﻿242 
  EEPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY, 
  

  

  Grand, 
  and 
  the 
  Green 
  Rivers. 
  We 
  may 
  think 
  of 
  these 
  streams 
  as 
  orig- 
  

   inally 
  sweeping 
  out 
  at 
  right 
  angles 
  to 
  the 
  trend 
  of 
  the 
  metamorphic 
  

   highland 
  and 
  connecting 
  with 
  the 
  main 
  drainage-coarse 
  of 
  the 
  Eio 
  Col- 
  

   orado, 
  which 
  runs 
  from 
  north 
  to 
  south. 
  It 
  is 
  useless 
  to 
  speculate 
  as 
  to 
  

   what 
  geologic 
  horizon 
  formed 
  the 
  surface 
  of 
  the 
  plain 
  upon 
  which 
  these 
  

   drainage-systems 
  were 
  first 
  laid 
  down; 
  it 
  was 
  doubtless, 
  however, 
  high 
  

   in 
  the 
  Tertiaries. 
  

  

  At 
  tlie 
  present 
  time 
  there 
  remains 
  no 
  longer 
  a 
  great 
  plain, 
  but 
  in 
  its 
  

   place 
  we 
  have 
  a 
  grand 
  skeleton 
  only. 
  The 
  main 
  water-course 
  of 
  the 
  

   Colorado 
  has 
  cut 
  its 
  way 
  down 
  through 
  almost 
  the 
  entire 
  thickness 
  of 
  

   the 
  sedimentary 
  strata, 
  and 
  the 
  four 
  subordinate 
  streams 
  have 
  scored 
  

   out 
  huge 
  caiioued 
  valleys 
  in 
  whose 
  sides 
  are 
  exposed 
  the 
  rocks 
  of 
  all 
  

   periods 
  from 
  the 
  Tertiary 
  to 
  the 
  Archaean. 
  Such 
  enormous 
  quantities 
  

   of 
  the 
  strata 
  composing 
  the 
  upper 
  half 
  of 
  the 
  plateau 
  have 
  been 
  re- 
  

   moved 
  along 
  the 
  main 
  drainage-course 
  that 
  the 
  outcropping 
  edges 
  of 
  

   the 
  higher 
  strata 
  have 
  retreated 
  far 
  to 
  the 
  east, 
  and 
  the 
  Upper 
  Cre- 
  

   taceous 
  and 
  Tertiary 
  rocks 
  are 
  only 
  to 
  be 
  found 
  along 
  the 
  base 
  of 
  the 
  

   mountains 
  or 
  in 
  particularly 
  protected 
  spots. 
  

  

  The 
  district 
  which 
  I 
  have 
  examined 
  is 
  a 
  fragment 
  of 
  this 
  skeleton 
  

   plateau 
  which 
  includes 
  a 
  large 
  portion 
  of 
  the 
  drainage-system 
  of 
  the 
  

   San 
  Juan, 
  and 
  which 
  extends 
  from 
  the 
  base 
  of 
  the 
  mountains 
  to 
  within 
  

   thirty 
  miles 
  of 
  the 
  Rio 
  Colorado. 
  The 
  rocks 
  exposed 
  are 
  chiefly 
  Meso- 
  

   zoic 
  It 
  is 
  a 
  region 
  of 
  exceptional 
  interest 
  and 
  unusually 
  well 
  calculated 
  

   for 
  geologic 
  study. 
  

  

  GENERAL 
  SECTION, 
  ETC, 
  

  

  1 
  give 
  in 
  this 
  connection 
  a 
  general 
  section 
  of 
  the 
  strata 
  exposed 
  in 
  my 
  

   district 
  so 
  that 
  there 
  may 
  be 
  no 
  difficulty 
  in 
  identification. 
  In 
  most 
  

   cases 
  I 
  have 
  given 
  characteristic 
  local 
  names 
  to 
  the 
  more 
  marked 
  groups, 
  

   giving 
  at 
  the 
  same 
  time 
  such 
  correlations 
  with 
  other 
  sections 
  as 
  are 
  

   clearly 
  established. 
  

  

  The 
  entire 
  exposure 
  of 
  sedimentary 
  rocks 
  is 
  not 
  far 
  from 
  9,000 
  feet, 
  

   2,000 
  of 
  which 
  are 
  classed 
  as 
  Tertiary, 
  5,000 
  as 
  Cretaceous, 
  and 
  1,500 
  as 
  

   Jura-Trias. 
  A 
  considerable 
  exposure 
  of 
  rocks 
  of 
  Upper 
  Carboniferous 
  

   age 
  occurs 
  in 
  the 
  La 
  Plata 
  Mountains, 
  but 
  so 
  changed 
  by 
  metamorphism 
  

   that 
  nothing 
  definite 
  can 
  be 
  made 
  out. 
  The 
  Cretaceous 
  formations 
  

   occupy 
  the 
  major 
  part 
  of 
  the 
  district, 
  and 
  are 
  the 
  only 
  series 
  that 
  I 
  have 
  

   been 
  able 
  to 
  thoroughly 
  identify 
  from 
  the 
  fossil 
  remains. 
  

  

  The 
  section 
  given 
  in 
  Plate 
  XXXV, 
  was 
  made 
  principally 
  along 
  the 
  

   banks 
  of 
  the 
  Rio 
  San 
  Juan, 
  beginning 
  in 
  the 
  canon 
  near 
  the 
  mouth 
  of 
  

   the 
  Rio 
  Pinos 
  and 
  ending 
  in 
  Utah, 
  on 
  meridian 
  109° 
  30'. 
  The 
  dip 
  is 
  so 
  

   slight 
  that 
  in 
  this 
  distance 
  of 
  100 
  miles 
  only 
  these 
  9,000 
  feet 
  of 
  strata 
  

   rise 
  to 
  the 
  surface; 
  the 
  fall 
  of 
  the 
  river 
  in 
  the 
  same 
  distance 
  being 
  a 
  

   little 
  over 
  1,000 
  feet. 
  

  

  Notwithstanding 
  this 
  slight 
  dip 
  the 
  succession 
  of 
  strata 
  is 
  quite 
  easily 
  

   made 
  out 
  and 
  measurements 
  are 
  easily 
  made, 
  on 
  account 
  of 
  the 
  tendency 
  

   to 
  break 
  off 
  in 
  steps. 
  Each 
  group 
  of 
  hard 
  strata 
  is 
  carried 
  west 
  in 
  a 
  

   long, 
  gentle 
  slope 
  until 
  a 
  break 
  occurs, 
  when 
  a 
  precipitous 
  descent 
  is 
  

   made 
  across 
  the 
  outcropping 
  edges, 
  and 
  so 
  on 
  throughout 
  the 
  section. 
  

   It 
  will 
  be 
  seen 
  by 
  the 
  section 
  that 
  there 
  are 
  six 
  of 
  these 
  well-marked 
  

   layers 
  of 
  firm 
  rock, 
  in 
  all 
  cases 
  of 
  sandstone, 
  with 
  a 
  corresponding 
  num- 
  

   ber 
  of 
  soft 
  beds, 
  chiefly 
  shales. 
  There 
  are, 
  therefore, 
  six 
  well-marked 
  

   steps 
  observable 
  in 
  descending 
  the 
  river; 
  the 
  compact 
  groups 
  forming 
  

   each 
  a 
  floor 
  and 
  an 
  escarpment 
  and 
  the 
  soft 
  groups 
  a 
  slope 
  and 
  talus. 
  

  

  Frequent 
  reference 
  will 
  be 
  made 
  to 
  this 
  section 
  throughout 
  my 
  report, 
  

  

  