IDAHO    SPRINGS    MINING    DISTRICT,    COLORADO.  37 
On  the  south  side  of  Clear  Creek  an  old  river  terrace  or  bench  extends  from  the  eastern 
of  the  Georgetown  quadrangle  along  the  south  bank  of  Clear  Creek  to  the  western 
edge  of  the  town  of  Idaho  Springs.  This  terrace  is  cut  through  by  both  Soda  and  Chicago 
creeks  and  is  beveled  by  Clear  Creek.  The  upper  part  of  the  scarp  due  to  the  action  of  1  he 
present  stream  is  about  100  feet  above  the  present  valley,  while  the  upper  part  of  the  terrace, 
which  has  a  slope  making  a  considerable  angle  with  the  present  stream,  is  about  130  feet 
above  Clear  Creek. 
The  terrace  gravels,  which  are  now  covered  by  the  Arthur  and  Doherty  placer  claims, 
were  formerly  extensively  worked  with  very  profitable  results.  This  terrace,  which  is 
located  in  the  southeast  angle  formed  by  the  union  of  Soda  and  Clear  creeks,  is  a  bed  rock 
bench  of  porphyry  which  in  rare  instances  reaches  the  surface,  but  is  usually  covered  with  a 
xiver  deposit  averaging  40  feet  in  thickness.  The  lower  20  feet  of  this  deposit  consists  of 
large  rounded  bowlders,  gravel,  and  sand,  while  the  upper  portion  is  composed  of  finer 
gravels  and  silts  which  are  capped  by  talus  deposits  having  an  average  thickness  of  6  feet. 
The  bedding  of  the  materials  in  these  deposits  dips  at  a  considerable  angle  toward  Clear 
Creek. 
Good  values  were  found  in  these  deposits,  the  best  ones  usually  occurring  either  in  the 
deeper  gravel-filled  channels  or  right  on  bed  rock.  Where  the  detritus  coating  above  bed 
rock  was  very  thick  tunnels  were  run  into  the  stream  banks  or  else  shafts  were  sunk  and 
drift  mining  was  resorted  to  to  expose  the  gold,  which  occurred  on  bed  rock  in  the  natural 
riffles  due  to  the  roughness  of  the  old  stream  bed.  Many  hundreds  of  feet  of  tunneling  and 
drifting  on  bed  rock  were  run  during  the  period  of  active  development,  and  according  to 
Fossetta  there  was  one  tunnel  over  900  feet  long  extending  into  the  bank  and  hillside 
adjoining  the  creek.  That  the  deposits  are  fairly  well  consolidated  is  shown  by  the  fact 
that  untimbered  drifts  and  tunnels  run  some  40  or  50  years  ago  can  still  be  explored  to  a 
considerable  extent. 
The  high  bench  found  at  the  junction  of  Soda  and  Clear  creeks  is  not  visible  below  this 
point  on  the  south  side  of  Clear  Creek  but  appears  again  on  the  north  side  about  one-fourth 
mile  downstream.  At  the  latter  place  is  a  horizontal  or  slightly  sloping  shelf  or  ledge  of 
coarse  conglomerate  and  soft  sandstone  or  granitic  arkose  of  a  character  somewhat  similar 
to  the  placer  deposits  on  the  other  side  of  the  valley.  This  fairly  well-indurated  conglomer- 
ate, which  is  comprised  chiefly  of  rounded  masses  of  gneiss  and  granite  but  also  contains 
some  rounded  pebbles  of  porphyry,  is  of  limited  extent  and  is  plastered  unconformably 
upon  the  rather  steep  gneiss  walls  of  the  valley  at  this  point. 
Placer  mining  remained  an  important  industry  in  Clear  Creek  County  and  added  materi- 
ally to  the  gold  production  up  to  the  early  eighties.  Since  that  time,  however,  although 
considerable  work  has  been  done  at  various  times,  it  has  been  with  variable  success  and 
as  a  rule  with  little  profit.  At  present,  aside  from  the  desultory  labors  of  a  few  miners 
who  occasionally  make  fair  wages,  little  work  is  being  done  on  the  deposits. 
Hot  springs. — On  the  east  side  of  Soda  Creek,  about  half  a  mile  above  its  mouth,  is  a 
hot  spring  which  has  deposited  grayish  calc-sinter  or  travertine  to  a  thickness  of  4  or  5  feet. 
The  lower  several  inches  of  this  deposit  are  soft  and  heavily  iron  stained.  Similar  deposits 
cover  the  east  bank  of  Soda  Creek  for  several  hundred  feet  to  the  north  of  the  present 
spring.  The  gravel  near  the  spring  is  cemented  by  the  same  calcareous  material  and 
closely  resembles  the  conglomerates  previously  described.  The  temperature  of  the  wafer, 
as  observed  in  two  short  tunnels  that  tap  the  flow,  is  106°  and  110°  F.  According  to  Mr. 
J.  R.  Allison,  however,  water  with  a  temperature  as  high  as  121°  F.  was  obtained  from  two 
wells  sunk  to  a  depth  of  25  feet  below  the  bed  of  Soda  Creek. 
Faulting. — Minor  faults  are  frequent  in  the  mines,  but  the  cross  faults  rarely  displace 
the  veins  more  than  a  few  feet.  The  greatest  known  displacement ,  that  of  the  Lit  tie  Mattie 
vein,  where  faulted  by  the  "cross  vein/'  is  from  10  to  25  feet.  In  the  Newhouse  tunnel  a 
small  porphyry  dike  which  cuts  several  pyrite  and  quartz  veinlets  is  step  faulted  by  a  series 
a  Fossett,  Colorado,  1880,  p.  381. 
