﻿'208 
  REPOBT 
  UNiTED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  isfact^ory. 
  t'his 
  is 
  particularly 
  the 
  case 
  with 
  reference 
  to 
  the 
  early 
  geo-~ 
  

   logical 
  history 
  of 
  Saa 
  Luis 
  Valley? 
  

  

  Metamorphics. 
  — 
  Rocks 
  belongiug 
  to 
  this 
  group 
  crop 
  out 
  in 
  but 
  two 
  re- 
  

   gions, 
  in 
  the 
  Sangre 
  de 
  Cristo 
  and 
  the 
  Saw^tch 
  Eauges. 
  In 
  the 
  former 
  

   •they 
  occupy 
  a 
  centrar 
  position, 
  being 
  flanked 
  oa 
  either 
  side 
  by 
  sedimen- 
  

   tary 
  or 
  by 
  volcanic 
  formations. 
  In 
  their 
  structure 
  they 
  resemble 
  sedi- 
  

   mentary 
  beds 
  at 
  some 
  i:)laces, 
  but 
  at 
  others 
  any 
  similarity 
  that 
  might 
  be 
  

   so 
  construed 
  is 
  too 
  much 
  ol^itera^d 
  to 
  admit 
  of 
  direct 
  comparison. 
  

   Wherever 
  their 
  chamcter 
  is 
  sUch 
  as 
  to 
  permit 
  speaking 
  of 
  their 
  " 
  strata," 
  

   it 
  will 
  be*fonnd 
  that 
  the 
  latter 
  agree 
  in 
  their 
  relations 
  with 
  the 
  overly- 
  

   ing, 
  unchanged 
  sedimentary 
  beds. 
  This 
  we 
  find 
  to 
  be 
  the 
  case 
  more 
  

   particularly 
  on- 
  the 
  .western 
  slope 
  of 
  the 
  range. 
  Following 
  the 
  meta- 
  

   morphic 
  outcrops 
  *northward, 
  we 
  establish 
  a 
  connection 
  with 
  those 
  oc- 
  

   curring 
  on 
  the 
  Arkansas 
  River, 
  and 
  it 
  is 
  there 
  that 
  we 
  obtain 
  a 
  clew 
  as 
  

   to 
  their 
  origin. 
  In 
  the 
  report 
  of 
  1873 
  (p. 
  308) 
  Silurian 
  beds 
  identified 
  

   by 
  fossils 
  and 
  gfiognostic 
  position 
  have 
  been 
  described 
  as 
  ovei^lying 
  the 
  

   granite 
  of 
  the 
  immediate 
  Arkansas 
  region. 
  In 
  the 
  northern 
  portion 
  of 
  

   the 
  Sdiigre 
  de 
  Cristo 
  Range, 
  however, 
  they 
  disappear 
  in 
  the 
  vicinity 
  of 
  

   Hunt's 
  Peak. 
  South 
  of 
  that 
  it 
  may 
  be 
  observed 
  that 
  the 
  metamorphic 
  

   rocks 
  change. 
  Instead 
  of 
  an 
  unbroken 
  series 
  of 
  granite, 
  we 
  find 
  

   schists, 
  goeissic, 
  micaceous, 
  and 
  chloritic, 
  showing 
  evidence 
  of 
  having 
  

   been 
  subjected 
  to 
  v^ry 
  intense 
  metamorphosing 
  influences. 
  At 
  Mosco 
  

   Pass 
  the 
  number 
  of 
  varieties 
  reaches 
  its 
  maximum, 
  continuing 
  south- 
  

   ward 
  into 
  the 
  Blanca 
  group. 
  No 
  Silurian 
  whatever 
  is 
  found 
  in 
  the 
  

   southern 
  extension 
  of 
  the 
  range. 
  The 
  general 
  lithological 
  character 
  of 
  

   the 
  rocks 
  agrees 
  with 
  that 
  observed 
  farther 
  north, 
  but 
  differs 
  from 
  that 
  

   (if 
  the 
  Arkansas 
  vicinity. 
  Considering 
  the 
  conformability 
  of 
  younger 
  

   ibrmations 
  with 
  the 
  Silurian 
  strata 
  on 
  that 
  river, 
  and 
  couvsidering 
  

   furthermore 
  ^he 
  cdijformability 
  metamorphic 
  strata 
  show 
  after 
  the 
  

   disappearance 
  of 
  the 
  Silurian, 
  ^t 
  the 
  same 
  time 
  noting 
  the 
  change 
  that 
  

   takes 
  ]fiace 
  in 
  the 
  lithological 
  character 
  of 
  these 
  strata, 
  south 
  of 
  the 
  

   Silurian 
  outcrops, 
  I 
  have 
  come 
  to 
  the 
  conclusion 
  that 
  the 
  metamorphic 
  

   rocks 
  of 
  the 
  Sangre 
  de 
  Cristo 
  Range 
  represent 
  the 
  original 
  Silurian 
  

   beds. 
  By 
  following 
  the 
  dips 
  and 
  general 
  courses 
  of 
  the 
  strikes, 
  and 
  

   comparing 
  them 
  with 
  those 
  of 
  the 
  superincumbent, 
  younger 
  strata, 
  the 
  

   similarity 
  of 
  arrangement 
  l^etween 
  the 
  two 
  expresses 
  itself 
  very 
  defi.- 
  

   nitely. 
  " 
  > 
  * 
  

  

  In 
  the 
  Sawatch 
  Range 
  the 
  metamorphic 
  outcrop 
  is 
  not 
  so 
  extensive, 
  

   but 
  of 
  great 
  interest. 
  In 
  Chapter 
  III 
  mentigu 
  has 
  been 
  made 
  of 
  its 
  strat- 
  

   igraphical 
  conditions, 
  as 
  well 
  as 
  of 
  its 
  lithological 
  character. 
  Granite, 
  

   coarsegrained, 
  with 
  a 
  flesh-colored 
  feldspar, 
  forms 
  the 
  higher 
  members 
  

   of 
  the 
  group, 
  overlying 
  pure 
  quartzites, 
  anttquarlzites 
  gradually 
  merg- 
  

   ing 
  into 
  micaceous 
  and-sgneissic 
  schists. 
  We 
  have 
  here, 
  therefore, 
  the 
  

   same 
  relative 
  position 
  that 
  was 
  observed 
  in 
  the 
  Quartzite 
  Mountains 
  

   during 
  the 
  precedihg 
  year. 
  This 
  might 
  point 
  to 
  the 
  fact 
  of 
  the 
  two 
  

   having 
  been 
  formed 
  by 
  the 
  m.etamorphosisof 
  at 
  least 
  similarly 
  arranged 
  

   if 
  not 
  i^lentical 
  beds. 
  It 
  is 
  a 
  noticeable 
  feature, 
  that 
  none 
  of 
  the 
  older 
  

   formations 
  occur 
  along 
  the 
  westt?rn 
  base 
  of 
  the 
  range, 
  so 
  far 
  as 
  I 
  have 
  

   been 
  able 
  to 
  determine. 
  From 
  the 
  character 
  of 
  the 
  younger 
  ones 
  (Creta- 
  

   ceous 
  and 
  Post-Cretaceous) 
  it 
  must 
  be 
  inferred 
  that 
  if 
  they 
  exist 
  tkere, 
  

   \t 
  is 
  at 
  (considerable 
  d-epth. 
  In 
  .the 
  Quartzite 
  Mountains 
  we 
  have 
  defi- 
  

   nite 
  observations,* 
  showing 
  the 
  metamorphic 
  granite 
  and 
  a 
  portion 
  of 
  the 
  

   underlying 
  schists 
  and 
  quiU'tzites^to 
  have 
  been 
  formed 
  by 
  an 
  alteration 
  

   o^ 
  the 
  Silurian, 
  and, 
  in 
  part, 
  Devonian 
  strata. 
  This 
  same 
  origin 
  I 
  as- 
  

  

  * 
  <5aite(l«tate3 
  Gftplogical 
  Report 
  1874, 
  p. 
  li)l. 
  

  

  