﻿40 
  REPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  After 
  the 
  branches 
  of 
  Blue 
  Creek 
  unite, 
  the 
  stream 
  produced 
  by 
  their 
  

   union 
  flows 
  in 
  a 
  straight 
  course 
  to 
  the 
  Gunnison. 
  On 
  both 
  sides 
  are 
  

   plateaus, 
  or 
  rather 
  mesas, 
  with 
  flat 
  tops, 
  (the 
  Twin 
  Mesas,) 
  which 
  have 
  

   approximately 
  the 
  same 
  level. 
  Under 
  this 
  capping, 
  along 
  the 
  Gunnison, 
  

   there 
  appear 
  to 
  be 
  both 
  breccia 
  and 
  sandstones, 
  as 
  seen 
  from 
  the 
  opposite 
  

   side 
  of 
  the 
  river. 
  

  

  On 
  the 
  northern 
  sides 
  I 
  was 
  unable 
  to 
  detect 
  their 
  presence. 
  In 
  many 
  

   places, 
  however, 
  if 
  not 
  in 
  all, 
  I 
  am 
  inclined 
  to 
  think 
  the 
  trachyte 
  rests 
  

   immediately 
  on 
  the 
  archsean 
  rocks, 
  without 
  any 
  interposed 
  layers. 
  

  

  CEBOLLA 
  AND 
  CIMMAEON 
  CREEKS. 
  

  

  The 
  Cebolla 
  is 
  the 
  main 
  stream 
  of 
  the 
  two 
  whose 
  consideration 
  we 
  

   now 
  take 
  up. 
  Together 
  they 
  drain 
  about 
  216 
  square 
  miles 
  of 
  country. 
  

   Cimmaron 
  Creek, 
  the 
  eastern 
  branch, 
  is 
  the 
  creek 
  next 
  west 
  of 
  Blue 
  

   Creek. 
  Both 
  branches 
  rise 
  in 
  the 
  Uncompahgre 
  Mountains, 
  draining 
  

   the 
  north 
  faces 
  of 
  Uncompahgre 
  Peak 
  and 
  the 
  peaks 
  west 
  and 
  north 
  in 
  

   the 
  immediate 
  vicinity. 
  About 
  five 
  miles 
  from 
  the 
  Gunnison 
  the 
  two 
  

   creeks 
  unite 
  in 
  an 
  open 
  valley. 
  This 
  valley 
  is 
  a 
  depression 
  in 
  Cretaceous 
  

   shales. 
  Both 
  rivers 
  are 
  terraced. 
  On 
  the 
  north 
  the 
  shales 
  seem 
  to 
  

   abut 
  directly 
  against 
  the 
  archsean 
  rocks. 
  It 
  is 
  altogether 
  probable 
  that 
  

   farther 
  south 
  the 
  older 
  formations 
  come 
  in 
  gradually 
  beneath 
  the 
  shales, 
  

   but 
  the 
  country 
  in 
  that 
  direction 
  soon 
  rises 
  into 
  high 
  hills 
  and 
  mount- 
  

   ains, 
  in 
  which 
  the 
  sedimentary 
  formations 
  are 
  covered 
  with 
  the 
  volcanic 
  

   rocks. 
  Bowlders 
  of 
  red 
  sandstone 
  are 
  seen 
  in 
  the 
  beds 
  of 
  the 
  creeks. 
  

   The 
  Cretaceous 
  shales 
  continue 
  uninterruptedly 
  across 
  to 
  the 
  Uncom- 
  

   pahgre 
  Valley. 
  Between 
  the 
  Cimmaron 
  and 
  Cebolla 
  is 
  a 
  high 
  plateau 
  

   which 
  seems 
  to 
  have 
  a 
  very 
  uniform 
  level. 
  It 
  is 
  covered 
  with 
  a 
  dense 
  

   growth 
  of 
  timber. 
  Between 
  the 
  Cebolla 
  and 
  the 
  Gunnison 
  are 
  high 
  

   granitic 
  hills. 
  There 
  are 
  three 
  summits. 
  The 
  two 
  most 
  eastern 
  have 
  a 
  

   capping 
  of 
  trachyte, 
  a 
  remnant 
  of 
  a 
  former 
  plateau-level 
  which 
  was 
  doubt- 
  

   less 
  a 
  continuation 
  of 
  the 
  mesa 
  on 
  the 
  west 
  side 
  of 
  Blue 
  Creek. 
  There 
  

   appears 
  to 
  be 
  no 
  breccia 
  between 
  the 
  schists 
  and 
  trachyte, 
  although 
  on 
  

   the 
  south 
  breccia 
  deposits 
  rest 
  on 
  the 
  Cretaceous 
  rocks 
  and 
  underlie 
  the 
  

   volcanic 
  mesas 
  that 
  stand 
  between 
  the 
  two 
  creeks 
  as 
  well 
  as 
  in 
  Tongue 
  

   Mesa 
  on 
  the 
  west 
  side 
  of 
  Cebolla 
  Creek. 
  The 
  strata 
  in 
  the 
  divide 
  be- 
  

   tween 
  Cebolla 
  Creek 
  and 
  the 
  Uncompahgre 
  belong 
  to 
  the 
  Upper 
  Creta- 
  

   ceous, 
  and 
  contain 
  layers 
  of 
  lignite. 
  

  

  UKCOMPAHGEE 
  VALLEY. 
  

  

  The 
  Uncompahgre 
  River 
  rises 
  in 
  the 
  Uncompahgre 
  Mountains, 
  its 
  total 
  

   length 
  from 
  the 
  mountains 
  to 
  the 
  mouth 
  in 
  the 
  Gunnison 
  being 
  about 
  65 
  

   miles. 
  It 
  is 
  formed 
  by 
  the 
  union 
  of 
  two 
  streams, 
  one 
  rising 
  in 
  the 
  heart 
  

   of 
  the 
  San 
  Juan 
  Mountains 
  and 
  the 
  other 
  between 
  Uncompahgre 
  Peak 
  

   and 
  Wilson's 
  station 
  10 
  of 
  1874. 
  On 
  the 
  east 
  side 
  of 
  the 
  latter, 
  (Eio 
  

   de 
  las 
  Vacas,) 
  which 
  is 
  the 
  most 
  eastern 
  branch, 
  is 
  a 
  long 
  narrow 
  

   mesa 
  about 
  14 
  miles 
  in 
  length. 
  It 
  preserves 
  the 
  same 
  general 
  level 
  for 
  

   that 
  distance, 
  breaking 
  off 
  to 
  the 
  south 
  into 
  a 
  ridge 
  of 
  curiously-shaped 
  

   peaks. 
  The 
  mesa 
  marks 
  the 
  portion 
  of 
  the 
  ridge 
  that 
  is 
  capped, 
  while 
  

   the 
  peaks 
  are 
  probably 
  composed 
  of 
  breccias, 
  with 
  perhaps, 
  in 
  some 
  

   places, 
  remnants 
  of 
  the 
  trachytic 
  flow 
  capping 
  them. 
  This 
  accounts 
  

   for 
  the 
  square 
  tops 
  that 
  are 
  noticed 
  on 
  some 
  of 
  them. 
  The 
  trachytic 
  

   capping 
  on 
  the 
  plateau 
  must 
  be 
  exceedingly 
  narrow 
  in 
  some 
  places. 
  

   -It 
  is 
  underlaid 
  with 
  breccia, 
  which 
  in 
  turn 
  rests 
  on 
  Cretaceous 
  strata, 
  

   below 
  which, 
  if 
  we 
  could 
  penetrate, 
  we 
  would 
  doubtless 
  find 
  strata 
  as 
  far 
  

  

  