﻿HAYDEN.] 
  LETTEE 
  TO 
  THE 
  SECRETARY. 
  9 
  

  

  South 
  of 
  the 
  Sierra 
  La 
  Sal 
  are 
  fine 
  valleys 
  extending 
  nearly 
  to 
  the 
  

   head 
  of 
  the 
  Dolores. 
  Farther 
  west, 
  the 
  country 
  is 
  a 
  plateau, 
  without 
  

   water, 
  covered 
  with 
  sage 
  and 
  piSon-pine, 
  and 
  cut 
  by 
  numberless 
  dry 
  

   canons. 
  

  

  The 
  geological 
  features 
  of 
  the 
  district 
  surveyed 
  by 
  the 
  Grand 
  River 
  

   divisiou 
  during 
  the 
  season 
  of 
  1875 
  are 
  comparatively 
  simple, 
  there 
  being 
  

   no 
  great 
  uplifts 
  nor 
  many 
  local 
  disturbances. 
  The 
  sedimentary 
  forma- 
  

   tions 
  represented 
  are 
  all 
  included 
  under 
  Carboniferous 
  red 
  beds 
  (Trias- 
  

   sic?), 
  Jurassic, 
  and 
  Cretaceous. 
  Exposures 
  of 
  metamorphic 
  rocks 
  are 
  

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

   canons, 
  the 
  streams 
  having 
  cut 
  through 
  the 
  overlying 
  sedimentaries. 
  

   The 
  eruptive 
  areas 
  are 
  also 
  limited. 
  In 
  the 
  southern 
  part 
  of 
  the 
  district, 
  

   we 
  had 
  the 
  overlapping 
  edges 
  of 
  various 
  trachytic 
  flows, 
  whose 
  sources 
  

   of 
  origin 
  were 
  in 
  the 
  Uncompahgre 
  Mountains, 
  still 
  farther 
  south. 
  Be- 
  

   sides 
  these, 
  there 
  are 
  three 
  distinct 
  centers 
  of 
  eruption, 
  viz, 
  the 
  Lone 
  

   Cone 
  group 
  of 
  mountains 
  on 
  the 
  south, 
  the 
  Abajo 
  Mountains 
  in 
  the 
  

   southwest, 
  and 
  the 
  Sierra 
  La 
  Sal 
  Mountains 
  toward 
  the 
  northwest. 
  These 
  

   are 
  of 
  porphyritic 
  trachyte, 
  and 
  have 
  been 
  pushed 
  up 
  through 
  the 
  Cre- 
  

   taceous 
  layers, 
  which 
  dip 
  gently 
  from 
  them. 
  The 
  greater 
  part 
  of 
  the 
  

   district, 
  however, 
  is 
  covered 
  with 
  sedimentary 
  rocks, 
  generally 
  hori- 
  

   zontal, 
  or, 
  if 
  dipping, 
  but 
  little 
  inclined. 
  In 
  these 
  beds, 
  the 
  drainage 
  is 
  

   outlined 
  by 
  canons, 
  which 
  are 
  from 
  a 
  few 
  hundred 
  to 
  over 
  a 
  thousand 
  

   feet 
  in 
  depth. 
  During 
  the 
  summer 
  months, 
  the 
  streams 
  are 
  dry. 
  

  

  Leaving 
  the 
  LosPinos 
  Indian 
  agency, 
  the 
  first 
  work 
  was 
  on 
  the 
  south 
  

   side 
  of 
  the 
  Gunnison 
  Eiver, 
  in 
  a 
  narrow 
  strip 
  of 
  country 
  lying 
  between 
  

   Mr. 
  Gannett's 
  district 
  of 
  1874 
  and 
  that 
  of 
  Mr. 
  Wilson 
  for 
  the 
  same 
  year. 
  

   The 
  rocks 
  here 
  are 
  trachytes, 
  interlaminated 
  with 
  tuffs 
  in 
  horizontal 
  

   layers. 
  They 
  rest 
  partly 
  on 
  metamorphic 
  rocks, 
  and 
  partly 
  on 
  the 
  rem- 
  

   nants 
  of 
  Cretaceous 
  sandstones. 
  Previous 
  to 
  the 
  outpouring 
  of 
  these 
  

   trachytes, 
  the 
  country 
  was 
  evidently 
  subjected 
  to 
  considerable 
  erosion, 
  

   the 
  sandstones 
  being 
  in 
  many 
  places 
  entirely 
  removed, 
  exposing 
  the 
  

   gneissic 
  rocks 
  upon 
  which 
  they 
  were 
  deposited. 
  Going 
  westward 
  

   toward 
  the 
  Uncompahgre 
  River, 
  the 
  volcanic 
  rocks 
  disappear, 
  and 
  

   rocks 
  of 
  Upper 
  Cretaceous 
  age 
  show 
  in 
  bluffs 
  on 
  the 
  east 
  side. 
  The 
  

   weathering 
  of 
  these 
  beds 
  has 
  produced 
  a 
  barren 
  alkaline 
  soil, 
  in 
  

   which 
  there 
  is 
  no 
  vegetation. 
  In 
  the 
  immediate 
  river-bottom, 
  there 
  

   is 
  some 
  good 
  soil, 
  but 
  it 
  is 
  limited 
  in 
  extent. 
  The 
  course 
  of 
  the 
  Un- 
  

   compahgre 
  is 
  a 
  few 
  degrees 
  west 
  of 
  north, 
  and 
  between 
  it 
  and 
  the 
  

   drainage 
  of 
  the 
  San 
  Miguel 
  and 
  Dolores 
  Rivers, 
  which 
  has 
  approxi- 
  

   mately 
  the 
  same 
  direction, 
  is 
  a 
  plateau-like 
  country, 
  with 
  a 
  gentle 
  slope 
  

   to 
  the 
  eastward, 
  toward 
  the 
  Uncompahgre, 
  and 
  breaking 
  off 
  in 
  benches 
  on 
  

   the 
  Dolores 
  side. 
  Seen 
  from 
  the 
  mountains, 
  this 
  plateau 
  appears 
  very 
  

   regular; 
  nevertheless, 
  it 
  is 
  very 
  much 
  cut 
  up 
  by 
  numerous 
  caSons, 
  which 
  

   carry 
  water 
  only 
  in 
  wet 
  seasons. 
  The 
  floor 
  of 
  the 
  plateau 
  is 
  composed 
  

   chiefly 
  of 
  sandstones 
  of 
  the 
  Dakota 
  group 
  (Cretaceous 
  No. 
  1), 
  underlaid 
  

   by 
  Jurassic 
  shales 
  and 
  red 
  beds 
  (Triassic?), 
  which 
  rest 
  upon 
  metamorphic 
  

  

  