﻿PEALE.] 
  AKCH^AN 
  AREA. 
  69 
  

  

  fact 
  we 
  have 
  already 
  noted. 
  If 
  what 
  is 
  now 
  the 
  plateau 
  was 
  under 
  the 
  

   sea-level 
  before 
  this 
  elevation, 
  it 
  then 
  appeared 
  as 
  land, 
  continuing 
  so 
  

   until 
  the 
  end 
  of 
  the 
  Carboniferous, 
  gradually 
  subsiding, 
  however; 
  for 
  

   there 
  are 
  evidences 
  of 
  a 
  gradually 
  encroaching 
  shore-line 
  from 
  the 
  west- 
  

   ward. 
  During 
  Carboniferous 
  times, 
  the 
  west 
  edge 
  of 
  the 
  plateau 
  was 
  a 
  

   shore-line. 
  In 
  Triassic 
  times, 
  it 
  was 
  moved 
  much 
  farther 
  toward 
  the 
  

   east. 
  How 
  much 
  farther 
  toward 
  the 
  south 
  it 
  moved 
  we 
  cannot 
  tell. 
  

  

  The 
  granitic 
  area 
  of 
  which 
  we 
  have 
  noted 
  exposures 
  in 
  our 
  district 
  

   was, 
  therefore, 
  a 
  portion 
  at 
  least 
  of 
  the 
  land 
  from 
  which 
  the 
  materials 
  

   of 
  Pre-Triassic 
  strata 
  were 
  derived. 
  I 
  have 
  several 
  times 
  referred 
  to 
  

   the 
  fact 
  of 
  there 
  being 
  a 
  gradual 
  subsidence. 
  There 
  may 
  have 
  been 
  

   oscillations, 
  some 
  of 
  which 
  were 
  merely 
  local, 
  but 
  the 
  general 
  move- 
  

   ment 
  from 
  the 
  beginning 
  of 
  Silurian 
  times 
  until 
  the 
  period 
  of 
  eleva- 
  

   tion 
  at 
  the 
  end 
  of 
  the 
  formation 
  of 
  the 
  Lignite 
  Tertiary 
  was 
  that 
  of 
  

   subsidence. 
  

  

  The 
  Archsean 
  area 
  along 
  the 
  northern 
  edge 
  of 
  the 
  San 
  Juan 
  Mountains 
  

   and 
  south 
  of 
  the 
  Gunnison 
  Eiver 
  probably 
  formed 
  a 
  shore-line 
  in 
  Cre- 
  

   taceous 
  times, 
  and 
  this 
  shore-line 
  was 
  probably 
  much 
  farther 
  north 
  in 
  

   more 
  ancient 
  times, 
  though 
  how 
  far 
  we 
  cannot, 
  with 
  the 
  present 
  limited 
  

   amount 
  of 
  knowledge, 
  say. 
  The 
  area 
  of 
  the 
  ArchsBan 
  continent 
  was 
  

   probably 
  of 
  some 
  considerable 
  extent, 
  and 
  the 
  area 
  in 
  our 
  district 
  was 
  

   probably 
  an 
  extension 
  of 
  that 
  farther 
  east, 
  where 
  the 
  main 
  chain 
  of 
  the 
  

   Eocky 
  Mountains 
  is 
  now, 
  rather 
  than 
  a 
  separate 
  island. 
  Its 
  boundaries 
  

   cannot 
  be 
  determined 
  until 
  the 
  entire 
  Eocky 
  Mountain 
  region 
  is 
  studied 
  

   more 
  in 
  detail. 
  

  

  The 
  general 
  level 
  above 
  the 
  sea 
  may 
  not 
  have 
  been 
  very 
  great, 
  but 
  

   the 
  western 
  North 
  American 
  continent 
  was 
  outlined 
  and 
  we 
  had 
  indi« 
  

   cations 
  of 
  the 
  future 
  Eocky 
  Mountains. 
  

  

  