  southern  extremity  of  the  present  outcrop.  The  other 
four,  east  and  north  of  it,  include  the  Hawkeye,  Monitor,  and  Gentle 
Annie.  One  of  these  lies  just  east  of  the  Caledonia  open  cut;  another 
on  the  divide  between  Blacktail  and  Dead  wood  gulches;  the  third  on  the 
divide  between  Blacktail  and  Deadwood  gulches,  and  the  fourth  on  the 
north  side  of  Blacktail  Gulch  beneath  a  heavy  capping  of  rhyolite  which 
forms  the  high  ridge  beyond.  The  gold-bearing  conglomerate  occupies 
irregular  depressions  in  the  old  schist  surface,  and  was  probably  not 
uniformly  distributed  along  an  old  shore  line.  It  thins  out  to  nothing 
along  the  strike  of  the  Homestake  lode,  and  allows  the  higher  measures 
