﻿34 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  they 
  will 
  be 
  referred 
  to 
  in 
  more 
  detail. 
  The 
  higher 
  plateaus 
  are 
  tim- 
  

   bered 
  with 
  pines 
  and 
  quaking-aspens 
  ; 
  the 
  lower 
  ones 
  are 
  generally 
  

   destitute 
  of 
  timber. 
  Along 
  the 
  courses 
  of 
  the 
  streams 
  are 
  groves 
  of 
  

   cottonwoods, 
  which, 
  however, 
  are 
  not 
  numerous. 
  

  

  The 
  geological 
  features 
  of 
  the 
  district 
  are 
  comparatively 
  simple, 
  there 
  

   being 
  no 
  great 
  uplifts 
  nor 
  many 
  local 
  disturbances. 
  The 
  sedimentary 
  

   formations 
  represented 
  are 
  all 
  included 
  under 
  Carboniferous 
  Triassic 
  "? 
  

   (red-beds), 
  Jurassic?, 
  and 
  Cretaceous. 
  Exposures 
  of 
  metamorphic 
  rocks 
  

   are 
  seen 
  in 
  several 
  parts 
  of 
  the 
  district, 
  limited 
  mainly 
  to 
  the 
  bottoms 
  of 
  

   canons 
  where 
  the 
  streams 
  have 
  cut 
  through 
  the 
  overlying 
  sedimenta- 
  

   ries. 
  The 
  areas 
  covered 
  with 
  eruptive 
  rocks 
  are 
  also 
  limited. 
  

  

  In 
  the 
  southern 
  and 
  southeastern 
  portions 
  of 
  the 
  district 
  are 
  the 
  

   overlying 
  edges 
  of 
  the 
  various 
  trachorheite 
  flows, 
  whose 
  sources 
  of 
  origin 
  

   are 
  the 
  hign 
  peaks 
  of 
  the 
  Uucompahgre 
  Mountains, 
  still 
  farther 
  south, 
  

   m 
  Mr. 
  Wilson's 
  district 
  for 
  1874. 
  The 
  rocks 
  are 
  trachytes 
  interlam- 
  

   inated 
  with 
  tufas 
  in 
  horizontal 
  layers. 
  South 
  of 
  the 
  Gunnison, 
  between 
  

   our 
  district 
  of 
  1874' 
  and 
  Mr. 
  Wilson's 
  district 
  for 
  the 
  same 
  year, 
  they 
  are 
  

   underlaid 
  by 
  breccia 
  deposits 
  which 
  rest 
  partly 
  on 
  metamorphic 
  rocks 
  

   and 
  partly 
  on 
  remnants 
  of 
  sandstone 
  which 
  are 
  in 
  all 
  probability 
  of 
  

   Cretaceous 
  age. 
  Previous 
  to 
  the 
  outpouring 
  of 
  these 
  trachytic 
  flows 
  

   the 
  country 
  had 
  evidently 
  been 
  subjected 
  to 
  considerable 
  erosion, 
  the 
  

   sandstones 
  in 
  many 
  places 
  having 
  been 
  completely 
  eroded, 
  exposing 
  the 
  

   gneissic 
  rocks 
  upon 
  which 
  they 
  were 
  deposited. 
  Besides 
  this 
  lava-cov- 
  

   ered 
  region 
  there 
  are 
  three 
  distinct 
  centres 
  of 
  eruption, 
  viz, 
  the 
  Sierra 
  

   la 
  Sal, 
  Abajo 
  Mountains, 
  and 
  the 
  Lone 
  Cone 
  group. 
  The 
  Sierra 
  la 
  Sal 
  

   is 
  typical 
  of 
  them 
  all, 
  and 
  is 
  composed 
  of 
  porphyritic 
  trachyte 
  which 
  

   has 
  been 
  pushed 
  up 
  through 
  the 
  sedimentary 
  layers 
  which 
  now 
  dip 
  

   away 
  from 
  the 
  mountains. 
  The 
  greater 
  part 
  of 
  the 
  district 
  is 
  covered 
  

   with 
  sedimentary 
  rocks, 
  of 
  which 
  the 
  Cretaceous 
  formation 
  .is 
  most 
  

   largely 
  represented 
  on 
  the 
  surface. 
  

  

  South 
  of 
  the 
  Gunnison 
  Elver, 
  as 
  far 
  west 
  as 
  the 
  Uncompahgre 
  Eiver, 
  

   the 
  prevailing 
  rocks, 
  as 
  we 
  have 
  already 
  noted, 
  are 
  volcanic. 
  The 
  

   Cretaceous 
  sandstones 
  beneath 
  show 
  only 
  occasionally 
  in 
  some 
  of 
  the 
  

   canons, 
  and 
  are 
  rather 
  of 
  Lower 
  Cretaceous 
  than 
  of 
  Upper 
  Cretaceous 
  

   age. 
  

  

  In 
  the 
  Uncompahgre 
  Valley, 
  however, 
  especially 
  on 
  the 
  eastern 
  side, 
  

   Upper 
  Cretaceous 
  rocks 
  prevail, 
  outcropping 
  in 
  bluffs 
  between 
  the 
  river 
  

   and 
  the 
  caSon 
  of 
  the 
  Gunnison, 
  forming 
  mauvaises 
  terres. 
  In 
  the 
  valley 
  

   of 
  the 
  Gunnison, 
  opposite 
  the 
  mouth 
  of 
  the 
  Uucompahgre, 
  in 
  the 
  district 
  

   of 
  1874, 
  they 
  are 
  well 
  marked 
  and 
  were 
  described 
  in 
  the 
  report 
  for 
  1874. 
  

   The 
  erosion 
  of 
  these 
  beds 
  has 
  produced 
  a 
  barren, 
  alkaline 
  soil, 
  in 
  which, 
  

   partly 
  from 
  want 
  of 
  water, 
  vegetation 
  does 
  not 
  flourish. 
  In 
  the 
  immediate 
  

   river-bottom, 
  especially 
  at 
  the 
  upper 
  part 
  of 
  the 
  stream, 
  there 
  is 
  some 
  

   good 
  soil, 
  but 
  it 
  is 
  linjited 
  in 
  extent. 
  The 
  Uncompahgre, 
  San 
  Miguel, 
  

   and 
  Dolores 
  Elvers 
  are 
  approximately 
  parallel, 
  the 
  general 
  direction 
  

   being 
  a 
  few 
  degrees 
  west 
  of 
  north. 
  Between 
  the 
  San 
  Miguel 
  and 
  Un- 
  

   compahgre 
  Elvers 
  the 
  countrj^ 
  is 
  a 
  plateau 
  which 
  slopes 
  gently 
  toward 
  

   the 
  Uucompahgre 
  from 
  a 
  point 
  about 
  25 
  miles 
  west 
  of 
  the 
  river. 
  At 
  

   this 
  distance 
  is 
  a 
  crest 
  parallel 
  to 
  the 
  San 
  Miguel. 
  It 
  is 
  the 
  edge 
  of 
  a 
  

   monoclinal 
  fold 
  which, 
  as 
  we 
  go 
  toward 
  the 
  north, 
  becomes 
  faulted 
  in 
  

   places. 
  Between 
  the 
  Uncompahgre 
  plateau 
  and 
  the 
  San 
  Miguel 
  .is 
  

   another 
  plateau, 
  (the 
  San 
  Miguel,) 
  very 
  nearly 
  level, 
  and 
  from 
  eight 
  to 
  

   ten 
  miles 
  wide. 
  The 
  floor 
  of 
  both 
  these 
  plateaus 
  is 
  the 
  sandstone 
  of 
  

   the 
  Dakota 
  group, 
  (Cretaceous 
  'No. 
  1.) 
  Farther 
  north, 
  between 
  the 
  Gun- 
  

   nison 
  and 
  Dolores, 
  the 
  Uncompahgre 
  plateau 
  has 
  the 
  same 
  width 
  as 
  

   it 
  has 
  farther 
  south, 
  but 
  on 
  the 
  Dolores 
  side 
  there 
  is 
  oftener 
  a 
  fault 
  than 
  

  

  