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  EEPOKT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  beyond 
  by 
  a 
  promiuent 
  ridge 
  composed 
  of 
  tracbytic 
  strata. 
  Eugged 
  

   mountains 
  are 
  placed 
  in 
  a 
  row 
  trending 
  north 
  to 
  south. 
  The 
  boundary 
  

   between 
  the 
  stratoid 
  trachytes 
  and 
  the 
  underlying 
  Cretaceous 
  beds 
  is 
  

   very 
  clearly 
  defined, 
  and 
  can 
  be 
  recognized 
  even 
  from 
  a 
  distance, 
  on 
  ac- 
  

   count 
  of 
  the 
  characteristic 
  forms 
  and 
  color 
  of 
  the 
  volcanic 
  rocks. 
  Con- 
  

   glomerate 
  crops 
  out, 
  in 
  the 
  canon, 
  a 
  continuation 
  of 
  the 
  strata 
  farther 
  

   east 
  and 
  south, 
  showing 
  the 
  same 
  features 
  that 
  may 
  there 
  be 
  observed. 
  

  

  Eio 
  Blanco 
  rises 
  south 
  of 
  station 
  19, 
  and 
  flows 
  from 
  there 
  in 
  a 
  south- 
  

   westerly 
  direction. 
  It 
  passes 
  through 
  the 
  high 
  trachytes 
  of 
  the 
  mount- 
  

   ains, 
  until 
  it 
  leaves 
  them 
  south 
  of 
  station 
  77. 
  This 
  station 
  is 
  located 
  

   on 
  a 
  prominent 
  trachytic 
  peak, 
  belonging 
  to 
  that 
  row 
  nearest 
  to 
  the 
  

   valley. 
  Its 
  altitude 
  is 
  12,514 
  feet. 
  Ascending 
  it 
  we 
  first 
  find 
  the 
  heavy 
  

   beds 
  of 
  trachyte, 
  compact, 
  and 
  of 
  brown 
  color. 
  Light-brown 
  to 
  greyish- 
  

   brown 
  conglomerate 
  covers 
  it, 
  and 
  produces 
  locally 
  more 
  gentle 
  slopes. 
  

   Above 
  this 
  follow 
  the 
  variegated 
  bands 
  described 
  from 
  the 
  Chama 
  

   region. 
  Conspicuous 
  among 
  them 
  is 
  one 
  black 
  stratum, 
  resembling 
  

   basalt. 
  Upon 
  examination, 
  however, 
  it 
  proved 
  to 
  be 
  a 
  trachyte 
  contain- 
  

   ing 
  a 
  large 
  percentage 
  of 
  magnetite, 
  and 
  numerous 
  very 
  thin 
  transparent 
  

   crystals 
  of 
  sauidite. 
  These 
  strata 
  show 
  some 
  evidence 
  of 
  having 
  been 
  

   reheated; 
  they 
  are 
  hard, 
  very 
  compact, 
  brittle, 
  and 
  fragments 
  have 
  a 
  

   submetallic 
  ring. 
  Columnar 
  structure 
  indicates 
  reheating 
  also, 
  or 
  slow 
  

   cooling. 
  Not 
  much 
  regularity 
  is 
  shown 
  by 
  these 
  columns, 
  among 
  which 
  

   the 
  six-sided 
  ones 
  predominate, 
  but 
  they 
  produce, 
  in 
  consequence 
  of 
  

   easy 
  removal, 
  steep, 
  precipitous 
  sides, 
  on 
  which 
  the 
  structure 
  is 
  visible 
  

   from 
  a 
  long 
  distance. 
  Covering 
  these 
  strata, 
  that 
  remain 
  constant 
  in 
  

   their 
  occurrence 
  throughout 
  the 
  southern 
  end 
  of 
  the 
  Sawatch 
  Range, 
  we 
  

   find 
  a 
  massive 
  brown 
  trachyte, 
  containing 
  crystals 
  of 
  sanidite, 
  some 
  

   oligoclase, 
  and 
  small 
  six-sided 
  crystals 
  of 
  a 
  splendent 
  brown 
  mica. 
  

   Here 
  and 
  along 
  the 
  entire 
  western 
  edge 
  of 
  the 
  range 
  the 
  mountains 
  fall 
  

   off 
  very 
  steeply, 
  so 
  that 
  in 
  this 
  instance, 
  camp 
  55, 
  located 
  five 
  miles 
  

   west 
  of 
  station 
  77, 
  is 
  more 
  than 
  4,000 
  feet 
  lower. 
  It 
  is 
  this 
  fact 
  that 
  im- 
  

   parts 
  to 
  the 
  mountains 
  the 
  character 
  of 
  a 
  high, 
  rugged 
  range, 
  when 
  seen 
  

   from 
  below. 
  The 
  general 
  easterly 
  dip 
  of 
  the 
  strata, 
  2^-6^ 
  at 
  this 
  lo- 
  

   cality, 
  effectually 
  prevents 
  any 
  view 
  of 
  the 
  sloping 
  plateau 
  behind 
  them. 
  

   Fissures 
  and 
  eroded 
  caves 
  traverse 
  the 
  upper 
  beds 
  of 
  the 
  trachyte 
  

   and 
  add 
  to 
  the 
  rugged 
  appearance 
  of 
  the 
  mountains. 
  The 
  facility 
  with 
  

   which 
  these 
  strata 
  can 
  be 
  eroded 
  in 
  a 
  manner 
  to 
  form 
  shallow 
  caves, 
  is 
  

   noticeable 
  wherever 
  they 
  occur. 
  It 
  is 
  due 
  probably 
  to 
  a 
  want 
  of 
  homo- 
  

   geneousness 
  of 
  the 
  physical 
  constitution, 
  in 
  consequence 
  of 
  which 
  

   erosive 
  agents 
  can 
  more 
  readily 
  attack 
  certain 
  portions 
  than 
  others. 
  

   Similar 
  features 
  to 
  these 
  found 
  here, 
  have 
  been 
  observed 
  at 
  rhany 
  locali- 
  

   ties 
  of 
  the 
  1874 
  district, 
  in 
  the 
  analogous 
  strata. 
  In 
  but 
  a 
  few 
  instances 
  

   were 
  the 
  caves 
  found 
  to 
  be 
  of 
  any 
  depth, 
  and 
  mostly 
  the 
  mouth 
  was 
  

   larger 
  than 
  any 
  portion 
  of 
  the 
  interior. 
  In 
  case 
  of 
  rain 
  or 
  hail, 
  so 
  fre- 
  

   quent 
  in 
  these 
  volcanic 
  regions, 
  they 
  afford 
  temporary 
  shelter, 
  and 
  

   some 
  found 
  farther 
  west 
  showed 
  evidence 
  of 
  having 
  been 
  utilized 
  for 
  

   similar 
  purposes 
  by 
  Indians. 
  

  

  Kio 
  San 
  Juan 
  is 
  the 
  largest 
  river 
  of 
  the 
  1875 
  district 
  on 
  the 
  Pacific 
  

   side 
  of 
  the 
  divide. 
  Several 
  good-sized 
  streams 
  join 
  about 
  four 
  miles 
  

   northeast 
  of 
  station 
  76, 
  and 
  make 
  up 
  the 
  river. 
  The 
  largest 
  one 
  of 
  these 
  

   heads 
  just 
  north 
  of 
  station 
  19, 
  at 
  the 
  divide, 
  and 
  flowing 
  in 
  a 
  northerly 
  

   curve 
  emerges 
  from 
  the 
  mountains 
  near 
  station 
  76, 
  Another 
  branch, 
  

   almost 
  of 
  equal 
  length, 
  heads 
  south 
  of 
  station 
  28, 
  and 
  flows 
  in 
  a 
  south- 
  

   erly 
  direction, 
  joining 
  the 
  first 
  at 
  the 
  place 
  mentioned. 
  Both 
  receive 
  

   creeks 
  carrying 
  a 
  considerable 
  amount 
  of 
  water, 
  so 
  that 
  the 
  San 
  Juan 
  

   is 
  a 
  stream 
  of 
  some 
  size 
  already 
  upon 
  entering 
  the 
  lower 
  country. 
  From 
  

  

  