DIATOMACEOUS    DEPOSITS    IN    CALIFORNIA.  439 
me-celled  plants  that  adapt  themselves  to  a  wide  range  of  conditions 
>f  depth  and  temperature  in  fresh  and  salt  water  and  secrete  siliceous 
jasings  or  frustules  around  their  organic  material.  They  must  have 
ived  in  extreme  abundance  in  the  ancient  sea,  for  it  is  evident  that 
hie  deposit  was  built  up  by  the  little  shells  of  these  plants  which 
hopped  to  the  sea  bottom.  The  name  "  chalk  rock  "  is  inappropriate, 
or  although  the  deposits  closely  resemble  soft  chalk  in  appearance 
hey  are  made  of  silica  instead  of  calcareous  material.  Examination 
frith  a  hand  lens  almost  always  reveals  a  large  number  of  the  round 
onus  of  the  diatom  shells  thickly  embedded  in  the  shale,  many  of 
them  in  a  very  good  state  of  preservation.  In  some  of  the  material 
:hey  can  be  plainly  made  out  with  the  naked  eye.  Here  and  there  in 
petroliferous  portions  of  the  diatomaceous  shale,  individual  diatoms 
will  be  seen  completely  filled  wTith  petroleum. 
GEOLOGIC     RELATIONS     OF    THE     DEPOSITS. 
By  far  the  greater  part  of  the  diatomaceous  material  occurring  in 
the  territory  under  discussion  is  found  in  the  Monterey  (middle  Mio- 
cene) formation.  Such  is  the  age  of  the  deposits  in  the  hills  south  and 
north  of  the  Santa  Ynez  River,  in  the  Burton  Mesa,  Casmalia  and 
Santa  Maria  oil  field  areas,  and  in  the  region  northeast  of  Santa  Maria 
Valley  and  Sisquoc  River. 
Fairbanks"  also  reports  diatomaceous  earth  in  the  Monterey  forma- 
tion south  of  Morro  Bay,  northeast  of  Port  Harford,  north  of  Pismo, 
and  southeast  of  Edna,  in  San  Luis  Obispo  County.  The  principal 
deposits  are  found  toward  the  top  of  the  Monterey,  those  in  the  lower 
portion  being  local  in  extent.  The  soft  material  is  usually  found  in 
series  of  beds  of  considerable  thickness,  in  which  are  a  fewT  beds  of 
impure  shale  and  some  limy  layers,  the  latter  being  common  toward 
the  bottom  of  the  Monterey.  All  gradations  from  the  soft  earthy 
diatomaceous  material  to  the  hardest  of  black  flints  may  be  found,  in 
some  places  within  very  short  distances,  thus  indicating  the  extreme 
localization  of  the  conditions  that  produced  the  harder  varieties  from 
the  unaltered  deposits. 
Diatomaceous  shale  is  found  in  the  lower  part  of  the  Fernando 
formation,  which  overlies  the  Monterey  unconformably,  in  the  region 
between  the  head  of  Howard  Canyon  and  Sisquoc  and  at  one  or  two 
places  along  the  north  side  of  the  Casmalia  Hills.  This  portion  of  the 
Fernando  is  doubtless  of  upper  Miocene  age  and  is  the  equivalent  of 
the  Pismo  and  Santa  Margarita  formations  described  by  Fairbanks, 
which  contain  diatomaceous  earth  in  the  region  north  of  Arroyo 
Grande,  southeast  of  Edna,  and  along  Salinas  River  as  far  up  as  Rin- 
conada  Creek,  all  in  the  San  Luis  quadrangle. 
a  Fairbanks,  H.  W.,  Description  of  the  San  Luis  quadrangle:  Geologic  Atlas  II.  S.,  folio  101,  U.  S. 
Geol.  Survey,  1904,  p.  14. 
