﻿HOLJiEs] 
  LA 
  PLATA 
  MOUNTAINS. 
  269 
  

  

  mass, 
  locally 
  developed 
  by 
  causes 
  which 
  have 
  operated 
  only 
  over 
  a 
  

   small 
  area. 
  " 
  These 
  causes 
  are 
  uot 
  easily 
  ascertained, 
  unless 
  we 
  conclude 
  

   that 
  the 
  intrusion 
  and 
  outflow 
  of 
  the 
  great 
  masses 
  of 
  trachyte 
  would 
  be 
  

   sufficient 
  to 
  produce 
  all 
  local 
  peculiarities. 
  

  

  Against 
  attributing 
  any 
  great 
  amount 
  of 
  change 
  in 
  the 
  sedimentary 
  

   rocks 
  to 
  the 
  presence 
  of 
  the 
  trachyte, 
  is 
  the 
  fact 
  that 
  in 
  the 
  neighboring 
  

   groups 
  of 
  mountains 
  of 
  trachytic 
  origin 
  there 
  is 
  little 
  or 
  no 
  metamor- 
  

   phism 
  apparent. 
  At 
  the 
  same 
  time 
  there 
  are 
  examples 
  all 
  along 
  the 
  

   western 
  border 
  of 
  metamorphism 
  by 
  contact 
  with 
  masses 
  or 
  sheets 
  

   of 
  trachyte, 
  notably 
  in 
  the 
  Elk 
  Mountains. 
  In 
  the 
  group 
  under 
  consid- 
  

   eration, 
  there 
  are 
  many 
  localities 
  where 
  it 
  is 
  clearly 
  evident 
  that 
  great 
  

   changes 
  have 
  been 
  produced. 
  It 
  is 
  very 
  probable, 
  however, 
  that 
  the 
  

   main 
  part 
  of 
  the 
  elevation 
  of 
  the 
  sedimentary 
  strata 
  belongs 
  to 
  the 
  pe- 
  

   riod 
  of 
  the 
  general 
  elevation 
  of 
  the 
  quartzite 
  group 
  to 
  the 
  east, 
  and 
  that 
  

   the 
  elevating 
  influence 
  of 
  the 
  trachytes 
  is 
  in 
  no 
  case 
  of 
  very 
  great 
  im- 
  

   portance. 
  

  

  The 
  metamorphic 
  core 
  as 
  exposed 
  by 
  the 
  deep-cut 
  valleys 
  would 
  prob- 
  

   ably 
  be 
  included 
  in 
  a 
  circle 
  six 
  or 
  seven 
  miles 
  in 
  diameter. 
  In 
  passing 
  

   up 
  the 
  La 
  Plata 
  River 
  from 
  Parrott 
  City, 
  we 
  tind 
  at 
  the 
  opening 
  of 
  the 
  

   cafLon, 
  that 
  is, 
  where 
  the 
  river 
  issues 
  irom 
  the 
  mountains, 
  that 
  the 
  

   Lower 
  Cretaceous 
  sandstones 
  dip 
  away 
  from 
  the 
  mountains 
  at 
  an 
  angle 
  

   of 
  from 
  20° 
  to 
  40°, 
  and 
  that 
  these 
  lie 
  upon 
  the 
  red 
  sandstones. 
  We 
  

   notice, 
  however, 
  that 
  these 
  beds 
  have 
  been 
  considerably 
  changed 
  and 
  

   are 
  quite 
  dark. 
  A 
  little 
  farther 
  up, 
  the 
  beds 
  that 
  were 
  red 
  below 
  become 
  

   gray, 
  and 
  soon 
  lose 
  in 
  a 
  great 
  measure 
  their 
  bedded 
  character, 
  so 
  that 
  

   within 
  a 
  mile 
  of 
  the 
  opening 
  of 
  the 
  valley 
  all 
  appearance 
  of 
  structure 
  

   is 
  lost. 
  High 
  up 
  toward 
  the 
  summit 
  of 
  the 
  western 
  ridge, 
  however, 
  

   where 
  the 
  trachytes 
  have 
  not 
  obscured 
  the 
  outcrops, 
  the 
  bedded 
  charac- 
  

   ter, 
  and 
  even 
  the 
  color 
  in 
  places, 
  are 
  preserved. 
  From 
  Comstock 
  Gulch 
  

   to 
  the 
  mouth 
  of 
  Lewis 
  Creek, 
  a 
  distance 
  of 
  five 
  miles, 
  approximately, 
  

   there 
  is 
  uot 
  the 
  slightest 
  trace 
  of 
  bedded 
  structure, 
  and 
  so 
  complete 
  has 
  

   been 
  the 
  obliteration 
  of 
  all 
  the 
  sedimentary 
  characters 
  and 
  so 
  intricate 
  

   is 
  the 
  net-work 
  of 
  mineral 
  veins 
  and 
  porphyritic 
  dikes, 
  that 
  an 
  analy- 
  

   sis 
  of 
  the 
  geologic 
  structure 
  is 
  out 
  of 
  the 
  question. 
  Having 
  crossed 
  

   this 
  area, 
  we 
  again 
  encounter 
  the 
  red 
  sandstone, 
  probably 
  Carboniferous 
  

   here, 
  at 
  first 
  hardly 
  recognizable, 
  but 
  farther 
  on, 
  near 
  Basin 
  Creek, 
  

   recovering 
  their 
  original 
  bedded 
  character 
  and 
  color. 
  

  

  In 
  the 
  central 
  part 
  of 
  the 
  altered 
  area 
  the 
  metamorphic 
  mass 
  proba- 
  

   bly 
  extends 
  up 
  to 
  and 
  includes 
  the 
  Red 
  Beds^, 
  but 
  the 
  higher 
  portions 
  

   are 
  so 
  obscured 
  by 
  trachytic 
  outflows 
  that 
  nothing 
  was 
  determined. 
  

   All 
  along 
  the 
  western 
  slopes 
  of 
  the 
  mountains, 
  as 
  far 
  as 
  I 
  examined, 
  the 
  

   trachytic 
  rocks 
  are 
  associated 
  with 
  the 
  Middle 
  Cretaceous 
  shales. 
  On 
  

   the 
  eastern 
  side, 
  according 
  to 
  Dr. 
  Endlich, 
  they 
  rest 
  upon 
  the 
  Carbon- 
  

   iferous. 
  On 
  the 
  north 
  and 
  south, 
  in 
  a 
  number 
  of 
  cases, 
  they 
  are 
  in 
  con- 
  

   tact 
  with 
  the 
  Lower 
  Cretaceous 
  and 
  Jura-Trias 
  rocks, 
  but 
  it 
  is 
  hard 
  

   to 
  say 
  whether 
  the 
  masses 
  which 
  now 
  cap 
  the 
  summits 
  were 
  poured 
  out 
  

   over 
  the 
  already 
  eroded 
  surfaces, 
  or 
  whether 
  they 
  have 
  been 
  intruded 
  

   among 
  the 
  sedimentary 
  rocks 
  so 
  that 
  their 
  present 
  positions 
  on 
  the 
  sum- 
  

   mit 
  regions 
  is 
  the 
  result 
  of 
  their 
  tendency 
  to 
  retard 
  erosion 
  over 
  the 
  

   area 
  where 
  their 
  bulk 
  is 
  greatest. 
  Be 
  this 
  as 
  it 
  may, 
  it 
  is 
  at 
  least 
  evi- 
  

   dent 
  that 
  the 
  channels 
  or 
  vents 
  by 
  which 
  the 
  volcanic 
  materials 
  reached 
  

   their 
  present 
  resting-places 
  are 
  within 
  the 
  La 
  Plata 
  Mountains 
  them- 
  

   selves. 
  To 
  this 
  the 
  positions 
  of 
  the 
  various 
  outlying 
  masses 
  attest, 
  as 
  

   they 
  increase 
  in 
  number 
  and 
  importance 
  as 
  we 
  approach 
  the 
  main 
  

   mountain 
  mass. 
  Also 
  the 
  dikes, 
  such 
  as 
  were 
  observed, 
  seem 
  to 
  extend 
  

   out 
  into 
  the 
  surrounding 
  rocks 
  from 
  the 
  same 
  mass. 
  

  

  