HISTORY  OF  PETROLEUM  OR  ROCK  OIL. 
527 
ON  THE  HISTORY  OF  PETROLEUM  OR  ROCK  OIL. 
By  T.  Sterry  Hunt,  M.  A.,  F.R.S.,  of  the  Geological  Survey  of  Canada. 
Public  attention  has  lately  been  drawn  to  the  petroleum  fur- 
nished by  the  oil  wells  in  Canada  and  the  United  States,  and  we 
have,  therefore,  thought  it  well  to  bring  together  some  few  facts 
which  may  serve  to  explain  the  origin  of  this  and  of  similar  sub- 
stances, including  naphtha,  petroleum,  or  rock  oil,  and  asphalt, 
or  mineral  pitch,  all  of  which  are  forms  of  bitumen,  the  one 
being  solid,  and  the  others  fluid  at  ordinary  temperatures. 
These  differences  are,  in  many  cases  at  least,  due  to  subsequent 
alterations  ;  the  more  liquid  of  these  substances  are  mixtures 
of  oils  differing  in  volatility,  and  by  exposure  to  the  air  become 
less  fluid,  and  partly  by  evaporation,  partly  by  oxidation  from 
the  air,  eventually  become  solid,  and  are  changed  into  mineral 
pitch.  These  substances,  which  are,  doubtless,  of  organic  ori- 
gin, occur  in  rocks  of  all  ages,  from  the  Lower  Silurian  to  the 
tertiary  period  inclusive,  and  are  generally  found  impregnating 
limestones,  and,  more  rarely,  sandstones,  and  shales.  Their 
presence  in  the  lower  palaeozoic  rocks,  which  contain  no  traces 
of  land  plants,  show  that  they  have  not  been  in  all  cases  de- 
rived from  terrestrial  vegetation,  but  may  have  been  formed  from 
marine  plants  or  animals  :  the  latter  is  not  surprising  when  we 
consider  that  a  considerable  portion  of  the  tissues  of  the  lower 
marine  animals  is  destitute  of  nitrogen,  and  very  similar  in 
chemical  composition  to  the  woody  fibre  of  plants.  Besides  the 
rocks  which  contain  true  bitumen,  we  have  what  are  called  bi- 
tuminous shales,  which  when  heated  burn  with  flame,  and  by 
distillation  at  a  high  temperature  yield,  besides  inflammable 
gases,  a  portion  of  oil  not  unlike  in  its  characters  to  petroleum. 
These  are,  in  fact,  argillaceous  rocks  intermixed  with  a  portion 
of  organic  matter  allied  to  peat  or  lignite,  which  by  heat  is  de- 
composed, and  gives  rise  to  oily  hydrocarbons.  These  inflam- 
mable or  lignitic  shales,  which  maybe  conveniently  distinguished 
by  the  name  of  Pyroschists  (the  brandschiefer  of  the  Germans,) 
are  to  be  carefully  distinguished  from  rocks  containing  ready- 
formed  bitumen  ;  this  being  easily  soluble  in  benzole  or  sulphuret 
of  carbon,  can  be  readily  dissolved  from  the  rocks  in  which  it 
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