﻿68 
  SANTA 
  MART 
  A 
  OIL 
  DISTRICT, 
  CALIFORNIA. 
  

  

  to 
  the 
  main 
  portions 
  of 
  the 
  quiet 
  sea 
  bottom 
  which 
  was 
  formerly 
  

   the 
  surface 
  of 
  the 
  land 
  and 
  which 
  had 
  been 
  given 
  a 
  comparatively 
  low 
  

   relief 
  by 
  the 
  long 
  period 
  of 
  erosion 
  that 
  preceded 
  the 
  submergence. 
  

  

  During 
  the 
  period 
  of 
  transition 
  between 
  the 
  Vaqueros 
  and 
  the 
  Mon- 
  

   terey, 
  limestone 
  was 
  formed 
  chiefly, 
  but 
  somewhat 
  inclosed 
  basins 
  

   where 
  deposits 
  of 
  alkaline 
  mud 
  were 
  laid 
  down 
  apparently 
  existed 
  in 
  

   places. 
  Such 
  a 
  basin 
  is 
  indicated 
  by 
  the 
  alkaline 
  gypsiferous 
  clays 
  oh 
  

   the 
  south 
  side 
  of 
  the 
  Casmalia 
  Hills, 
  probably 
  representing 
  upper 
  

   Vaqueros. 
  In 
  some 
  places, 
  as, 
  for 
  instance, 
  in 
  the 
  San 
  Rafael 
  Moun- 
  

   tains, 
  sandstone 
  beds 
  were 
  formed 
  early 
  in 
  Monterey 
  time, 
  probably 
  in 
  

   the 
  neighborhood 
  of 
  locally 
  unsubmerged 
  areas. 
  But 
  later 
  very 
  little 
  

   sand 
  was 
  deposited 
  anywhere. 
  Further 
  submergence 
  no 
  doubt 
  took 
  

   place 
  during 
  the 
  period, 
  removing 
  the 
  sources 
  of 
  this 
  sand 
  and 
  

   allowing 
  to 
  be 
  deposited 
  under 
  fairly 
  constant 
  conditions 
  a 
  thickness 
  

   of 
  beds 
  greater 
  than 
  a 
  mile. 
  It 
  is 
  not 
  probable, 
  however, 
  that 
  the 
  

   depth 
  of 
  the 
  sea 
  was 
  at 
  any 
  time 
  as 
  much 
  as 
  this, 
  being 
  more 
  likely 
  

   closer 
  to 
  half 
  a 
  mile. 
  

  

  During 
  the 
  early 
  part 
  of 
  Monterey 
  time 
  conditions 
  were 
  variable, 
  

   calcareous 
  and 
  siliceous 
  deposits 
  alternating, 
  probably 
  as 
  a 
  result 
  of 
  

   alternating 
  temporary 
  predominance 
  in 
  the 
  sea 
  of 
  organisms 
  with 
  

   calcareous 
  or 
  siliceous 
  shells. 
  As 
  the 
  period 
  progressed 
  the 
  siliceous 
  

   organisms 
  became 
  more 
  predominant 
  and 
  remained 
  so, 
  making 
  up 
  a 
  

   large 
  fraction 
  of 
  the 
  total 
  bulk 
  of 
  the 
  Monterey 
  formation. 
  It 
  was 
  

   an 
  age 
  of 
  diatoms. 
  These 
  small 
  marine 
  plants 
  lived 
  in 
  extreme 
  

   abundance 
  in 
  the 
  sea 
  and 
  fell 
  in 
  showers 
  with 
  their 
  siliceous 
  tests 
  

   to 
  add 
  to 
  the 
  accumulating 
  ooze 
  of 
  the 
  ocean 
  bottom, 
  just 
  as 
  they 
  are 
  

   forming 
  ooze 
  at 
  the 
  present 
  day 
  in 
  some 
  oceanic 
  waters. 
  It 
  is 
  well 
  

   known 
  that 
  diatoms 
  multiply 
  with 
  extreme 
  rapidity. 
  It 
  has 
  been 
  

   calculated 
  that 
  starting 
  with 
  a 
  single 
  individual 
  the 
  offspring 
  may 
  

   number 
  1,000,000 
  within 
  a 
  month. 
  One 
  can 
  conceive 
  that 
  under 
  

   very 
  favorable 
  life 
  conditions, 
  such 
  as 
  must 
  have 
  existed, 
  the 
  diatom 
  

   frustules 
  may 
  have 
  accumulated 
  rapidly 
  at 
  the 
  sea 
  bottom 
  and 
  aided 
  

   the 
  fine 
  siliceous 
  and 
  argillaceous 
  sediments 
  in 
  the 
  quick 
  building 
  

   up 
  of 
  the 
  thick 
  deposits 
  of 
  middle 
  Miocene 
  time. 
  A 
  principal 
  obstacle 
  

   to 
  the 
  rapid 
  accumulation 
  of 
  the 
  diatoms 
  might 
  be 
  the 
  limited 
  supply 
  

   of 
  silica 
  from 
  which 
  these 
  alga? 
  derive 
  the 
  material 
  of 
  their 
  tests. 
  

   Other 
  organisms 
  with 
  their 
  shells 
  and 
  skeletons 
  were 
  -also 
  present 
  

   to 
  aid 
  in 
  building 
  up 
  the 
  shale 
  beds. 
  They 
  were 
  Radiolaria 
  and 
  

   Foraminifera; 
  sponges 
  with 
  their 
  spicules, 
  which 
  were 
  abundant; 
  

   Crustacea; 
  fishes, 
  the 
  remains 
  of 
  which 
  are 
  numerous 
  in 
  the 
  shales; 
  

   and 
  mollusks 
  with 
  delicate 
  shells, 
  which 
  are 
  common, 
  though 
  poorly 
  

   preserved. 
  

  

  Volcanic 
  eruptions, 
  possibly 
  submarine, 
  broke 
  out 
  at 
  different 
  

   times 
  during 
  the 
  latter 
  part 
  of 
  the 
  lower 
  Miocene 
  (Vaqueros) 
  and 
  

   the 
  early 
  part 
  of 
  the 
  middle 
  Miocene 
  (Monterey.) 
  They 
  may 
  have 
  

   accompanied 
  movements 
  that 
  took 
  place 
  during 
  the 
  transition 
  

  

  