﻿HOLMES] 
  INTRUSION 
  OF 
  TRACHYTES. 
  273 
  

  

  clase 
  ; 
  crystals 
  of 
  sanidite 
  are 
  Very 
  minute 
  and 
  rare 
  ; 
  small 
  crystals 
  of 
  

   quartz 
  are 
  scattered 
  through 
  the 
  mass. 
  ISText 
  in 
  frequency 
  to 
  the 
  oligo- 
  

   clase 
  is 
  a 
  green 
  mineral 
  — 
  probably 
  some 
  variety 
  of 
  amphibole. 
  Bits 
  of 
  

   metamorphosed 
  rock 
  are 
  found 
  in 
  many 
  of 
  the 
  specimens. 
  

  

  Wheu 
  viewed 
  more 
  critically, 
  this 
  mountain 
  group 
  does 
  not 
  seem 
  

   closely 
  compacted, 
  as 
  if 
  formed 
  of 
  a 
  single 
  mass 
  of 
  trachyte, 
  but 
  rather 
  

   as 
  if 
  formed 
  of 
  a 
  number 
  of 
  distinct 
  bodies 
  that 
  had 
  reached 
  their 
  present 
  

   horizon 
  through 
  closely 
  associated 
  vents. 
  That 
  the 
  various 
  masses 
  of 
  

   trachyte 
  came 
  from 
  sources 
  located 
  directly 
  beneath 
  is 
  evident 
  from 
  the 
  

   fact 
  tbat 
  the 
  sedimentary 
  rock 
  are 
  frequently 
  bent 
  up 
  at 
  a 
  high 
  angle 
  

   around 
  the 
  borders. 
  The 
  upturned 
  strata 
  include 
  the 
  lower 
  part 
  of 
  the 
  

   Middle 
  Cretaceous 
  shales 
  and 
  portions 
  of 
  the 
  Dakota 
  sandstones. 
  

  

  It 
  is 
  a 
  fact 
  worthy 
  of 
  notice 
  that 
  in 
  all 
  parts 
  of 
  the 
  trachytic 
  mass 
  

   there 
  are 
  fragments 
  of 
  the 
  Cretaceous 
  shales 
  which 
  have 
  been 
  caught 
  

   up 
  in 
  the 
  rising 
  mass 
  and 
  distorted 
  and 
  methamorphosed, 
  but 
  that 
  no 
  

   other 
  rock 
  appears 
  similiarly 
  situated. 
  This 
  fact 
  would 
  seem 
  to 
  have 
  

   an 
  interesting 
  bearing 
  upon 
  the 
  question 
  of 
  the 
  manner 
  in 
  which 
  the 
  

   volcanic 
  matter 
  reached 
  the 
  position 
  now 
  occupied 
  by 
  it. 
  It 
  does 
  not 
  

   seem 
  possible 
  that 
  any 
  considerable 
  bulk 
  of 
  pasty 
  or 
  even 
  fluid 
  matter 
  

   could 
  rise 
  through 
  the 
  sedimentary 
  crust 
  without 
  carrying 
  up 
  quanti- 
  

   ties 
  of 
  fragments. 
  The 
  absence 
  of 
  all 
  fragments 
  excepting 
  of 
  the 
  shales 
  

   goes 
  to 
  prove 
  that 
  the 
  ascent 
  of 
  the 
  molten 
  material 
  up 
  to 
  the 
  base 
  of 
  the 
  

   shales 
  has 
  been 
  through 
  narrow, 
  firm- 
  walled 
  crevices, 
  and 
  that 
  the 
  lateral 
  

   spreadingand 
  consequent 
  stoppage 
  of 
  the 
  great 
  bulk 
  took 
  place 
  when 
  the 
  

   horizon 
  of 
  the 
  yielding 
  shales 
  was 
  reached. 
  So 
  far 
  as 
  my 
  observations 
  go, 
  

   the 
  trachyte 
  does 
  not 
  seem 
  to 
  have 
  been 
  deposited 
  as 
  a 
  single 
  compact 
  

   mass, 
  as 
  though 
  on 
  reaching 
  the 
  shale 
  horizon 
  it 
  had 
  opened 
  for 
  itself 
  

   a 
  great 
  cavity, 
  with 
  the 
  firm 
  sandstones 
  as 
  a 
  base, 
  and 
  an 
  unbroken 
  arch 
  

   or 
  dome 
  of 
  the 
  entire 
  series 
  of 
  superincumbent 
  strata 
  above, 
  but 
  rather 
  

   as 
  though 
  it 
  had 
  broken 
  irregularly 
  into 
  the 
  yielding 
  shales, 
  pushing 
  

   Ibem 
  back 
  upon 
  themselves, 
  penetrating 
  them 
  first 
  in 
  one 
  direction 
  and 
  

   then 
  in 
  another, 
  with 
  a 
  sort 
  of 
  irregular 
  radiation 
  of 
  masses 
  from 
  the 
  

   mouths 
  of 
  the 
  vents— 
  not 
  at 
  once, 
  perhaps, 
  but 
  with 
  a 
  rapidity 
  depend- 
  

   ing 
  upon 
  the 
  size 
  of 
  the 
  vents 
  and 
  the 
  character 
  of 
  the 
  forces 
  beneath. 
  

   By 
  such 
  a 
  method 
  of 
  intrusion 
  numerous 
  masses 
  of 
  the 
  shales 
  would 
  be 
  

   surrounded 
  and 
  held 
  suspended 
  in 
  the 
  mass 
  of 
  plastic 
  material, 
  or 
  sand- 
  

   wiched 
  between 
  masses 
  proceeding 
  from 
  distinct 
  flows. 
  In 
  no 
  case 
  did 
  

   I 
  notice 
  any 
  symmetrical 
  arching 
  of 
  strata 
  over 
  the 
  trachytes, 
  but 
  my 
  

   observations 
  all 
  tend 
  to 
  show 
  that 
  there 
  has 
  been 
  a 
  sort 
  of 
  absorption, 
  

   so 
  to 
  speak, 
  of 
  the 
  shales, 
  and 
  that 
  at 
  least 
  half 
  of 
  the 
  space 
  through 
  

   which 
  the 
  trachyte 
  is 
  distributed 
  is 
  occupied 
  by 
  the 
  crushed 
  and 
  meta- 
  

   morphosed 
  fragments 
  of 
  shale. 
  As 
  a 
  consequence, 
  the 
  height 
  of 
  the 
  

   arch 
  — 
  such 
  as 
  may 
  once 
  have 
  existed 
  — 
  would 
  not 
  be 
  equal 
  to 
  the 
  height 
  

   of 
  the 
  trachytic 
  mass, 
  as 
  only 
  the 
  higher 
  layers 
  of 
  shale 
  extend 
  entirely 
  

   over 
  it, 
  the 
  lower 
  layers 
  having 
  been 
  absorbed 
  by 
  it 
  and 
  really 
  forming 
  

   a 
  part 
  of 
  it. 
  The 
  three 
  figures 
  given 
  in 
  Plate 
  XL 
  VI 
  will 
  assist 
  in 
  mak- 
  

   ing 
  my 
  meaning 
  clearer. 
  

  

  Figure 
  1 
  illustrates 
  the 
  manner 
  in 
  which 
  the 
  molten 
  matter 
  seems 
  to 
  

   have 
  been 
  intruded 
  among 
  the 
  shales. 
  If 
  the 
  entire 
  mass 
  had 
  at 
  once 
  

   been 
  intruded 
  between 
  the 
  strata 
  at 
  a 
  given 
  horizon 
  arching 
  those 
  

   above, 
  the 
  result 
  would 
  probably 
  be 
  as 
  represented 
  in 
  Figure 
  2. 
  Figure 
  

   3 
  will 
  illustrate 
  the 
  combined 
  aVching 
  and 
  absorption 
  as 
  ordinarily 
  ex- 
  

   hibited 
  in 
  this 
  group. 
  On 
  the 
  north 
  side 
  of 
  Ute 
  Peak 
  a 
  large 
  vertical 
  

   dike 
  cuts 
  through 
  the 
  Jura-Trias 
  and 
  Lower 
  Cretaceous 
  sandstones, 
  and 
  

   connects, 
  apparently, 
  with 
  the 
  trachytic 
  mass 
  of 
  a 
  northwest 
  spur. 
  (See 
  

   Plate 
  XLVII.) 
  On 
  the 
  north 
  face 
  of 
  Ute 
  Peak 
  there 
  is 
  a 
  heavy 
  hori- 
  

  

  18 
  G 
  s 
  

  

  