﻿94 
  EEPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SUKVEY. 
  

  

  the 
  rocks 
  on 
  the 
  Gunnison 
  is 
  much 
  less 
  than 
  in 
  Dr. 
  Endlich's 
  districts 
  

   His 
  group 
  of 
  No. 
  2 
  is 
  1,200 
  feet 
  thick 
  farther 
  south, 
  while 
  the 
  beds 
  I 
  

   have 
  given 
  above 
  are 
  only 
  about 
  100 
  feet 
  where 
  measured, 
  and 
  proba- 
  

   bly 
  at 
  no 
  place 
  exceed 
  500 
  feet, 
  and 
  they 
  possibly 
  include 
  a 
  portion 
  of 
  

   the 
  groups 
  above 
  and 
  below. 
  

  

  The 
  rocks, 
  therefore, 
  represent 
  the 
  northern 
  and 
  northeastern 
  edges 
  of 
  

   one 
  flow, 
  the 
  one 
  preceding 
  and 
  the 
  one 
  following 
  scarcely 
  reaching 
  so 
  

   far, 
  or, 
  if 
  they 
  did, 
  having 
  been 
  entirely 
  removed 
  prior 
  and 
  subsequent 
  

   to 
  the 
  flow 
  of 
  No. 
  2. 
  

  

  Throughout 
  a 
  large 
  portion 
  of 
  the 
  area 
  the 
  volcanic 
  rocks 
  rest 
  imme- 
  

   diately 
  on 
  the 
  granites. 
  Along 
  the 
  Gunnison, 
  particularly 
  on 
  the 
  north 
  

   side, 
  the 
  following 
  is 
  the 
  order 
  of 
  the 
  rocks 
  : 
  

  

  1. 
  Trachytes. 
  

  

  2. 
  Breccia. 
  

  

  3. 
  Remnants 
  of 
  Cretaceous 
  sandstones. 
  

  

  4. 
  Granitic 
  rocks. 
  

  

  As 
  we 
  go 
  south 
  layer 
  No. 
  3 
  disappears 
  very 
  soon, 
  and 
  next 
  the 
  brec- 
  

   cia 
  is 
  absent, 
  leaving 
  the 
  trachytes 
  on 
  the 
  granite, 
  and 
  in 
  places 
  the 
  gran- 
  

   ite 
  projects 
  through, 
  forming 
  hills 
  that 
  rise 
  above 
  the 
  plateau-like 
  mass 
  

   of 
  trachyte. 
  

  

  Between 
  the 
  branches 
  of 
  Cebolla, 
  Cimmaron, 
  and 
  Blue 
  Creeks 
  are 
  

   long, 
  tongue-like 
  mesas, 
  which 
  appear 
  to 
  be 
  capped 
  with 
  trachyte 
  rest- 
  

   ing 
  on 
  breccia. 
  

  

  Between 
  the 
  Cimmaron 
  and 
  Blue 
  there 
  is 
  a 
  dike 
  of 
  trachyte 
  in 
  Cre- 
  

   taceous 
  rocks, 
  from 
  which 
  the 
  volcanic 
  material 
  appears 
  to 
  have 
  flowed 
  

   and 
  covered 
  an 
  area 
  of 
  some 
  extent. 
  This 
  area 
  is 
  at 
  a 
  lower 
  level 
  than 
  

   that 
  covered 
  by 
  the 
  flow 
  which 
  caps 
  the 
  mesas 
  to 
  the 
  south 
  and 
  be- 
  

   tween 
  the 
  Gunnison 
  and 
  Cimmaron. 
  In 
  the 
  latter 
  the 
  trachyte 
  rests 
  

   on 
  granite. 
  The 
  dike 
  may 
  be 
  of 
  more 
  modern 
  age. 
  More 
  particulars 
  

   in 
  regard 
  to 
  these 
  volcanic 
  rocks 
  will 
  be 
  found 
  in 
  the 
  Eeport 
  for 
  1874, 
  

   (pp. 
  168, 
  171, 
  193, 
  209.) 
  

  

  BKECCIA. 
  

  

  The 
  breccia 
  underlying 
  the 
  trachytes 
  just 
  described 
  is 
  best 
  exposed 
  

   north 
  of 
  the 
  Gunnison 
  Eiver, 
  in 
  which 
  region 
  it 
  was 
  described 
  in 
  the 
  

   Eeport 
  for 
  1874. 
  Its 
  stratified 
  character 
  indicates 
  its 
  deposition 
  in 
  

   water. 
  

  

  South 
  of 
  the 
  Gunnison 
  it 
  soon 
  disappears, 
  although 
  Dr. 
  Endlich 
  

   finds 
  it 
  again 
  south 
  of 
  the 
  San 
  Juan 
  Mountains. 
  The 
  shore-line 
  of 
  the 
  

   lake 
  or 
  sea 
  appears 
  to 
  have 
  been 
  irregular. 
  From 
  the 
  mouth 
  of 
  Coche- 
  

   topa 
  Creek 
  to 
  Mountain 
  Creek 
  it 
  seems 
  to 
  have 
  been 
  not 
  far 
  south 
  of 
  

   the 
  Gunnison 
  Eiver, 
  and 
  approximately 
  parallel 
  to 
  the 
  course 
  of 
  the 
  

   river. 
  As 
  we 
  approach 
  the 
  Cimmaron 
  and 
  Cebolla 
  Creeks, 
  the 
  breccia 
  

   extends 
  much 
  farther 
  southward, 
  but 
  it 
  is 
  impossible 
  to 
  trace 
  the 
  

   lines 
  beneath 
  the 
  accumulations 
  of 
  volcanic 
  rocks 
  resting 
  upon 
  it. 
  

   West 
  ot 
  the 
  Uncompahgre 
  the 
  breccia 
  is 
  entirely 
  absent 
  until 
  we 
  ap- 
  

   proach 
  the 
  mountains, 
  where 
  remnants 
  are 
  seen 
  on 
  the 
  lower 
  slopes. 
  

   The 
  matrix 
  of 
  the 
  breccia 
  is 
  of 
  a 
  gray 
  ashen 
  color, 
  and 
  the 
  included 
  

   masses 
  are 
  generally 
  dark 
  trachytes 
  of 
  all 
  shapes 
  and 
  sizes. 
  In 
  most 
  

   places 
  they 
  are 
  angular 
  and 
  of 
  large 
  size. 
  Along 
  the 
  Gunnison 
  a 
  pink 
  

   tufa 
  separates 
  the 
  breccia 
  from 
  the 
  overlying 
  trachytes. 
  

  

  POEPHYEITIC 
  TRACHYTE. 
  

  

  In 
  the 
  Report 
  for 
  1874, 
  under 
  this 
  head, 
  I 
  described 
  some 
  isolated 
  

   mountains 
  just 
  west 
  of 
  the 
  Elk 
  Mountains. 
  The 
  rocks 
  in 
  that 
  region 
  dif- 
  

  

  