﻿400 
  

  

  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  

  

  rare 
  towards 
  the 
  central 
  portion. 
  One 
  interpretation 
  which 
  could 
  

   be 
  placed 
  on 
  this 
  fact 
  is 
  that 
  only 
  along 
  the 
  margins 
  did 
  sediments 
  

   accumulate 
  fast 
  enough 
  to 
  bury 
  the 
  bone 
  before 
  weathering 
  or 
  other 
  

   factors 
  could 
  destroy 
  them. 
  However, 
  evidence 
  of 
  alluvial 
  fans 
  along 
  

   the 
  basin's 
  margin 
  have 
  not 
  been 
  observed. 
  The 
  alternative, 
  which 
  

   seems 
  the 
  more 
  probable, 
  is 
  that 
  the 
  accumulating 
  soil 
  of 
  the 
  central 
  

   portion 
  was 
  more 
  acid 
  from 
  decaying 
  vegetation 
  and 
  the 
  bones, 
  react- 
  

   ing 
  with 
  the 
  sulphides 
  of 
  vegetable 
  decay, 
  were 
  a 
  partial 
  som-ce 
  of 
  the 
  

   gypsum 
  in 
  these 
  deposits. 
  

  

  Throughout 
  their 
  observed 
  exposures 
  the 
  Oligocene 
  sediments 
  are 
  

   uniformly 
  fine 
  grained, 
  ranging 
  in 
  color 
  from 
  light 
  gray 
  to 
  light 
  buff. 
  

  

  

  .0 
  "^mL, 
  

  

  

  ■''.■\ 
  '•'' 
  

  

  ^> 
  • 
  ;,;rr. 
  

  

  

  

  ^^^^^-V^t, 
  

  

  ^^, 
  

  

  t 
  ^..%m^y^-^:::^^^'^^''^^rM 
  

  

  

  

  >^^'**PVr?- 
  ."''^/^'f^^i^i"' 
  ^"^^?f^>--v'-sS^ 
  

  

  Figure 
  42. 
  — 
  Locality 
  No. 
  24LC18 
  from 
  the 
  south. 
  

  

  No 
  lithological 
  distinction 
  could 
  be 
  made 
  between 
  the 
  Lower 
  and 
  

   Middle 
  Oligocene 
  sediments, 
  but 
  those 
  which 
  are 
  believed 
  to 
  represent 
  

   the 
  Upper 
  Oligocene 
  contain 
  considerably 
  more 
  ash, 
  are 
  less 
  consoli- 
  

   dated, 
  and 
  grayish 
  green 
  in 
  color. 
  The 
  Oligocene 
  sediments 
  show 
  

   rather 
  even 
  bedding 
  planes 
  which 
  are 
  indistinct 
  except 
  when 
  the 
  ex- 
  

   posures 
  are 
  wet. 
  Occasional 
  stream 
  gravels 
  are 
  found 
  at 
  nearly 
  all 
  

   levels 
  but 
  do 
  not 
  appear 
  to 
  have 
  cut 
  very 
  deep 
  into 
  the 
  Oligocene 
  

   deposits. 
  These 
  gravels 
  are 
  made 
  up 
  almost 
  entirely 
  of 
  angular 
  frag- 
  

   ments 
  of 
  the 
  Proterozoic 
  and 
  Paleozoic 
  sediments 
  which 
  form 
  the 
  

   Big 
  Belt 
  Mountains 
  and 
  the 
  Spokane 
  Hills. 
  

  

  The 
  Miocene 
  sediments, 
  like 
  the 
  Oligocene, 
  are 
  uniformly 
  fine 
  

   grained 
  but 
  contain 
  a 
  higher 
  percentage 
  of 
  sand 
  and 
  are 
  better 
  con- 
  

  

  