﻿134 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  diately 
  west 
  of 
  the 
  peaks 
  is 
  n 
  prominent 
  Lill, 
  trending 
  north 
  and 
  sontli, 
  

   which 
  belongs 
  to 
  the 
  dike 
  system 
  also. 
  It 
  must 
  have 
  been 
  one 
  of 
  those 
  in- 
  

   stances 
  where 
  the 
  lava 
  overflowed 
  and 
  caused 
  the 
  formation 
  of 
  a 
  capping 
  

   of 
  small 
  extent 
  on 
  the 
  strata 
  through 
  which 
  it 
  had 
  been 
  ejected. 
  Several 
  

   small 
  outcrops 
  of 
  volcanic 
  rocks 
  occur 
  in 
  this 
  vicinity 
  that 
  belong 
  to 
  the 
  

   class 
  of 
  dikes. 
  One 
  butte 
  in 
  particular, 
  a 
  little 
  west 
  of 
  station 
  121, 
  near 
  

   Arapahoe 
  Creek, 
  may 
  here 
  be 
  mentioned. 
  It 
  {a) 
  is 
  probably 
  in 
  direct 
  

   connection 
  with 
  the 
  long 
  dike 
  (ft) 
  issuing 
  from 
  the 
  mountain, 
  but 
  it 
  is 
  

   somewhat 
  obscured, 
  owing 
  to 
  the 
  crumbling 
  down 
  of 
  the 
  prominent 
  

   wall. 
  The 
  butte 
  stands 
  isolated, 
  shows 
  vertical 
  walls 
  on 
  nearly 
  every 
  

   side, 
  and 
  the 
  greyish-brown 
  trachyte 
  composing 
  it 
  shows 
  indications 
  of 
  

   columnar 
  structure. 
  Although 
  so 
  many 
  dikes 
  originate 
  from 
  station 
  

   ]21,all 
  of 
  them 
  passing 
  through 
  the 
  red 
  sandstone 
  and 
  metamorphosing 
  

   it 
  wherever 
  the 
  two 
  were 
  in 
  contact, 
  but 
  few 
  are 
  found 
  near 
  East 
  Span- 
  

   ish. 
  There 
  the 
  strain 
  upon 
  the 
  strata 
  was 
  by 
  no 
  means 
  so 
  great 
  as 
  it 
  

   was 
  a 
  few 
  miles 
  farther 
  west, 
  and 
  as 
  a 
  result 
  they 
  were 
  not 
  cracked 
  and 
  

   broken 
  to 
  the 
  same 
  extent. 
  In 
  view 
  of 
  so 
  large 
  a 
  number 
  of 
  dikes 
  and 
  

   the 
  prol)ability 
  of 
  the 
  strata 
  parting 
  while 
  breaking, 
  examinations 
  were 
  

   made 
  to 
  discover 
  instances 
  of 
  intrusion. 
  Only 
  a 
  few 
  cases 
  of 
  this 
  kind 
  

   were 
  observed, 
  however, 
  except 
  higher 
  np 
  on 
  West 
  Spanish 
  Peak, 
  

   Several 
  dikes, 
  or 
  rather 
  one 
  main 
  dike 
  continues 
  from 
  Spanish 
  Peaks 
  

   southward 
  into 
  Stonewall 
  Valley. 
  It 
  is 
  broken 
  into 
  many 
  fragments 
  by 
  

   the 
  passage 
  of 
  water 
  at 
  right 
  angles 
  to 
  its 
  strike. 
  In 
  each 
  instance 
  the 
  

   protection 
  aftbrded 
  by 
  the 
  hard 
  dike-rock 
  to 
  the 
  surrounding 
  softer 
  

   material 
  has 
  resulted 
  in 
  the 
  formation 
  of 
  hogback 
  shaped 
  hills 
  several 
  

   hundred 
  feet 
  in 
  height. 
  Opposite 
  station 
  123 
  several 
  accessories 
  either 
  

   join 
  or 
  emanate 
  from 
  the 
  main 
  dike, 
  but 
  they 
  are 
  all 
  by 
  far 
  smaller 
  and 
  

   do 
  not 
  produce 
  the 
  same 
  hil.s 
  as 
  the 
  former. 
  Viewed 
  irom 
  a 
  distance, 
  

   the 
  effect 
  is 
  similar 
  to 
  that 
  of 
  a 
  long-continued 
  row 
  of 
  very 
  regular 
  

   hogbacks. 
  

  

  All 
  of 
  these 
  dikes 
  in 
  question 
  are 
  composed 
  of 
  trachyte. 
  Mostly 
  it 
  

   is 
  of 
  a 
  light-colored 
  variety, 
  containing 
  small 
  crystals 
  of 
  sanidite 
  and 
  

   oligoclase 
  in 
  a 
  microcrystalline 
  or 
  nearly 
  amorphous, 
  compact 
  paste. 
  

   Other 
  varieties, 
  with 
  mica, 
  hornblende, 
  &c., 
  are 
  not 
  w^anting, 
  however. 
  

   The 
  uniformity 
  is 
  such, 
  though, 
  that 
  their 
  correlation 
  among 
  each 
  other 
  

   is 
  evident 
  at 
  a 
  glance, 
  were 
  that 
  not 
  proved 
  already 
  by 
  their 
  occurrence 
  

   and 
  relative 
  position. 
  Owing 
  to 
  an 
  admixture 
  of 
  magnetite 
  decomposi- 
  

   tion 
  has 
  produced 
  a 
  variety 
  of 
  colors, 
  among 
  which 
  green 
  and 
  reddish- 
  

   brown 
  are 
  the 
  predominating. 
  Opposite 
  station 
  123 
  the 
  color 
  of 
  the 
  

   trachyte 
  is 
  by 
  far 
  darker 
  than 
  farther 
  north, 
  and 
  the 
  oligoclase 
  is 
  not 
  

   found. 
  The 
  former 
  may 
  be 
  due 
  to 
  a 
  greater 
  percentage 
  of 
  magnetic 
  iron. 
  

   As 
  a 
  rule, 
  however, 
  the 
  colors 
  are 
  light, 
  and 
  add 
  to 
  the 
  ijrominency 
  of 
  

   the 
  outstanding 
  walls. 
  In 
  a 
  number 
  of 
  instances 
  a 
  cast 
  of 
  the 
  strata, 
  

   through 
  which 
  the 
  molten 
  lava 
  has 
  passed, 
  and 
  where 
  it 
  cooled, 
  can 
  be 
  

   observed 
  on 
  the 
  dikes, 
  more 
  particularly 
  on 
  those 
  that 
  were 
  injected 
  

   into 
  the 
  fissures 
  of 
  the 
  hard, 
  red 
  sandstone. 
  Eegarding 
  the 
  process 
  

   through 
  which 
  the 
  dikes 
  have 
  attained 
  their 
  present 
  prominent 
  appear- 
  

   ance, 
  it 
  may 
  be 
  very 
  easily 
  explained. 
  Assuming 
  that 
  the 
  fissures 
  were 
  

   formed 
  by 
  some 
  volcanic 
  activity, 
  and 
  reached 
  to 
  suf&bient 
  depth 
  to 
  be 
  

   in 
  connection 
  with 
  the 
  molten 
  material 
  below, 
  it 
  is 
  evident 
  that 
  the 
  

   l)ressure 
  exerted 
  upon 
  this 
  material, 
  which 
  was 
  sufficient 
  to 
  produce 
  

   a 
  rupture, 
  was 
  sufficient 
  also 
  to 
  force 
  the 
  liquefied 
  rock 
  upward 
  into 
  

   the 
  tissures. 
  That 
  this 
  process 
  took 
  place 
  very 
  nearly 
  iu 
  the 
  manner 
  

   described 
  is 
  indicated 
  by 
  the 
  fact 
  that 
  no 
  disturbances 
  of 
  strata 
  occur 
  

   in 
  the 
  immediate 
  vicinity 
  of 
  these 
  small 
  dikes, 
  and 
  that 
  the 
  strata 
  on 
  

   either 
  side 
  are 
  mostly 
  thoroughly 
  baked 
  and 
  fritted. 
  Exhibiting 
  on 
  

  

  