﻿CHAPTER 
  IV. 
  

  

  AEOH^AN 
  EOCKS. 
  

  

  The 
  areas 
  iu 
  which 
  archeean 
  rocks 
  are 
  shown 
  throughout 
  our 
  district 
  

   are 
  limited 
  to 
  those 
  places 
  where 
  the 
  overlying 
  sedimentary 
  beds 
  have 
  

   been 
  removed. 
  This 
  is 
  generally 
  along 
  the, 
  courses 
  of 
  streams 
  flowing 
  in 
  

   canons. 
  The 
  rocks 
  are 
  metamorphic, 
  and 
  their 
  characters 
  will 
  be 
  treated 
  

   of 
  as 
  we 
  refer 
  to 
  the 
  particular 
  localities 
  in 
  which 
  they 
  occur. 
  In 
  many- 
  

   places 
  the 
  schistose 
  character 
  is 
  very 
  distinct 
  and 
  the 
  bedding 
  very 
  

   clearly 
  seen, 
  but 
  in 
  most 
  cases 
  we 
  were 
  unable 
  to 
  see 
  any 
  traces 
  of 
  bed- 
  

   ding, 
  the 
  rocks 
  being 
  granitoid. 
  From 
  the 
  number 
  of 
  exposures 
  noticed 
  

   it 
  is 
  evident 
  that 
  the 
  rocks 
  underlie 
  the 
  entire 
  district. 
  From 
  the 
  lim- 
  

   ited 
  and 
  isolated 
  exposures, 
  it 
  was 
  not 
  possible 
  to 
  trace 
  connections 
  

   from 
  one 
  place 
  to 
  another. 
  

  

  The 
  limited 
  amount 
  of 
  time, 
  and 
  loss 
  of 
  all 
  specimens 
  collected, 
  pre- 
  

   clude 
  the 
  possibility 
  of 
  giving 
  many 
  lithological 
  details. 
  

  

  The 
  strata 
  have 
  all 
  undergone 
  such 
  changes 
  since 
  their 
  deposition 
  as 
  

   sediments, 
  that 
  the 
  only 
  way 
  in 
  which 
  we 
  can 
  approximate 
  their 
  age 
  is 
  

   by 
  position 
  in 
  reference 
  to 
  the 
  overlying 
  sedimencaries. 
  In 
  our 
  district 
  

   the 
  oldest 
  recognized 
  sedimentary 
  rocks 
  resting 
  upon 
  the 
  metamorphic 
  

   series 
  were 
  of 
  Carboniferous 
  or 
  Permo-Carboniferous 
  age 
  this 
  indicates 
  

   that 
  they 
  are 
  at 
  least 
  of 
  Pre-Carboniferous 
  age. 
  Farther 
  east, 
  and 
  north 
  

   in 
  Colorado, 
  we 
  find 
  them 
  covered 
  with 
  the 
  representative 
  of 
  the 
  Pots- 
  

   dam 
  sandstone, 
  so 
  there 
  they 
  are 
  Pre-Silurian, 
  or 
  of 
  archgean 
  age. 
  So, 
  

   also, 
  far 
  to 
  the 
  north, 
  primordial 
  rocks 
  rest 
  upon 
  them, 
  proving 
  them 
  

   again 
  of 
  Pre-Silurian 
  age. 
  

  

  Although, 
  in 
  these 
  different 
  localities 
  the 
  lithological 
  peculiarities 
  

   may 
  differ 
  slightly, 
  they 
  belong 
  to 
  the 
  same 
  metamorphic 
  series. 
  As 
  Mr. 
  

   Marvine 
  says 
  in 
  the 
  report 
  for 
  1873,* 
  " 
  The 
  conclusion 
  as 
  to 
  their 
  ar- 
  

   chsean 
  age 
  is 
  also 
  rendered 
  almost 
  necessary 
  on 
  the 
  independent 
  ground 
  

   of 
  the 
  extent, 
  uniformity, 
  and 
  completeness 
  of 
  the 
  metamorphism 
  

   which 
  has 
  affected 
  the 
  mass. 
  For 
  it 
  is 
  no 
  case 
  of 
  local 
  metamorphism, 
  

   nor 
  one 
  of 
  supposed 
  dependence 
  upon 
  adjacent 
  masses 
  of 
  eruptive 
  rocks, 
  

   nor 
  of 
  the 
  accidental 
  presence 
  of 
  mineral 
  waters. 
  The 
  metamorphism 
  

   is 
  regular, 
  or 
  normal, 
  affecting 
  a 
  great 
  system 
  of 
  bedded 
  rocks 
  of 
  un- 
  

   known 
  thickness 
  and 
  indefinite 
  extent." 
  

  

  Sufficient 
  data 
  have 
  not 
  yet 
  been 
  obtained 
  to 
  determine 
  the 
  exact 
  age 
  

   of 
  the 
  metamorphic 
  series, 
  although, 
  as 
  Marvine 
  remarks 
  of 
  those 
  farther 
  

   east, 
  " 
  the 
  prevalence 
  of 
  siliceous 
  and 
  granitic 
  types 
  recalls 
  the 
  de- 
  

   scriptions 
  of 
  Laurentian 
  areas." 
  In 
  one 
  place 
  the 
  schists 
  are 
  very 
  dis- 
  

   tinctly 
  stratified, 
  consisting 
  of 
  dark 
  micaceous 
  schists, 
  with 
  seams 
  of 
  

   quartz 
  and 
  feldspar. 
  These 
  may 
  be 
  of 
  Huronian 
  age, 
  although 
  we 
  can- 
  

   not 
  trace 
  their 
  relations 
  to 
  those 
  of 
  the 
  other 
  ATchaean 
  rocks, 
  as 
  they 
  

   are 
  exposed 
  in 
  an 
  isolated 
  area 
  at 
  the 
  bottom 
  of 
  canons 
  distant 
  from 
  the 
  

   other 
  outcrops. 
  In 
  the 
  caiiou 
  of 
  the 
  Colorado, 
  according 
  to 
  Newberry 
  

  

  *Page 
  138 
  

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