﻿174 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  appearance, 
  and 
  from 
  the 
  relative 
  position 
  they 
  occupy 
  to 
  the 
  superincum- 
  

   bent 
  trachytes, 
  we 
  can 
  see 
  that 
  at 
  the 
  time 
  of 
  the 
  eruptions 
  they 
  pre- 
  

   sented 
  a 
  varied 
  surface, 
  mountains 
  and 
  valleys. 
  Inasmuch 
  as 
  the 
  volcanic 
  

   flows 
  bave 
  shown 
  themselves 
  to 
  be 
  very 
  constant 
  in 
  their 
  vertical 
  dimen- 
  

   sions, 
  it 
  may 
  be 
  deduced 
  that 
  the 
  first 
  drainage 
  existing 
  after 
  their 
  

   deposition, 
  before 
  the 
  older 
  formations 
  had 
  been 
  entirely 
  covered, 
  will 
  

   have 
  followed 
  approximately 
  its 
  old 
  courses. 
  In 
  accordance 
  with 
  tbis 
  

   view 
  we 
  find 
  the 
  distribution 
  of 
  the 
  conglomerate. 
  Northeast 
  of 
  the 
  

   nearest 
  metamorphic 
  outcrops 
  of 
  the 
  Quartzite 
  group, 
  we 
  find 
  the 
  con- 
  

   glomerates 
  of 
  South 
  Eiver 
  and 
  its 
  vicinity. 
  Beyond 
  that, 
  down 
  the 
  Eio 
  

   Grande, 
  they 
  disappear. 
  Southwest 
  of 
  the 
  group 
  mentioned, 
  the 
  same 
  

   formation 
  occurs 
  along 
  the 
  edge 
  of 
  the 
  mountains. 
  Again, 
  we 
  find 
  that, 
  

   in 
  the 
  neighborhood 
  of 
  the 
  station 
  94 
  metamorphics, 
  the 
  conglomerate 
  

   reaches 
  an 
  extraordinary 
  development 
  both 
  horizontally 
  and 
  vertically. 
  

   Following 
  the 
  outcrops 
  of 
  the 
  conglomerate, 
  and 
  keeping 
  in 
  view 
  its 
  

   thickness, 
  I 
  would 
  infer 
  that 
  it 
  has 
  been 
  composed 
  mainly 
  of 
  material 
  

   derived 
  from 
  the 
  range 
  itself, 
  and 
  that 
  those 
  places 
  afibrding 
  the 
  great- 
  

   est 
  facilities 
  for 
  its 
  deposition 
  by 
  water 
  at 
  present 
  show 
  both 
  the 
  great- 
  

   est 
  areas 
  and 
  the 
  greatest 
  thickness 
  of 
  it. 
  From 
  the 
  fact 
  that, 
  wherever 
  

   found, 
  the 
  conglomerate 
  is 
  covered 
  by 
  subsequent 
  flows 
  of 
  volcanic 
  rocks, 
  

   it 
  must 
  be 
  inferred 
  that 
  its 
  level 
  at 
  the 
  time 
  of 
  these 
  second 
  eruptions 
  

   was 
  a 
  very 
  uniform 
  one. 
  Had 
  it 
  been 
  deposited 
  by 
  flowing 
  water 
  only, 
  

   this 
  would 
  scarcely 
  have 
  been 
  the 
  case 
  ; 
  local 
  accumulations 
  would 
  have 
  

   raised 
  hills 
  at 
  one 
  point, 
  while 
  erosion 
  would 
  have 
  produced 
  depressions 
  

   at 
  another. 
  It 
  is 
  i^robable, 
  therefore, 
  that 
  the 
  drift-material 
  was 
  depos- 
  

   ited 
  into 
  a 
  large 
  body 
  of 
  still 
  water. 
  This 
  would 
  account 
  for 
  the 
  char- 
  

   acteristic 
  features 
  of 
  the 
  conglomerate 
  analogous 
  to 
  such 
  as 
  are 
  often 
  

   observed 
  in 
  sandstones. 
  

  

  Considering 
  the 
  basalt, 
  we 
  meet 
  with 
  a 
  difficult 
  question. 
  It 
  is 
  the 
  

   one 
  referring 
  to 
  its 
  place 
  of 
  eruption. 
  Are 
  we 
  to 
  assume 
  that 
  the 
  entire 
  

   mass, 
  some 
  of 
  which 
  reaches 
  to 
  an 
  elevation 
  of 
  more 
  than 
  12,000 
  feet, 
  

   should 
  have 
  flowed 
  from 
  such 
  points 
  of 
  eruption 
  as 
  Mount 
  San 
  Antonio, 
  

   10,900 
  feet 
  high, 
  and 
  subsequently 
  have 
  been 
  raised 
  to 
  its 
  present 
  alti- 
  

   tude? 
  It 
  certainly 
  is 
  possible, 
  and 
  would 
  explain 
  the 
  general 
  easterly 
  

   dip 
  of 
  the 
  Sawatch 
  Range, 
  but 
  where 
  the 
  force 
  producing 
  this 
  uplift 
  came 
  

   from, 
  or 
  where 
  else 
  it 
  manifested 
  itself, 
  is 
  not 
  completely 
  answered 
  by 
  the 
  

   study 
  of 
  the 
  surrounding 
  country. 
  We 
  have 
  farther 
  to 
  the 
  northwest, 
  

   a 
  long 
  distance 
  off, 
  it 
  is 
  true, 
  high 
  basaltic 
  plateaus 
  upon 
  which 
  stations 
  

   (stations 
  3, 
  4, 
  5, 
  19, 
  and 
  20) 
  were 
  located 
  during 
  1874. 
  It 
  seems 
  more 
  

   probable 
  to 
  me 
  that 
  the 
  lower 
  strata 
  of 
  basalt 
  that 
  we 
  find 
  in 
  San 
  Luis 
  

   Valley, 
  and 
  the 
  strata 
  of 
  the 
  southern 
  end 
  of 
  the 
  Sawatch 
  Eange, 
  should 
  

   have 
  originated 
  near 
  those 
  plateaus 
  and 
  extended 
  southward 
  to 
  the 
  

   localities 
  where 
  now 
  we 
  find 
  them. 
  All 
  the 
  more 
  does 
  this 
  seem 
  prob- 
  

   able, 
  as 
  we 
  find 
  them 
  in 
  both 
  places 
  overlying 
  the 
  same 
  tracbytic 
  strata, 
  

   a 
  direct 
  connection 
  between 
  which 
  can 
  by 
  far 
  more 
  readily 
  be 
  estab- 
  

   lished. 
  Those 
  points 
  of 
  outflow 
  that 
  we 
  iSnd 
  in 
  San 
  Luis 
  Valley 
  are 
  

   essentially 
  of 
  local 
  significance 
  only, 
  and 
  although 
  their 
  influence 
  is 
  felt 
  

   in 
  the 
  volcanic 
  beds 
  occurring 
  in 
  the 
  valley, 
  they 
  have 
  scarcely 
  supplied 
  

   the 
  material 
  for 
  the 
  beds 
  that 
  cover 
  plateaus 
  1,200 
  feet 
  above 
  their 
  own 
  

   summits. 
  So 
  far 
  as 
  lithological 
  characters 
  are 
  concerned 
  no 
  definite 
  

   opinion 
  can 
  be 
  reached, 
  as 
  the 
  varieties 
  at 
  every 
  point 
  of 
  eruption 
  are 
  

   e'xceediugly 
  numerous, 
  and 
  among 
  these 
  varieties 
  such 
  will 
  certainly 
  be 
  

   found 
  that 
  correspond 
  with 
  others 
  from 
  distant 
  localities. 
  Comparing 
  

   the 
  rocks 
  from 
  separated 
  plateaus 
  where 
  they 
  occur 
  merely 
  as 
  a 
  cover- 
  

   ing, 
  or 
  from 
  the 
  capping 
  of 
  mountaius, 
  will 
  furnish 
  valuable 
  hints, 
  but 
  

  

  