﻿GEOLOGICAL 
  REPORT 
  ON 
  THE 
  SAN 
  J 
  DAN 
  DISTRICT. 
  

  

  CHAPTER 
  I. 
  

  

  GENERAL 
  REVIEW 
  OF 
  THE 
  DISTRICT— 
  OROGRAPHIC 
  FEA- 
  

   TURES. 
  

  

  On 
  descending 
  from 
  the 
  Weminuche 
  Pass 
  we 
  come 
  out, 
  at 
  an 
  elevation 
  

   of 
  8,000 
  feet, 
  upon 
  the 
  border 
  of 
  a 
  vast 
  broken 
  plain 
  which, 
  to 
  the 
  south 
  

   and 
  west, 
  seems 
  almost 
  limitless 
  ; 
  but 
  not 
  until 
  we 
  have 
  reached 
  the 
  

   banks 
  of 
  the 
  Rio 
  Mancos, 
  50 
  miles 
  farther 
  west, 
  and 
  have 
  passed 
  the 
  

   La 
  Plata 
  Mountains, 
  the 
  last 
  of 
  the 
  high 
  promontories 
  of 
  the 
  Colorado 
  

   or 
  Park 
  Mountains, 
  do 
  we 
  obtain 
  a 
  full 
  view 
  of 
  the 
  plateau 
  region. 
  

  

  To 
  one 
  who 
  has 
  been 
  long 
  among 
  the 
  great 
  highland 
  regions 
  of 
  Cen- 
  

   tral 
  Colorado 
  the 
  name 
  plateau 
  seems 
  almost 
  a 
  misnomer 
  when 
  applied 
  

   to 
  the 
  lowland 
  country 
  about 
  the 
  valley 
  of 
  the 
  Rio 
  Colorado 
  ; 
  but 
  when 
  

   considered 
  in 
  reference 
  to 
  the 
  sea-level 
  it 
  becomes 
  indeed 
  a 
  plateau 
  quite 
  

   grand 
  in 
  dimensions, 
  although 
  broken 
  into 
  a 
  thousand 
  fragments 
  by 
  the 
  

   labyrinth 
  of 
  caiions. 
  I 
  have 
  in 
  a 
  measure 
  become 
  accustomed 
  to 
  think 
  

   of 
  the 
  numerous 
  groups 
  and 
  chains 
  composing 
  the 
  Rocky 
  Mountains 
  in 
  

   Colorado 
  as 
  so 
  many 
  remnants 
  of 
  a 
  great 
  plateau 
  — 
  a 
  continental-divide 
  

   plateau 
  of 
  great 
  width 
  which 
  has 
  been 
  encroached 
  upon 
  and 
  cut 
  down 
  

   by 
  tbe 
  waters 
  of 
  the 
  two 
  oceans 
  until 
  only 
  a 
  small 
  number 
  of 
  widely 
  

   scattered 
  summits 
  remain 
  to 
  index 
  a 
  former 
  general 
  level 
  of 
  at 
  least 
  

   14,000 
  feet. 
  North 
  and 
  south 
  along 
  this 
  remarkable 
  collection 
  of 
  rem- 
  

   nants 
  the 
  continental 
  divide 
  runs, 
  meandering 
  a 
  belt 
  100 
  miles 
  in 
  width 
  

   from 
  east 
  to 
  west. 
  On 
  the 
  east 
  and 
  west 
  this 
  plateau 
  falls 
  off 
  quite 
  

   abruptly 
  to 
  the 
  level 
  of 
  7,000 
  feet, 
  or 
  one-half 
  of 
  the 
  depth 
  to, 
  the 
  sea- 
  

   level. 
  On 
  the 
  east 
  the 
  descent 
  is 
  quite 
  regular 
  and 
  complete, 
  there 
  being 
  

   no 
  great 
  promontories 
  or 
  outlying 
  masses. 
  On 
  the 
  west 
  the 
  line 
  is 
  

   decidedly 
  irregular 
  ; 
  a 
  few 
  groups 
  extend 
  far 
  to 
  the 
  west, 
  and 
  many 
  iso- 
  

   lated 
  patches 
  of 
  highland 
  occur. 
  Of 
  the 
  promontory 
  groups, 
  the 
  Sierra 
  

   San 
  Juan, 
  the 
  Elk 
  Mountains, 
  and 
  the 
  White 
  River 
  Plateau 
  are 
  the 
  

   priucij)al. 
  Alternating 
  with 
  these 
  are 
  four 
  great 
  bays, 
  so 
  to 
  speak, 
  into 
  

   which 
  the 
  approximately 
  horizontal 
  strata 
  extend. 
  This 
  is 
  well 
  shown 
  

   in 
  the 
  accompanying 
  diagram, 
  Plate 
  XXXIV. 
  It 
  will 
  be 
  plainly 
  seen 
  by 
  

   reference 
  to 
  the 
  figure, 
  that 
  Colorado 
  consists 
  of 
  three 
  distinct 
  belts, 
  

   topographically 
  as 
  well 
  as 
  geologically 
  — 
  a 
  belt 
  of 
  highland 
  chiefly 
  meta- 
  

   morphic, 
  two 
  belts 
  of 
  lowland 
  chiefly 
  sedimentary. 
  If 
  we 
  consider 
  the 
  

   metamorphic 
  belt 
  only, 
  and 
  its 
  relations 
  to 
  the 
  horizontal 
  sedimentaries, 
  

   the 
  two 
  middle 
  bays 
  become 
  one, 
  as 
  the 
  western 
  extension 
  of 
  the 
  Elk 
  

   Mountains 
  is 
  but 
  a 
  group 
  of 
  volcanic 
  masses 
  resting 
  on 
  and 
  in 
  and 
  

   protecting 
  the 
  horizontal 
  beds 
  which 
  belong 
  to 
  the 
  eastern 
  border 
  of 
  

   the 
  Colorado-Plateau 
  region. 
  Considered 
  alone, 
  however, 
  in 
  reference 
  

   to 
  the 
  actual 
  arrangement 
  of 
  the 
  highlands 
  and 
  lowlands, 
  there 
  appear 
  

   along 
  the 
  western 
  border 
  three 
  well-defined 
  promontories 
  and 
  four 
  great 
  

   bays. 
  Each 
  of 
  these 
  great 
  bays 
  possesses 
  a 
  system 
  of 
  drainage 
  of 
  its 
  own. 
  

   These 
  systems 
  are 
  represented 
  by 
  the 
  San 
  Juan, 
  the 
  Gunnison, 
  the 
  

  

  241 
  

   16 
  G 
  S 
  

  

  