﻿PEALE.] 
  ARCH^AN 
  ROCKS 
  SOUTH 
  SIDE 
  OF 
  GUNNISON. 
  65 
  

  

  and 
  Powell, 
  the 
  metamorpliic 
  rocks 
  occur 
  beneath 
  the 
  sedimentary, 
  their 
  

   original 
  structure 
  greatly 
  obscured 
  or 
  entirely 
  destroyed. 
  

  

  I 
  will 
  now 
  take 
  up 
  the 
  different 
  localities 
  of 
  the 
  district 
  in 
  which 
  

   rocks 
  of 
  Archaean 
  age 
  were 
  observed. 
  

  

  The 
  granitic 
  and 
  gneissic 
  rocks 
  in 
  the 
  valley 
  of 
  the 
  Gunnison 
  from 
  

   Cochetopa 
  Creek 
  to 
  the 
  Lake 
  Fork 
  were 
  described 
  in 
  the 
  report 
  for 
  

   1874. 
  Along 
  the 
  courses 
  of 
  the 
  creeks 
  flowing 
  to 
  the 
  Gunnison 
  from 
  

   the 
  south 
  we 
  have 
  tongues 
  of 
  metamorphic 
  rocks 
  extending 
  southward 
  

   from 
  the 
  narrow 
  strip 
  exposed 
  along 
  the 
  main 
  river. 
  (See 
  Fig. 
  4, 
  Plate 
  I.) 
  

  

  As 
  we 
  have 
  noted 
  in 
  a 
  preceding 
  chapter 
  the 
  prevailing 
  rocks 
  from 
  

   the 
  Gunnison 
  southward 
  to 
  the 
  San 
  Juan 
  Mountains 
  are 
  volcanic, 
  

   Near 
  the 
  Gunnison 
  they 
  rest 
  on 
  Cretaceous 
  strata, 
  but 
  farther 
  back 
  

   Archaean 
  rocks 
  underlie 
  them. 
  On 
  White 
  Earth 
  Eiver 
  there 
  are 
  sev- 
  

   eral 
  hills 
  of 
  granitic 
  rocks 
  rising 
  above 
  the 
  volcanics. 
  Station 
  No. 
  1 
  is 
  

   located 
  on 
  such 
  a 
  hill. 
  Although 
  the 
  outcrops 
  were 
  very 
  indistinct, 
  

   enough 
  was 
  seen 
  to 
  note 
  the 
  fact 
  that 
  some 
  of 
  the 
  layers 
  were 
  quartzitic 
  

   and 
  others 
  softer, 
  with 
  red 
  and 
  gray 
  gneissic 
  layers. 
  The 
  slopes 
  of 
  the 
  

   hill 
  were 
  covered 
  with 
  reddish 
  debris. 
  South 
  of 
  stations 
  3 
  and 
  4 
  is 
  

   a 
  small 
  area 
  in 
  which 
  the 
  trachyte 
  had 
  been 
  removed 
  from 
  the 
  coarse, 
  

   red, 
  feldspathic 
  granite 
  rock 
  which 
  here 
  evidently 
  underlies 
  it. 
  No 
  trace 
  

   of 
  stratification 
  was 
  observed. 
  On 
  the 
  Gunnison 
  the 
  rocks 
  vary 
  from 
  

   coarse-grained 
  with 
  silvery 
  mica 
  (mnscovite) 
  to 
  dark 
  schistose 
  and 
  

   hornblendic 
  gneisses. 
  From 
  Lake 
  Fork 
  to 
  Blue 
  Creek 
  the 
  streams 
  join- 
  

   ing 
  the 
  Gunnison 
  from 
  the 
  south 
  come 
  in 
  through 
  deep 
  canons 
  cut 
  

   through 
  Archaean 
  rocks 
  which 
  are 
  capped 
  with 
  volcanic 
  layers. 
  

  

  Between 
  Cebolla 
  Creek 
  and 
  the 
  Gunnison 
  Eiver 
  is 
  a 
  granitic 
  hill 
  

   with 
  three 
  summits, 
  two 
  of 
  which 
  are 
  covered 
  with 
  a 
  horizontal 
  layer 
  

   of 
  trachyte. 
  This 
  hill 
  is 
  intersected 
  with 
  quartzitic 
  veins. 
  There 
  ap- 
  

   pears 
  to 
  be 
  an 
  indistinct 
  stratification, 
  w^hich 
  indicates 
  the 
  dip 
  to 
  be 
  to- 
  

   ward 
  the 
  southeast. 
  This 
  area 
  is 
  continuous 
  with 
  the 
  plateau 
  on 
  the 
  south- 
  

   west 
  side 
  of 
  the 
  Gunnison, 
  between 
  that 
  river 
  and 
  the 
  Uncompahgre. 
  

   Here 
  again 
  the 
  rocks 
  are 
  coarse-grained 
  and 
  reddish 
  in 
  color. 
  In 
  the 
  

   caiion 
  of 
  the 
  Gunnison, 
  the 
  bottom 
  of 
  which 
  is 
  nearly 
  3,000 
  feet 
  below 
  

   the 
  top 
  or 
  plateau 
  level, 
  veins, 
  probably 
  quartzitic, 
  can 
  be 
  seen 
  crossing 
  

   in 
  all 
  directions, 
  some 
  reaching 
  from 
  the 
  level 
  of 
  the 
  river 
  to 
  the 
  tops 
  of 
  

   the 
  walls, 
  which 
  are 
  perfectly 
  bare. 
  Against 
  these 
  rocks, 
  from 
  the 
  val- 
  

   ley 
  in 
  which 
  the 
  Cimmaron 
  and 
  Cebolla 
  unite 
  to 
  a 
  point 
  in 
  the 
  Uncom- 
  

   pahgre 
  Valley, 
  shales 
  of 
  Upper 
  Cretaceous 
  age 
  abut. 
  There 
  appear 
  to 
  

   be 
  no 
  older 
  beds 
  until 
  we 
  recede 
  some 
  distance 
  toward 
  the 
  southwest 
  

   and 
  also 
  the 
  north. 
  There 
  must 
  have 
  been 
  an 
  area 
  here, 
  which 
  in 
  Pre- 
  

   Cretaceous 
  times 
  was 
  above 
  water. 
  It 
  is 
  probably 
  also 
  of 
  Pre-Silurian 
  

   age, 
  as 
  we 
  have 
  already 
  noted. 
  A 
  gradual 
  subsidence 
  took 
  place, 
  and 
  

   we 
  find 
  the 
  sedimentary 
  layers 
  gradually 
  overlapping 
  each 
  other 
  until 
  

   the 
  Upper 
  Cretaceous 
  beds 
  rest 
  immediately 
  on 
  the 
  granite. 
  Some 
  

   points 
  may 
  have 
  been 
  above 
  water 
  also 
  during 
  Cretaceous 
  times. 
  After 
  

   the 
  deposition 
  of 
  the 
  shales 
  there 
  followed 
  an 
  elevation 
  (probably 
  in 
  

   Tertiary 
  time.) 
  I 
  do 
  not 
  mean 
  to 
  say 
  that 
  all 
  over 
  the 
  Eocky 
  Mountain 
  

   region 
  there 
  was 
  but 
  one 
  period 
  of 
  elevation 
  from 
  the 
  time 
  of 
  the 
  first 
  

   appearance 
  of 
  this 
  granitic 
  area 
  above 
  water 
  to 
  the 
  time 
  when 
  the 
  Cre- 
  

   taceous 
  beds 
  made 
  their 
  appearance 
  above 
  water, 
  but 
  merely 
  that 
  at 
  

   this 
  point 
  we 
  have 
  evidence 
  of 
  but 
  this 
  one, 
  and 
  evidence 
  also 
  of 
  the 
  

   gradual 
  subsidence 
  of 
  the 
  land 
  from 
  Pre-Silurian 
  times 
  to 
  at 
  least 
  the 
  

   end 
  of 
  the 
  Cretaceous 
  period. 
  Facts 
  observed 
  in 
  other 
  portions 
  of 
  Col- 
  

   orado 
  point 
  to 
  the 
  Tertiary 
  as 
  the 
  period 
  of 
  this 
  elevation. 
  I 
  shall 
  refer 
  

   to 
  these 
  points 
  again 
  in 
  another 
  portion 
  of 
  the 
  report. 
  

  

  I 
  will 
  next 
  take 
  up 
  the 
  Archaean 
  areas 
  of 
  the 
  Uncompahgre 
  Plateau 
  be- 
  

   5 
  G 
  s 
  

  

  