﻿214 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  This 
  observation 
  points 
  to 
  the 
  fact 
  that 
  different 
  processes 
  of 
  cooling 
  

   will 
  furnish 
  products 
  which 
  physically 
  differ, 
  though 
  chemically 
  they 
  

   may 
  show 
  the 
  same 
  ultimate 
  constituents. 
  

  

  The 
  second 
  group, 
  porphyritic 
  trachytes, 
  varies 
  from 
  the 
  first 
  mainly 
  

   by 
  their 
  mode 
  of 
  occurrence 
  and 
  age. 
  They 
  form 
  principally 
  isolated 
  

   volcanic 
  masses, 
  rising 
  considerably 
  above 
  the 
  level 
  of 
  the 
  surrounding 
  

   country. 
  A 
  number 
  of 
  the 
  detached 
  mountain 
  groups 
  in 
  Western 
  Colo- 
  

   rado 
  — 
  Sierra 
  La 
  Sal, 
  Sierra 
  Abajo, 
  La 
  Plata 
  Mountains, 
  and 
  others 
  — 
  

   are 
  composed 
  of 
  this 
  material. 
  In 
  the 
  eastern 
  portion 
  of 
  the 
  State 
  they 
  

   are 
  not 
  wanting, 
  although 
  not 
  so 
  frequently 
  met 
  with. 
  Dr. 
  Peale 
  is 
  pre- 
  

   paring 
  an 
  exhaustive 
  paper 
  on 
  this 
  subject 
  for 
  the 
  annual 
  report 
  of 
  

   1876, 
  which 
  will 
  give, 
  in 
  detail, 
  the 
  mode 
  of 
  occurrence 
  and 
  features 
  of 
  

   mountains 
  and 
  ranges 
  belonging 
  to 
  this 
  formation. 
  He 
  has 
  had 
  occasion 
  

   to 
  examine 
  quite 
  a 
  number 
  of 
  them, 
  and 
  has 
  obtained 
  data 
  that 
  enable 
  

   him 
  to 
  treat 
  of 
  the 
  subject 
  thoroughly. 
  

  

  Dolerite 
  and 
  basalt 
  comprise 
  the 
  third 
  group. 
  The 
  former 
  is 
  but 
  

   sparingly 
  represented, 
  while 
  the 
  latter 
  is 
  very 
  frequently 
  met 
  with. 
  In 
  

   the 
  southern 
  portion 
  of 
  San 
  Luis 
  Yalley 
  and 
  on 
  the 
  eastern 
  slope 
  of 
  the 
  

   southerly 
  extension 
  of 
  the 
  Sawatch 
  Eange 
  the 
  largest 
  mass 
  of 
  basalt 
  

   was 
  observed. 
  It 
  has 
  been 
  described 
  in 
  Chapter 
  II. 
  

  

  Both 
  the 
  porphyritic 
  trachyte 
  and 
  the 
  basalt 
  form 
  dikes, 
  that 
  often 
  

   extend 
  for 
  a 
  number 
  of 
  miles, 
  traversing 
  sedimentary 
  beds. 
  All 
  the 
  vol- 
  

   canic 
  eruptions 
  in 
  Southern 
  Colorado 
  are 
  "massive," 
  in 
  contradistinction 
  

   to 
  " 
  volcanic 
  " 
  eruptions. 
  I^^o 
  where 
  was 
  even 
  a 
  single 
  well-defined 
  crater 
  

   found. 
  Instead 
  of 
  the 
  volcanic 
  material 
  being 
  ejected, 
  as 
  we 
  see 
  it 
  done 
  

   to-day, 
  from 
  a 
  more 
  or 
  less 
  regular 
  cone, 
  building 
  up 
  a 
  crater 
  by 
  the 
  

   deposition 
  of 
  lava 
  around 
  the 
  orifice, 
  through 
  which 
  the 
  expulsion 
  takes 
  

   place, 
  we 
  have, 
  at 
  best, 
  hills 
  or 
  mountains, 
  most 
  frequently 
  of 
  irregular 
  

   shape. 
  Through 
  the 
  agency 
  of 
  either 
  plutonic 
  or 
  volcanic 
  earthquakes 
  

   (less 
  probably 
  through 
  contraction 
  of 
  the 
  earth's 
  crust 
  or 
  portions 
  thereof) 
  

   "cracks" 
  were 
  formed, 
  reaching 
  down 
  to 
  a 
  depth 
  where 
  the 
  liquid 
  or 
  plastic 
  

   material 
  occurred, 
  or 
  to 
  which 
  it 
  could 
  force 
  its 
  way. 
  This, 
  forced 
  upward, 
  

   passed 
  through 
  the 
  fissure, 
  and, 
  upon 
  its 
  arrival 
  at 
  the 
  surface, 
  spread 
  

   in 
  every 
  available 
  direction 
  commensurate 
  with 
  the 
  propelling 
  press- 
  

   ure 
  from 
  below. 
  In 
  the 
  dikes 
  we 
  have 
  evidence 
  that 
  the 
  mate- 
  

   rial 
  composing 
  them 
  must 
  have 
  reached 
  the 
  walls 
  of 
  the 
  fissure, 
  at 
  

   least 
  near 
  the 
  surface, 
  in 
  a 
  plastic 
  and 
  not 
  viscous 
  state. 
  This, 
  too, 
  

   was 
  probably 
  the 
  case 
  with 
  many 
  of 
  the 
  porphyritic 
  trachytes. 
  If 
  

   we 
  assume 
  that 
  the 
  pressure 
  producing 
  their 
  ejection 
  through 
  fis- 
  

   sures, 
  that 
  owed 
  their 
  existence 
  either 
  to 
  the 
  action 
  of 
  this 
  same 
  

   pressure 
  or 
  to 
  other 
  causes^ 
  was 
  but 
  little 
  more 
  than 
  adequate 
  to 
  

   force 
  the 
  liquefied 
  mass 
  to 
  the 
  surface, 
  we 
  can 
  infer 
  that 
  the 
  passage 
  

   of 
  this 
  mass 
  was 
  slower 
  than 
  if 
  the 
  pressure 
  had 
  been 
  greater. 
  Oppor- 
  

   tunity 
  is 
  therefore 
  given 
  to 
  the 
  material 
  to 
  cool 
  gradually 
  while 
  ascend- 
  

   ing, 
  and 
  to 
  assume 
  rigidity 
  more 
  rapidly 
  after 
  reaching 
  the 
  surface 
  than 
  

   otherwise 
  would 
  have 
  been 
  the 
  case. 
  Having 
  become 
  rigid, 
  it 
  would 
  

   naturally 
  require 
  a 
  by 
  far 
  greater 
  force 
  than 
  up 
  to 
  that 
  point 
  of 
  time 
  

   had 
  been 
  employed 
  to 
  produce 
  any 
  further 
  motion 
  in 
  the 
  mass. 
  Should, 
  

   then, 
  the 
  pressure 
  below 
  not 
  be 
  fully 
  expended, 
  although 
  no 
  longer 
  

   able 
  to 
  exert 
  any 
  direct 
  influence 
  upon 
  the 
  volcanic 
  body 
  it 
  has 
  brought 
  

   to 
  the 
  surface, 
  it 
  is 
  highly 
  probable 
  that 
  phenomena 
  comparable 
  to 
  

   present 
  seismic 
  action 
  would 
  take 
  place. 
  In 
  that 
  case 
  we 
  could 
  expect 
  

   accessory 
  fissures 
  to 
  be 
  formed, 
  into 
  which 
  the 
  liquid 
  or 
  plastic 
  material 
  

   would 
  be 
  injected 
  by 
  the 
  remnant 
  of 
  the 
  original 
  pressure. 
  If 
  the 
  vol- 
  

   canic 
  mass 
  reaches 
  the 
  surface 
  in 
  a 
  sufficiently 
  liquid 
  state 
  to 
  permit 
  

   its 
  flowing, 
  and 
  the 
  pressure 
  brought 
  to 
  bear 
  upon 
  it 
  from 
  below 
  still 
  

  

  