﻿264 
  REPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  ■will 
  average 
  about 
  G,000 
  feet 
  altitude. 
  The 
  general 
  dip 
  is 
  to 
  tlie 
  soutli, 
  

   at 
  a 
  gentle 
  angle, 
  so 
  that 
  in 
  going 
  north 
  from 
  this 
  line 
  we 
  rise 
  on 
  a 
  gen- 
  

   tle 
  slope 
  which 
  continues, 
  with 
  but 
  little 
  variation, 
  to 
  our 
  northern 
  line, 
  

   a 
  distance 
  of 
  40 
  miles, 
  where 
  it 
  will 
  be 
  found 
  that 
  the 
  upper 
  surface 
  of 
  

   the 
  Dakota 
  sandstones 
  lias 
  an 
  altitude 
  of 
  from 
  7,000 
  to 
  8,000 
  feet. 
  The 
  

   actual 
  surface 
  of 
  the 
  country 
  varies 
  but 
  little 
  from 
  this 
  plane. 
  Some 
  

   remnants 
  of 
  the 
  Middle 
  Cretaceous 
  occur 
  on 
  the 
  north 
  edge, 
  and 
  the 
  

   various 
  canon-beds 
  are 
  sunk 
  to 
  considerably 
  varying 
  depths 
  beneath 
  

   the 
  surface. 
  

  

  Kow 
  the 
  i)eculiar 
  relations 
  of 
  the 
  Eio 
  Dolores 
  to 
  this 
  plane 
  may 
  be 
  

   easily 
  understood 
  by 
  an 
  examination 
  of 
  Plate 
  XLII. 
  

  

  On 
  leaving 
  the 
  mountains 
  in 
  the 
  northeast 
  corner 
  (a), 
  which 
  is 
  con- 
  

   siderably 
  higher 
  than 
  the 
  northwest 
  (&), 
  the 
  river 
  flows 
  at 
  first 
  in 
  a 
  

   canon 
  over 
  2,000 
  feet 
  in 
  depth. 
  In 
  its 
  southward 
  course 
  its 
  angle 
  of 
  

   descent 
  is 
  so 
  much 
  less 
  than 
  that 
  of 
  the 
  surface 
  of 
  the 
  plane 
  that 
  it 
  

   rises 
  in 
  the 
  geologic 
  scale 
  from 
  the 
  JBed 
  Beds 
  to 
  the 
  upper 
  part 
  of 
  the 
  

   Upper 
  Dakota 
  sandstones. 
  When, 
  however, 
  it 
  is 
  within 
  one 
  hundred 
  

   feet 
  or 
  less 
  of 
  the 
  upper 
  surface 
  of 
  these 
  sandstones 
  (c) 
  — 
  which' 
  form 
  the 
  

   upper 
  stratum 
  of 
  the 
  confining 
  walls 
  throughout 
  the 
  entire 
  course 
  — 
  it 
  

   suddenly 
  changes 
  its 
  course, 
  turns 
  to 
  the 
  north, 
  and 
  cuts 
  back 
  into 
  the 
  

   plane, 
  and 
  descends 
  through 
  the 
  geologic 
  scale 
  in 
  a 
  gradually 
  deepening 
  

   caiion 
  which, 
  at 
  the 
  northern 
  edge 
  of 
  the 
  plane 
  (&), 
  is 
  2,000 
  feet 
  deep. 
  

  

  Why 
  this 
  stream 
  should 
  thus 
  stop 
  midway 
  in 
  a 
  great 
  slope, 
  which 
  

   presents 
  no 
  obstacle 
  to 
  its 
  continuous 
  flow 
  southward 
  to 
  the 
  San 
  Juan, 
  

   and 
  turn 
  back 
  upon 
  itself 
  against 
  the 
  slope, 
  is 
  an 
  interesting 
  question.. 
  

   At 
  the 
  extreme 
  southern 
  bend 
  (c), 
  as 
  will 
  be 
  seen 
  by 
  the 
  diagram, 
  the 
  

   bed 
  of 
  the 
  stream 
  is 
  within 
  a 
  few 
  feet 
  of 
  the 
  surface. 
  So 
  rapid 
  is 
  the 
  

   descent 
  of 
  the 
  plane 
  to 
  the 
  south 
  from 
  this 
  point, 
  that 
  if 
  the 
  stream 
  

   could 
  be 
  carried 
  one 
  mile 
  farther 
  south 
  it 
  would 
  reach 
  the 
  surface 
  of 
  

   the 
  sandstone 
  which 
  confines 
  it, 
  and 
  would 
  then 
  assume 
  its 
  natural 
  

   relations 
  to 
  the 
  present 
  configuration 
  of 
  the 
  country, 
  and 
  become 
  a 
  

   tributary 
  of 
  the 
  San 
  Juan, 
  since 
  from 
  the 
  point 
  e 
  to 
  the 
  San 
  Juan 
  

   there 
  is 
  an 
  unbroken 
  slope 
  which 
  descends 
  more 
  than 
  2,000 
  feet 
  in 
  45 
  

   miles. 
  

  

  With 
  tho 
  last-mentioned 
  fact 
  in 
  view, 
  it 
  will 
  readily 
  be 
  seen 
  why 
  the 
  

   many-armed 
  canons, 
  tributary 
  to 
  the 
  San 
  Juan, 
  reach 
  up 
  to 
  the 
  very 
  

   brink 
  of 
  the 
  cai3on' 
  of 
  the 
  Dolores, 
  and 
  also 
  why 
  the 
  direction 
  of 
  all 
  

   the 
  northern 
  tributaries 
  of 
  the 
  Dolores 
  is 
  to 
  the 
  south 
  ; 
  it 
  is 
  the 
  natural 
  

   drainage 
  of 
  the 
  country 
  as 
  regulated 
  bj^ 
  the 
  present 
  i)osition 
  of 
  the 
  

   surface 
  strata. 
  

  

  During 
  the 
  outlining 
  of 
  the 
  main 
  channel 
  of 
  the 
  Dolores, 
  however, 
  we 
  

   will 
  have 
  to 
  suppose 
  conditions 
  which 
  are 
  not 
  now 
  evident; 
  conditions 
  

   which 
  have 
  governed 
  the 
  course 
  of 
  the 
  main 
  channel 
  but 
  not 
  the 
  course 
  

   of 
  the 
  less 
  important 
  drainage, 
  or 
  at 
  least 
  that 
  the 
  latter 
  has 
  in 
  a 
  great 
  

   measure 
  conformed 
  to 
  the 
  new 
  conditions. 
  

  

  Let 
  us 
  suppose 
  that 
  the 
  course 
  of 
  the 
  river 
  was 
  originally 
  determined 
  

   wliile 
  the 
  sedimentary 
  rocks 
  were 
  in 
  a 
  horizontal 
  position 
  over 
  this 
  area, 
  

   or, 
  perhaps 
  better, 
  that 
  there 
  was 
  a 
  gentle 
  dip 
  to 
  the 
  northwest. 
  The 
  

   waters 
  accumulating 
  on 
  the 
  highlands 
  to 
  the 
  east 
  have 
  encountered 
  the 
  

   bodies 
  of 
  trachyte 
  which 
  at 
  one 
  time 
  doubtless 
  extended 
  westward 
  over 
  

   the 
  Dolores 
  Plateau, 
  and 
  have 
  thus 
  been 
  turned 
  to 
  the 
  southwest, 
  encoun- 
  

   tering 
  in 
  turn 
  the 
  belt 
  protected 
  by 
  the 
  La 
  Plata 
  trachytes; 
  when 
  finallj^ 
  

   the 
  open 
  country 
  has 
  been 
  reached 
  the 
  unimpeded 
  waters 
  would 
  turn 
  to 
  

   the 
  northwest 
  and 
  descend 
  the 
  even 
  slope. 
  Such 
  conditions 
  may 
  or 
  

   may 
  }iot 
  have 
  existed. 
  It 
  does 
  not 
  matter 
  whether 
  the 
  course 
  was 
  

   determined 
  in 
  this 
  way 
  or 
  in 
  some 
  other 
  way; 
  I 
  wish 
  only 
  to 
  give 
  the 
  

  

  