﻿CHAPTER 
  VII. 
  

  

  EEUPTIVE 
  EOCKS—TEACHORHEITES— 
  BRECCIA— 
  POE- 
  

   PHYEITIC 
  TEACHYTE— 
  BASALT. 
  

  

  The 
  volcanic 
  areas 
  of 
  the 
  district 
  are 
  readily 
  divisible 
  into 
  three 
  

   classes 
  : 
  

  

  1st. 
  Those 
  covered 
  with 
  a 
  flow 
  of 
  trachytes, 
  partially 
  described 
  under 
  

   the 
  head 
  of 
  Trachorheites 
  in 
  the 
  Eeport 
  for 
  1874. 
  These 
  are 
  in 
  many 
  

   places 
  underlaid 
  by 
  a 
  trachytic 
  breccia. 
  

  

  2d. 
  The 
  areas 
  of 
  porphyritic 
  trachyte. 
  

  

  3d. 
  Basaltic 
  areas. 
  

  

  TEACHORHEITES. 
  

  

  In 
  the 
  description 
  of 
  the 
  south 
  side 
  of 
  the 
  Gunnison 
  Eiver, 
  in 
  a 
  pre- 
  

   ceding 
  chapter, 
  I 
  mentioned 
  the 
  fact 
  that 
  a 
  large 
  portion 
  of 
  the 
  region 
  

   was 
  covered 
  with 
  trachytic 
  rock, 
  forming 
  plateaus 
  and 
  mesas 
  between 
  

   the 
  different 
  streams 
  tributary 
  to 
  the 
  Gunnison 
  from 
  the 
  south. 
  The 
  

   source 
  of 
  the 
  lava-flows 
  is 
  somewhere 
  in 
  Dr. 
  Endlich's 
  district, 
  where 
  

   these 
  volcanic 
  rocks 
  prevail 
  to 
  a 
  much 
  greater 
  extent. 
  Dr. 
  Endlich's 
  

   report 
  for 
  1874 
  contains 
  a 
  detailed 
  description 
  of 
  the 
  rocks. 
  He 
  has 
  

   divided 
  them 
  into 
  four 
  groups, 
  as 
  follows 
  : 
  

  

  No. 
  1. 
  White, 
  yellow, 
  green, 
  orange, 
  red, 
  brown, 
  and 
  gray 
  trachytes, 
  

   decomposing 
  readily, 
  frequently 
  weathering 
  in 
  picturesque 
  forms. 
  

  

  No. 
  2. 
  Eed 
  and 
  brownish 
  stratified 
  trachyte 
  ; 
  not 
  infrequently 
  con- 
  

   tained 
  interstrata 
  of 
  obsidian 
  and 
  of 
  pitchstone 
  resembling 
  obsidian. 
  

   Sanidite 
  is 
  the 
  predominant 
  one 
  of 
  the 
  segregated 
  minerals. 
  Small 
  

   crystals 
  of 
  brown 
  to 
  black 
  mica 
  occur 
  dispersed 
  throughout. 
  

  

  No. 
  3. 
  TTpper, 
  red 
  to 
  brown 
  trachyte 
  ; 
  lower, 
  red 
  to 
  brown 
  trachyte, 
  

   laminated. 
  

  

  No. 
  4. 
  Trachytes 
  variable 
  in 
  lithological 
  characteristics, 
  red 
  stratum 
  

   below, 
  variegated 
  beds 
  next, 
  and 
  rhyolite, 
  dolerite, 
  and 
  basalt 
  above, 
  

   the 
  latter 
  members 
  not 
  in 
  such 
  continuous 
  masses 
  nor 
  in 
  so 
  regular 
  suc- 
  

   cession 
  as 
  those 
  below. 
  

  

  Along 
  the 
  Gunnison 
  the 
  rocks 
  seem 
  to 
  represent 
  a 
  portion 
  of 
  No. 
  2. 
  

   The 
  following 
  section 
  gives 
  the 
  succession 
  of 
  the 
  rocks, 
  and 
  it 
  will 
  be 
  

   seen 
  that 
  they 
  agree 
  more 
  closely 
  with 
  those 
  of 
  Dr. 
  Endlich's 
  group 
  No. 
  

   2 
  than 
  with 
  any 
  of 
  the 
  others. 
  It 
  is 
  possible 
  that 
  a 
  portion 
  of 
  No. 
  3 
  

   may 
  be 
  represented 
  in 
  some 
  places 
  : 
  

  

  1. 
  Hard 
  gray 
  laminated 
  trachyte. 
  

  

  2. 
  Obsidian 
  porphyry. 
  

  

  3. 
  Spherulitic 
  and 
  porphyritic 
  obsidian. 
  

  

  4. 
  Brown 
  to 
  purplish 
  red 
  rhyolitic 
  trachyte. 
  

  

  5. 
  Purple 
  vesicular 
  rhyolitic 
  trachyte. 
  

  

  6. 
  Bluish-gray 
  laminated 
  rhyolitic 
  trachyte. 
  

  

  It 
  will 
  be 
  noticed, 
  when 
  the 
  heights 
  of 
  the 
  plateaus 
  are 
  compared, 
  

   that 
  there 
  is 
  a 
  general 
  slope 
  to 
  the 
  east 
  and 
  northeast. 
  The 
  thickness 
  of 
  

  

  93 
  

  

  