528 
HISTORY  OF  PETROLEUM  OR  ROCK  OIL. 
occurs,  while  the  pyroschists  in  question  yield,  like  coal  and 
lignite,  little  or  nothing  to  these  liquids. 
It  is  the  more  necessary  to  insist  upon  the  distinction  between 
lignitic  and  bituminous  rocks,  inasmuch  as  some  have  been  dis- 
posed to  regard  the  former  as  the  source  of  the  bitumen  found 
in  Nature,  which  they  conceive  to  have  originated  from  a  slow 
distillation  of  these  matters.  The  result  of  a  careful  examina- 
tion of  the  question  has,  however,  led  us  to  the  conclusion  that 
the  formation  of  the  one  excludes  more  or  less  completely  that 
of  the  other,  and  that  bitumen  has  been  generated  under  con- 
ditions different  from  those  which  have  transformed  organic 
matters  into  coal  and  lignite,  and  probably  in  deep  water  de- 
posits from  which  atmospheric  oxygen  was  excluded.  Thus,  in 
the  palaeozoic  strata  of  North  America  we  find  in  the  Utica  and 
Hamilton  formations  highly  inflammable  pyroschists  which  con- 
tain no  soluble  bitumen,  and  the  same  is  true  to  a  certain  ex- 
tent of  some  limestones ;  while  the  Trenton  and  Corniferous 
limestones  of  the  same  series  are  impregnated  with  petroleum 
or  mineral  pitch,  and,  as  we  shall  show,  give  rise  to  petroleum 
springs.  The  fact  that  intermediate  porous  strata  of  similar 
mineral  characters  are  destitute  of  bitumen,  shows  that  this 
material  cannot  have  been  derived  from  overlying  or  underlying 
beds,  but  has  been  generated  by  the  transformation  of  organic 
matters  in  the  strata  in  which  it  is  met  with.  This  conclusion 
is  in  accordance  with  that  arrived  at  by  Mr.  S.  P.  Wall  in  his 
recent  investigations  in  Trinidad.  He  has  shown  that  the  as. 
phalt  of  that  island  and  of  Venezuela  belongs  to  strata  of  the 
tertiary  formation  (of  upper  miocene  or  lower  pliocene  age,) 
which  consists  of  limestones,  sandstones,  and  shales,  associated 
with  beds  of  lignite.  The  bitumen  is  found  not  only  in  the  fa- 
mous pitch  lake,  but  in  situ  where  it  is  confined  to  particular 
strata  which  were  originally  shales,  containing  vegetable  remains. 
These  have  undergone  "  a  special  mineralization,  producing  a 
bituminous  matter  instead  of  coal  or  lignite.  This  operation  is 
not  attributable  to  heat,  nor  of  the  nature  of  a  distillation,  but 
is  due  to  chemical  reactions  at  the  ordinary  temperature,  and 
under  the  normal  conditions  of  climate."  He  also  describes  wood 
partially  converted  into  bitumen,  which  last,  when  removed  by 
solution,  leaves  a  portion  of  woody  tissue.  (Proc.  Geol.  Soc. 
London,  May,  1860.) 
