26  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
of  the  shale  were  sandy  and  after  metamorphism  resulted  in  a  hard,  dense,  and  easily 
fractured  rock,  much  like  the  quartzite. 
The  pre-Cambrian  rocks  are  rather  closely  folded,  the  axes  of  the  folds  trending  gen- 
erally a  few  degrees  south  of  east.  They  are  also  faulted,  the  faults  running  nearly  east 
and  west,  approximately  with  the  strike  of  the  beds.  The  andesite  tuffs  and  flows  rest 
upon  a  very  irregular  eroded  surface  of  the  pre-Cambrian  rock.  They  have  suffered  little 
change  since  their  deposition. 
A  long,  narrow  fault  block  of  the  Uncompahgre  quartzite  enters  the  area  just  south  of 
Beartown  and  extends  nearly  due  west  to  the  head  of  Elk  Creek.  On  the  north  it  is  in 
faulted  contact  with  the  Archcan  schist  and  on  the  south  with  other  Uncompahgre  schist 
and  quartzite.  It  is  partially  covered  by  the  Tertiary  volcanics.  Where  it  is  the  surface 
rock  it  stands  up  conspicuously  above  the  surrounding  country  and  is  very  generally  mis- 
taken for  a  dike,  which  form  is  strongly  suggested  by  its  long,  narrow,  regular  outline.  A 
number  of  claims  are  located  along  this  fault  block,  among  which  are  the  Good  Hope,  the 
New  York  and  Brooklyn,  and  the  Silverton.  The  lodes  cross  this  fault  block  nearly  at 
right  angles  to  the  faults.  They  are  more  conspicuous  in  the  quartzite  than  in  the  sur- 
rounding slate  and  schists. 
Ore,  minerals.— The  ore  deposits  are  narrow  fissure  veins  composed  chiefly  of  white 
quart/.,  in  which  are  scattered  small  masses  or  pockets  ol  gray  ore  minerals.  While  quartz 
is  by  far  the  most  important  gangue  mineral,  calcite,  barite,  and  a  soft  white  material, 
probably  kaolinite,  are  also  present.  The  metallic  minerals  are  a  telluride  of  gold  and 
silver  (probably  petzite),  tetrahedrite  (or  gray  copper),  irop  pyrite,  marcasite  (or  white- 
iron  pyrite),  copper  pyrites,  bornite  (or  peacock  ore),  galena,  sphalerite,  arsenopyrite, 
limonite,  hematite,  malachite,  and  azurite.  None  of  these  minerals  have  been  discovered 
in  sufficient  quantities  to  be  of  economic  importance  except  the  telluride  and  the  gray 
copper.  These  occur  very  intimately  associated.  What  appears  to  be  a  single  mineral 
from  the  Good  Hope  mine  gave  tests  for  tellurium,  copper,  arsenic,  antimony,  and  sulphur. 
This  mineral  has  the  physical  properties  ol'  gray  copper.  The  tellurium  mineral  probably 
occurs  also  in  a  much  purer  state,  since  roasted  specimens  which  have  been  taken  from 
the  Good  Hope  mine  show  light-yellow  globules  of  considerable  size  sticking  to  the  white- 
quartz  gangue. 
Wnite  iron  pyrite  is  found  in  relative  abundance,  especially  at  the  Sylvanite  and  Kan- 
kakee properties.  Its  occurrence  is  similar  to  that  of  the  ore  minerals  and  it  has  been 
mistaken  for  a  telluride.  It  may  be  distinguished  from  the  tellurium  ores  by  the  darker 
color  of  its  powder,  its  greater  hardness,  and  its  tendency  to  assume  botryoidal  or  roundish 
form. « 
Pyrite  and  chalcopyrite  (copper  pyrites)  occur  as  small  masses  and  crystals  in  the  gangu<> 
and  to  a  certain  extent  in  the  country  rock.  Assays  of  these  minerals  show  that  they  carry 
very  low  values  in  gold  and  silver.  The  copper,  lead,  and  zinc  minerals  have  not  been 
found  in  sufficient  quantities  for  profitable  exploitation. 
Ore  deposits. — Some  of  the  veins  arc  well  defined  and  their  decomposed  outcrops  marked 
by  yellow  iron  stains  may  be  followed  along  the  surface  almost  continuously  for  several 
hundred  feet.  Their  usual  trend  is  nearly  north  and  south.  Those  of  an  important  group 
run  about  S.  7°  W.  The  veins  are  usually  vertical  or  are  inclined  less  than  10°  from  that 
position.  The  fissures  do  not  appear  to  be  closely  related  to  the  larger  structural  features 
of  the  country,  but  cut  the  faults,  the  axes  of  the  folds,  and  the  fissility  of  the  rocks  at  a 
high  angle.  The  fissures  cut  all  the  rocks,  but  are  most  clearly  defined  in  the  quartzite 
or  in  the  siliceous  portion  of  the  slate.  These  rocks  are  strong  and  brittle,  and  are  better 
adapted  to  forming  and  holding  open  the  cavities  in  which  the  quartz  and  ore  may  he 
deposited  than  are  the  slates.  Along  the  outcrops  of  the  Gold  Bug  lead  there  are  a  number 
of  shallow  surface  workings.     Some  of  them  are  said  to  have  produced  several  hundred 
n  Tost  for  tellurium:  Take  a  piece  of  the  mineral  about  the  size  of  a'  0.22  ball  crush  to  due  powder, 
place  in  a  glass  vessel  and  pour  in  about  ;is  much  concentrated  sulphuric  acid  as  would  (ill  an  ordinary 
thimble.    Heat  gently.    If  tellurium  is  present,  the  solution  will  assume  a  reddish-violel  color. 
