﻿130 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  rocks, 
  and 
  they 
  are 
  well 
  exposed. 
  Metamorphic 
  rocks 
  underlie 
  the 
  

   trachyte 
  here, 
  which 
  has 
  a 
  thickuess 
  of 
  300 
  to 
  500 
  feet. 
  It 
  is 
  evident, 
  

   both 
  from 
  this 
  ieature 
  and 
  from 
  the 
  identity 
  of 
  its 
  lithological 
  constitu- 
  

   tion, 
  that 
  this 
  is 
  but 
  a 
  flow 
  from 
  the 
  south. 
  On 
  the 
  west 
  side 
  drift 
  of 
  

   the 
  San 
  Pedro 
  Valley 
  covers 
  the 
  dipping 
  strata 
  or 
  flows, 
  and 
  they 
  are 
  

   hidden 
  from 
  view. 
  It 
  could 
  not 
  be 
  determined 
  with 
  certainty 
  whether 
  

   they 
  appear 
  at 
  the 
  base 
  of 
  the 
  San 
  Pedro 
  Mesa, 
  but 
  this 
  seems 
  probable 
  ; 
  

   it 
  is 
  also 
  probable 
  that 
  their 
  continuation 
  is 
  to 
  be 
  looked 
  for 
  in 
  the 
  bluffs 
  

   northeast 
  of 
  station 
  105 
  in 
  San 
  Luis 
  Valley. 
  At 
  the 
  south 
  end 
  of 
  the 
  mesa 
  

   the 
  connection 
  between 
  the 
  two 
  former 
  is 
  established, 
  and 
  there 
  the 
  

   trachyte 
  does 
  appear, 
  but 
  it 
  seems 
  to 
  dip 
  under 
  too 
  far 
  to 
  show 
  any 
  

   broad 
  area 
  farther 
  north. 
  The 
  connection 
  between 
  this 
  and 
  the 
  southern 
  

   mass 
  has 
  been 
  severed 
  by 
  erosion, 
  and 
  Eio 
  Costilla 
  forms 
  the 
  boundary 
  

   between 
  them, 
  running 
  in 
  metamorphic 
  rocks 
  at 
  that 
  place. 
  Of 
  the 
  

   large 
  volcanic 
  area 
  to 
  the 
  south, 
  merely 
  the 
  northern 
  end 
  came 
  within 
  

   the 
  boundary 
  of 
  our 
  district. 
  Along 
  the 
  western 
  edge 
  of 
  tiie 
  mountains 
  

   metamorphic 
  granite 
  occurs 
  from 
  Rio 
  Costilla 
  southward. 
  This 
  reaches 
  

   up 
  to 
  about 
  timber 
  line 
  as 
  a 
  rule, 
  though 
  not 
  always, 
  and 
  from 
  there 
  

   upward 
  the 
  trachyte 
  sets 
  in. 
  It 
  follows 
  in 
  its 
  western 
  border 
  the 
  edge 
  

   of 
  the 
  valley, 
  and 
  produces 
  a 
  number 
  of 
  sharp, 
  high 
  peaks. 
  Generally 
  

   it 
  remains 
  on 
  the 
  highest 
  portions, 
  but 
  west 
  of 
  station 
  110 
  crosses 
  the 
  

   Costilla, 
  and 
  forms 
  a 
  number 
  of 
  low 
  hills 
  with 
  steep, 
  blufty 
  sides. 
  Here 
  

   it 
  is 
  nearest 
  to 
  the 
  station 
  114 
  group, 
  being 
  only 
  four 
  miles 
  distant. 
  

   The 
  elevations 
  of 
  the 
  two 
  groups, 
  this 
  lowest 
  one 
  and 
  the 
  former 
  also, 
  

   agree, 
  as 
  well 
  as 
  their 
  structure 
  and 
  texture. 
  A 
  few 
  isolated 
  outcrops, 
  

   that 
  may 
  merely 
  be 
  fragments, 
  are 
  found 
  higher 
  up 
  on 
  the 
  Costilla, 
  but 
  

   the 
  main 
  mass 
  recedes 
  southward. 
  Immediately^ 
  east 
  of 
  Costilla 
  Peak, 
  

   however, 
  (station 
  111,) 
  it 
  projects 
  to 
  the 
  north, 
  forming 
  the 
  prominent 
  

   hill 
  (d) 
  in 
  section 
  V. 
  It 
  seems 
  highly 
  probable 
  that 
  this 
  is 
  one 
  of 
  the 
  

   points 
  of 
  minor 
  outflow, 
  as 
  the 
  character 
  of 
  the 
  trachyte 
  differs 
  from 
  

   that 
  observed 
  farther 
  south. 
  From 
  there 
  it 
  extends 
  eastward, 
  covering 
  

   some 
  of 
  the 
  sandstone 
  bluffs. 
  Prior 
  to 
  the 
  erosion 
  which 
  produced 
  the 
  

   l)resent 
  bluffs, 
  it 
  must 
  have 
  covered 
  a 
  by 
  far 
  greater 
  area, 
  as 
  remnants 
  

   of 
  it 
  are 
  found 
  at 
  station 
  120, 
  nine 
  miles 
  beyond 
  its 
  present 
  extreme 
  

   northern 
  point. 
  Steep 
  walls 
  and 
  sharp 
  ridges, 
  similar 
  to 
  those 
  of 
  the 
  

   Sawatch 
  Eange, 
  indicate 
  the 
  presence 
  of 
  this 
  volcanic 
  rock. 
  So 
  far 
  as 
  

   could 
  be 
  determined 
  from 
  a 
  distance, 
  it 
  extends 
  much 
  farther 
  south, 
  

   forming 
  the 
  mountains 
  of 
  the 
  continuation 
  of 
  the 
  Saugre 
  de 
  Cristo 
  

   Eange. 
  At 
  all 
  points 
  along 
  the 
  northern 
  and 
  western 
  edge 
  of 
  this 
  mass 
  

   it 
  was 
  found 
  to 
  overlie 
  metamorphic 
  rocks, 
  having 
  adapted 
  itself 
  in 
  its 
  

   flow 
  to 
  the 
  various 
  features 
  of 
  erosion 
  that 
  existed 
  at 
  that 
  time. 
  When- 
  

   ever 
  a 
  mountain 
  was 
  too 
  high 
  to 
  be 
  reached, 
  or 
  a 
  ridge 
  barred 
  its 
  

   progress, 
  the 
  trachyte 
  has 
  altered 
  the 
  direction 
  of 
  its 
  course 
  and 
  sought 
  

   more 
  readily 
  accessible 
  localities. 
  In 
  the 
  upper 
  valley 
  of 
  the 
  Costilla, 
  

   aqueous 
  erosion 
  has 
  no 
  doubt 
  removed 
  a 
  very 
  great 
  portion 
  of 
  the 
  vol- 
  

   canic 
  rock, 
  and 
  the 
  river 
  has 
  deposited 
  it 
  either 
  in 
  the 
  form 
  of 
  bowlders 
  

   or 
  sand 
  at 
  the 
  base 
  of 
  the 
  mountains 
  in 
  San 
  Luis 
  Valley. 
  Columnar 
  

   structure 
  can 
  be 
  observed 
  in 
  some 
  of 
  the 
  small 
  outcrops 
  in 
  the 
  valley, 
  

   but 
  not 
  so 
  frequently 
  in 
  the 
  mass 
  higher 
  up. 
  The 
  trachyte 
  bears 
  

   evidence 
  of 
  having 
  been 
  very 
  thoroughly 
  fused, 
  and 
  is 
  as 
  a 
  rule 
  very 
  

   bard, 
  brittle, 
  compact 
  of 
  texture, 
  and 
  has 
  a 
  semi-metallic 
  ring 
  when 
  

   found 
  in 
  slabs. 
  Station 
  108 
  is 
  located 
  on 
  a 
  sharp 
  peak 
  of 
  this 
  group, 
  

   near 
  the 
  head 
  of 
  Kio 
  Colorado, 
  and 
  is 
  12,467 
  feet 
  above 
  sea- 
  level. 
  It 
  is 
  

   on 
  the 
  western 
  spur 
  of 
  the 
  main 
  range, 
  which, 
  I'arther 
  north, 
  separates 
  

   into 
  two 
  branches. 
  Here 
  the 
  trachyte 
  is 
  very 
  characteristic, 
  closely 
  

   resembling 
  that 
  of 
  the 
  Sawatch 
  Eange; 
  not 
  that, 
  however, 
  found 
  in 
  the 
  

  

  