ipving.]  OKE    DEPOSITS    NEAR    LAKE    CITY,    COLO.  79 
rock  a  strong  resemblance  to  sedimentary  material,  but  the  bands  are 
undulating  and  their  continuity  is  broken  by  innumerable  inclusions, 
either  lenticular — owing  to  the  action  of  flow — or  sharply  angular 
and  scattered  without  regularity  through  the  rock.  These  inclusions 
are  either  broken  fragments  of  the  rhyolite  itself  or  masses  of  differ- 
ent rocks  which  have  been  caught  up  during  the  movement  of  the  rock 
magma.  They  vary  in  size  from  merely  microscopic  pieces  to  10  or 
15  feet  in  diameter.  In  most  cases  their  diameter  does  not  exceed  an 
inch.  Into  and  through  these  rhyolite  breccias  intrusions  of  andesite- 
porphyry  have  forced  their  way  and  may  now  be  seen  either  in  great 
irregular  masses,  such  as  near  the  Red  River  mill,  in  irregular  dikes 
which  often  show  columnar  structure,  and  in  widely  extended,  nearly 
horizontal  sills,  which  frequently  break  down  and  form  abrupt  cliffs. 
One  of  these  can  be  seen  in  the  vicinity  of  the  mine  workings  in 
Yellowstone  Gulch. 
Irregular  beds  of  volcanic  tuff  and  agglomerate  that  belong  to  both 
the  rhyolite  and  andesite  groups  are  mingled  with  the  rhyolite  brec- 
cias in  a  confused  manner.  South  of  Lake  City  the  geology  is 
slightly  different.  While  the  flow-breccias  extend  in  that  direction 
to  the  limit  of  the  mining  region,  they  become  subordinate  in  amount 
to  the  rhyolitic  and  andesitic  tuffs  which  are  very  largely  developed. 
The  andesite-porphyries  seem  to  be  absent  in  this  vicinity.  The  hill 
above  the  Golden  Fleece  mine  is  capped  with  a  very  thick  sheet  of 
latite,  a  pink  porphyritic  rock  of  very  massive  character.  At  the 
base  of  the  cliffs  formed  by  this  rock  alternating  rhyolitic  and  ande- 
sitic tuffs  form  the  canyon  walls.  A  small  dike  of  diabase  or  related 
rock  is  also  found  in  the  vicinity  of  the  Golden  Fleece  mine. 
Other  eruptive  rocks  undoubtedly  occur  in  the  region,  but  they  arc 
not  exposed  in  the  vicinity  of  the  mines.  The  entire  rock  series  is 
traversed  by  faults,  which  in  some  cases  involve  considerable  dis- 
placement. These  faults  are  at  some  places  heavily  mineralized  and 
at  others  are  without  any  indications  of  alteration.  Their  age  does 
not  seem  to  be  determinable  with  accuracy. 
The  geologic  age  of  the  volcanic  rocks  can  not  be  determined  from 
the  exposures  in  the  region  under  discussion,  but  the  westward  exten- 
sion of  some  of  them  into  the  Ouray  quadrangle  furnishes  evidence 
that  they  are  probably  of  Eocene  or  post-Eocene  age. 
In  the  vicinity  of  Lake  City  landslides  have  brought  great  masses 
of  apparently  massive  rock  from  higher  levels  into  the  bed  of  the 
canyon. 
HISTORY. 
The  Lake  City  mining  region  is  among  the  oldest  mining  camps  in 
Colorado.  It  is  not  at  present  a  heavy  producer,  but  the  mines  which 
have  supported  the,  town  in' former  years  have  been  some  of  the  most 
