438  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
unloading  has  afforded  opportunity  for  these  small  and  superficial 
folds.  Even  in  the  highest  cliffs  no  such  folds  were  observed  at 
greater  depths  than  about  20  feet.  It  is  highly  important  that  these 
superficial  folds  be  carefully  distinguished  from  the  profounder 
structural  features. 
Strata  containing  oil. — The  oil  is  contained  within  the  Pierre  (Cre- 
taceous) beds,  which  at  this  place  have  a  thickness  estimated  by 
Eldridge  to  be  about  4,450  feet.  These  beds  furnish  both  the  so- 
called  "  cap  rock  "  and  the  reservoirs  for  the  oil.  The  latter  consist  of 
certain  more  arenaceous  beds,  very  limited  in  lateral  extent.  From 
the  isolated  positions  of  these  arenaceous  beds  it  will  be  seen  that  the 
part  played  by  folding  in  determining  the  limits  and  distribution  of 
the  oil  bodies  is  by  no  means  the  same  in  this  case  as  in  the  case 
where  the  oil-bearing  sand  is  continuous.  There  is  good  reason,  how- 
ever, for  attaching  much  importance  to  the  folds,  and  they  demand 
close  attention. 
Oil  has  been  found  at  almost  all  depths  between  1,300  and  3,100 
feet.  The  deepest  well  in  the  field  is  3,G50  feet,  but  no  oil  is  known 
to  have  been  found  below  3,090  feet.  There  is  no  depth  at  which 
oil,  or  even  a  bed  capable  of  containing  oil,  is  certain  to  be  found. 
While  there  is  within  the  limits  named  no  considerable  thickness 
of  rock  which  does  not  sIioav  indications  of  oil  in  some  part  of  the 
field,  yet  oil  is  not  equally  abundant  at  all  horizons.  Several  at- 
tempts have  been  made  to  range  the  several  occurrences  or  showings 
of  oil  into  certain  limited  thicknesses  of  beds  separated  by  relatively 
barren  bands.  An  oil-bearing  horizon  as  thus  defined  is,  of  course, 
not  horizontal,  but  parallel  to  the  bedding.  Some  years  have  elapsed 
and  many  wells  have  been  drilled  since  the  last  revision  of  such  a 
classification  of  horizons.  Any  future  revision  should  not  only  take 
into  account  the  newer  wells,  but  should  be  based  on  carefully 
revised  data  of  dips  and  strikes,  showing  the  position  in  which  the 
deeper  beds  probably  lie. 
Communication  between  oil  bodies. — The  cases  in  which  two  wells 
appeal*  to  have  tapped  the  same  oil  body,  or  in  which  the  production 
of  one  well  has  affected  that  of  another,  are  rare  indeed.  One  oc- 
currence has  been  interpreted,  apparently  with  good  reason,  as  evi- 
dence of  an  open  fissure  at  the  bottom  of  a  deep  well.  The  cavity 
was  revealed  by  a  sudden  drop  of  the  drilling  tools.  Upon  attempt- 
ing to  fill  this  supposed  crevice,  at  least  two  Avagon  loads  of  gravel 
were  thrown  in  and  apparently  disappeared.  If  the  interpretation 
of  this  phenomenon  as  an  open  crevice  be  correct,  it  suggests  the' 
probable  existence  of  other  similar  openings.  These  should  be  of 
the  first  importance  in  the  vertical  distribution  of  the  oils.  The 
objection  that  sucli  cracks,  which,  on  account  of  the  synclinal  fold- 
