﻿222 
  REPORT 
  UJsITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  existence 
  of 
  these 
  glaciers 
  at 
  different 
  localities 
  indirectly 
  only. 
  By 
  

   studying 
  the 
  causes 
  that 
  produced 
  glaciers 
  in 
  Southern 
  Colorado, 
  and 
  

   arriving 
  at 
  a 
  satisfactory 
  conclusion 
  as 
  to 
  their 
  efficacy, 
  we 
  may 
  judge 
  

   that 
  like 
  cause 
  produces 
  like 
  effect, 
  and 
  may 
  thus 
  indirectly 
  deduce 
  for 
  

   allour 
  former 
  glaciers 
  in 
  that 
  region, 
  the 
  result 
  that 
  they 
  were 
  active 
  

   at 
  the 
  same 
  time. 
  I 
  am 
  of 
  the 
  opinion 
  that, 
  although 
  a 
  general 
  glacial 
  

   period 
  may 
  readily 
  be 
  accepted, 
  local 
  influences 
  determine 
  in 
  many 
  

   instances 
  the 
  perpetuation 
  of 
  glaciers 
  at 
  given 
  points. 
  The 
  glaciers 
  of 
  

   our 
  own 
  country, 
  those 
  of 
  Switzerland, 
  and 
  numerous 
  other 
  regions 
  are 
  

   but 
  in 
  support 
  of 
  this 
  view. 
  Eemove 
  the 
  cause 
  of 
  their 
  perpetuation 
  

   either 
  by 
  a 
  change 
  of 
  the 
  mean 
  annual 
  temperature 
  or 
  an 
  inadequate 
  

   supply 
  of 
  precipitated 
  moisture, 
  and 
  the 
  glacier 
  will 
  gradually 
  disappear. 
  

   It 
  has 
  been 
  my 
  endeavor 
  to 
  draw 
  conclusions 
  from 
  analogous 
  occur-* 
  

   rences 
  of 
  other 
  localities, 
  as 
  to 
  the 
  origin 
  and 
  fate 
  of 
  our 
  glaciers 
  in 
  

   Southern 
  Colorado, 
  and 
  the 
  result 
  thereof 
  is 
  contained 
  in 
  the 
  subjoined 
  

   pages. 
  

  

  Before 
  proceeding 
  to 
  a 
  more 
  general 
  discussion 
  as 
  to 
  the 
  causes 
  and 
  

   the 
  origin 
  of 
  the 
  glaciers 
  in 
  Southwestern 
  Colorado, 
  I 
  wish 
  to 
  make 
  

   reference 
  to 
  an 
  interesting 
  group 
  of 
  drift 
  on 
  the 
  eastern 
  side 
  of 
  San 
  Luis 
  

   VaJley. 
  In 
  Chapter 
  II 
  mention 
  has 
  already 
  been 
  made 
  of 
  the 
  " 
  compact 
  

   drift" 
  observed 
  at 
  station 
  115, 
  extending 
  from 
  there 
  northward 
  beyond 
  

   station 
  118. 
  It 
  is 
  by 
  no 
  means 
  an 
  easy 
  matter 
  to 
  explain 
  an 
  accumula- 
  

   tion 
  of 
  drift 
  bowlders 
  covering 
  so 
  extensive 
  an 
  area, 
  and 
  reaching 
  a 
  

   thickness 
  of 
  more 
  than 
  1,000 
  feet. 
  I 
  have 
  not 
  seen 
  the 
  "regular 
  strati- 
  

   fication" 
  mentioned 
  by 
  others 
  as 
  existing 
  in 
  these 
  bluffs. 
  There 
  is 
  an 
  

   attempt 
  at 
  stratoid 
  arrangement 
  noticeable, 
  but 
  it 
  is 
  too 
  obscure 
  to 
  ad- 
  

   mit 
  of 
  terming 
  the 
  poorly-defined 
  layers 
  strata. 
  We 
  found 
  (station 
  118) 
  

   that 
  this 
  compact 
  drift 
  was 
  there 
  covered 
  by 
  about 
  200 
  feet 
  of 
  bas£tlt. 
  

   At 
  the 
  same 
  time 
  pebbles 
  and 
  bowlders 
  of 
  basalt 
  were 
  found 
  in 
  the 
  con- 
  

   glomeritic 
  mass 
  underlying 
  the 
  volcanic 
  cap. 
  Metamorphic, 
  sedi- 
  

   mentary 
  (Carboniferous), 
  trachytic, 
  and, 
  as 
  stated, 
  basaltic 
  material 
  

   compose 
  the 
  " 
  compact 
  drift." 
  On 
  the 
  western 
  slope 
  of 
  the 
  Sangre 
  de 
  

   Cristo 
  Eange, 
  at 
  the 
  immediate 
  base 
  of 
  the 
  high 
  mountains 
  forming 
  its 
  

   crest, 
  canons 
  are 
  cut 
  into 
  the 
  metamorphic 
  rocks, 
  showing 
  detail- 
  

   features 
  that 
  closely 
  resemble 
  those 
  produced 
  by 
  glacial 
  erosion. 
  This 
  

   is 
  particularly 
  noticeable 
  due 
  east 
  of 
  the 
  drift-bluffs. 
  It 
  seems 
  highly 
  

   probable 
  to 
  me, 
  therefore, 
  that 
  the 
  drift 
  in 
  question 
  was 
  deposited 
  in 
  

   the 
  region 
  where 
  we 
  now 
  find 
  it, 
  by 
  glaciers 
  extending 
  and 
  moving 
  

   from 
  east 
  to 
  west. 
  In 
  Chapter 
  II 
  the 
  former 
  lakes 
  of 
  San 
  Luis 
  Yalley 
  

   have 
  been 
  treated 
  of, 
  and 
  with 
  reference 
  thereto 
  I 
  explain 
  the 
  stratoid 
  

   appearance 
  of 
  a 
  portion 
  of 
  this 
  drift 
  by 
  assuming 
  that 
  such 
  portion 
  was 
  ■ 
  

   deposited 
  into 
  still 
  water, 
  into 
  the 
  waters 
  of 
  the 
  great 
  lake. 
  It 
  may 
  

   seem 
  rash 
  to 
  assign 
  an 
  accumulation 
  of 
  drift 
  capped 
  by 
  hasaJt 
  to 
  glacial 
  

   action. 
  This 
  appears 
  all 
  the 
  more 
  unorthodox 
  when 
  we 
  remember 
  that 
  

   farther 
  west 
  glacial 
  erosion 
  was 
  observed 
  on 
  the 
  basalt 
  itself. 
  Taking 
  

   into 
  consideration, 
  however, 
  the 
  long 
  period 
  of 
  time 
  that 
  must 
  have 
  

   elapsed 
  before 
  the 
  last 
  of 
  the 
  basaltic 
  flows 
  made 
  its 
  appearance, 
  and 
  

   taking 
  into 
  consideration, 
  furthermore, 
  the 
  intermissions 
  of 
  suspended 
  

   volcanic 
  activity 
  between 
  many 
  of 
  these 
  flows, 
  there 
  is 
  no 
  reason 
  why 
  

   glaciers 
  should 
  not 
  have 
  formed 
  and 
  removed 
  large 
  masses 
  of 
  rocky 
  ma- 
  

   terial 
  during 
  such 
  period 
  of 
  inactivity. 
  These 
  redeposited 
  masses 
  were 
  

   eventually 
  reached 
  by 
  the 
  lava 
  of 
  the 
  latest 
  volcanic 
  eruptions. 
  This 
  

   case, 
  or 
  rather 
  this 
  interpretation 
  of 
  facts 
  observed, 
  is 
  not 
  isolated, 
  

   although 
  I 
  am 
  not 
  aware 
  of 
  any 
  similar 
  or 
  identical 
  occurrence 
  having 
  

   been 
  observed 
  on 
  the 
  North 
  American 
  continent. 
  

  

  