﻿4C> 
  SANTA 
  MAIM 
  \ 
  OIL 
  DISTEICT, 
  CALIFORNIA. 
  

  

  opaline 
  rock 
  of 
  analysis 
  No. 
  (> 
  were 
  samples 
  taken 
  from 
  the 
  same 
  

   hand 
  specimen 
  within 
  1 
  inch 
  of 
  each 
  other. 
  The 
  mass 
  of 
  the 
  deposit 
  

   from 
  which 
  the 
  specimen 
  came 
  was 
  soft 
  white 
  shale 
  belonging 
  high 
  

   in 
  the 
  formation 
  and 
  contained 
  a 
  rough 
  layer, 
  a 
  few 
  inches 
  thick, 
  of 
  

   the 
  harder 
  material 
  between 
  two 
  beds 
  of 
  the 
  soft 
  rock. 
  

  

  The 
  soft 
  shale 
  has 
  been 
  described 
  in 
  the 
  preceding 
  pages 
  as 
  " 
  unal- 
  

   tered," 
  and 
  in 
  referring 
  to 
  the 
  harder 
  varieties 
  different 
  degrees 
  of 
  

   " 
  alteration 
  " 
  have 
  been 
  mentioned, 
  for 
  the 
  reason 
  that 
  the 
  best 
  

   explanation 
  of 
  the 
  origin 
  of 
  the 
  harder 
  rocks 
  appears 
  to 
  be 
  that 
  they 
  

   are 
  products 
  of 
  metamorphism 
  of 
  the 
  soft 
  variety. 
  It 
  is 
  believed 
  

   that 
  the 
  soft 
  white 
  and 
  chocolate-colored 
  organic 
  shale 
  represents 
  

   the 
  original 
  slate 
  of 
  the 
  beds 
  of 
  the 
  whole 
  formation, 
  and 
  that 
  a 
  proc- 
  

   ess 
  of 
  silicification 
  and 
  crystallization 
  has 
  caused 
  the 
  changes, 
  this 
  

   process 
  having 
  been 
  aided 
  possibly 
  by 
  structural 
  disturbances 
  and 
  

   pressure. 
  The 
  beds 
  of 
  soft 
  shale 
  are 
  usually 
  found 
  in 
  attitudes 
  only 
  

   gently 
  disturbed, 
  whereas 
  the 
  harder 
  shale 
  is 
  most 
  commonly 
  much 
  

   folded 
  and 
  is 
  invariably 
  the 
  component 
  rock 
  of 
  folds 
  where 
  the 
  

   forces 
  have 
  been 
  especially 
  intense. 
  This 
  fact 
  may 
  throw 
  light 
  on 
  

   the 
  problem 
  of 
  the 
  alteration 
  of 
  the 
  shale, 
  and 
  yet 
  it 
  may 
  be 
  simply 
  

   the 
  outcome 
  of 
  the 
  removal 
  of 
  the 
  softer 
  portion 
  of 
  the 
  formation 
  

   in 
  the 
  regions 
  of 
  greatest 
  uplift 
  and 
  disturbance. 
  The 
  chief 
  agent 
  

   in 
  causing 
  the 
  change 
  was 
  probably 
  infiltrating 
  water 
  carrying 
  

   silica 
  in 
  solution. 
  In 
  some 
  places 
  the 
  process 
  may 
  have 
  been 
  simply 
  

   or 
  largely 
  infiltration 
  in 
  the 
  extremely 
  porous 
  original 
  shale 
  and 
  

   deposition 
  of 
  silica 
  in 
  the 
  interspaces, 
  thus 
  giving 
  rise 
  to 
  hardened 
  

   and 
  compacted 
  irregular 
  granular 
  aggregates 
  of 
  the 
  original 
  amor- 
  

   phous 
  silica 
  and 
  the 
  new 
  crystalline 
  silica 
  combined, 
  the 
  result 
  being 
  

   an 
  increase 
  in 
  the 
  total 
  percentage 
  of 
  silica. 
  In 
  more 
  extreme 
  cases 
  

   the 
  original 
  material 
  was 
  probably 
  partly 
  taken 
  in 
  solution 
  and 
  rede- 
  

   posited, 
  being 
  replaced 
  almost 
  entirely 
  along 
  bands 
  or 
  in 
  spots, 
  and 
  the 
  

   change 
  being 
  carried 
  to 
  a 
  less 
  extent 
  along 
  other 
  layers 
  and 
  in 
  other 
  

   areas, 
  or 
  else 
  the 
  replacement 
  was 
  almost 
  complete 
  throughout. 
  As 
  

   the 
  rock 
  was 
  rendered 
  more 
  compact 
  in 
  this 
  process 
  a 
  shrinkage 
  may 
  

   have 
  been 
  the 
  result, 
  or 
  the 
  same 
  volume 
  may 
  have 
  been 
  retained 
  

   and 
  the 
  pores 
  filled. 
  That 
  solution 
  took 
  place 
  along 
  with 
  deposition 
  

   seems 
  to 
  be 
  shown 
  by 
  the 
  almost 
  complete 
  destruction 
  of 
  the 
  forms 
  

   of 
  organisms. 
  

  

  It 
  is 
  possible 
  that 
  the 
  differences 
  in 
  the 
  shales 
  may 
  be 
  original, 
  the 
  

   result 
  of 
  variation 
  in 
  the 
  material 
  deposited. 
  Whole 
  series 
  of 
  beds 
  of 
  

   different 
  material 
  might 
  have 
  been 
  deposited, 
  giving 
  rise 
  to 
  harder, 
  

   more 
  siliceous 
  rocks 
  than 
  the 
  soft 
  varieties, 
  and 
  the 
  same 
  material 
  

   might 
  have 
  been 
  locally 
  deposited 
  in 
  thin 
  beds 
  or 
  in 
  lenses 
  and 
  

   nodules, 
  or 
  have 
  been 
  intermingled 
  with 
  the 
  others 
  to 
  form 
  the 
  inter- 
  

   mediate 
  varieties. 
  But 
  it 
  would 
  be 
  difficult 
  to 
  say 
  what 
  this 
  mate- 
  

  

  tie 
  

  

  