40G  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225. 
of  vegetable  origin  and  is  derived  from  organic  matter  disseminated 
through  bituminous  shales.  It  is  also  believed  that  the  natural  gas 
found  in  the  sedimentary  rocks  of  Pennsylvania  is  commonly  derived 
from  petroleum. 
CONDITIONS    FAVORING    ACCUMULATION    OF    OIL    AND    GAS. 
These  two  hydrocarbons  are  widely  distributed  through  the  rocks 
which  form  the  earth's  crust,  but  usually  in  small  quantities.  Certain 
conditions  are  essential  to  the  accumulation  of  either  gas  or  oil  in 
quantities  sufficiently  large  to  be  of  commercial  importance.  These 
conditions  are  practically  the  same  for  both  the  fluid  and  the  gas. 
The  most  important  conditions  governing  the  natural  storage  of  oil 
and  gas  are  (1)  a  sufficient  supply  from  some  source,  (2)  a  porous  res- 
ervoir rock  in  which  it  may  be  stored,  and  (3)  an  impervious  cap  rock 
which  will  prevent  its  escape. 
Further  conditions,  not  necessary,  but  favoring,  are  (1)  gentle  but 
considerable  undulations  of  the  strata,  forming  anticlinal  arches,  (2) 
the  complete  saturation  of  the  rocks  with  water  and  its  slow  circulation. 
All  of  the  conditions  which  favor  the  accumulation  of  oil  and  gas 
are  too  well  known  or  too  readily  apparent  to  need  much  elaboration. 
That  there  must  be  a  source  of  supply  is  apparent.  Organic  material, 
either  animal  or  vegetable,  is  generally  to  be  found  in  some  of  the 
beds  of  any  great  deposit  of  sedimentary  rocks,  and  from  these 
remains  an  abundant  supply  of  hydrocarbon  might  be  derived. 
The  reservoir  rocks  in  which  the  pay  streaks  of  oil  and  gas  are  found 
in  Pennsylvania  are  usually  porous  sandstone.  The  porosity  depends 
upon  the  shape  of  the  grains,  their  uniformity  in  size,  and  the  amount 
of  cementing  material.  The  porosity  or  vacant  space  of  an  ordinary 
compact  fine-grained  sandstone  is  from  8  to  10  per  cent. 
The  impervious  cover  which  retains  oil  and  gas  in  the  sandrock  and 
prevents  its  escape  is  shale.  These  shales  are  interbedded  with  the 
several  sandstone  strata  which  act  as  reservoirs. 
The  geologic  structure  in  Pennsylvania  plays  an  important  part  in 
the  accumulation.  The  gentle  folding  of  the  rocks  into  broad  arclies 
which  lie  generally  parallel  gives  a  repetition  of  conditions  which  favor 
the  formation  of  pools.  It  was  here  that  the  anticlinal  theory  was 
developed  and  proved.  This  theory  is  that  when  gas  and  oil  occur  in 
a  porous  bed  along  with  water,  they  are  forced  to  move  upward  toward 
the  surface,  and  will  rise  until  the  surface  is  reached  or  some  obstruc- 
tion is  met.  A  horizontal  layer  of  impervious  material  is  the  most 
formidable  obstruction .  It  not  only  checks  the  progress,  but  keeps 
the  oil  and  gas  spread  out  over  a  large  area  instead  of  allowing  it  to 
collect  in  a  pool.  If,  however,  the  impervious  stratum  is  inclined,  the 
movement  will  continue  upward  along  the  under  surface  of  the  imper- 
