fenneman.]  BOULDER   OIL    FIELD,   COLORADO.  389 
deposit  into  so-called  " pools."  The  intermittence  of  good  oil  territory 
along  the  line  thus  far  developed  from  sees.  8  and  9  on  the  north  to 
the  Buffalo  gas  well  on  the  south  is  already  known.  It  can  not,  there- 
fore, be  assumed  that  one  or  two  dry  holes  indicate  that  a  line  marked 
by  the  above-described  structure  is  entirely  barren. 
Possible  finding  of  other  similar  deposits. — If,  then,  the  promise  of 
future  developments  in  this  oil  field  is  limited  to  this  relationship  with 
interrupted  dips,  it  is  highly  important  that  similar  occurrences  beyond 
the  limits  already  developed  be  examined.  At  the  outset  the  further 
extension  of  this  immediate  group  of  wells  demands  attention.  It 
must  always  be  taken  into  account  in  this  field  that  the  beds  capable  of 
containing  deposits  are  of  small  and  always  uncertain  lateral  extent. 
This  feature  was  fully  stated  in  the  former  report/'  and  that  statement 
needs  no  modification. 
The  axis  of  the  fainter  of  the  two  folds  whose  combined  effects  pro- 
duce the  monocline  in  and  south  of  sec.  8  runs  north-northwest  from 
the  present  developed  field.  The  very  few  exposures  along  this  north- 
ern portion  have  not  shown  dips  which  indicate  a  structure  similar  to 
that  farther  south  in  the  vicinity  of  the  successful  wells. 
The  stronger  fold  to  the  east  completely  neutralizes  the  eastward 
dip  as  far  south  as  sec.  4,  while  farther  north  it  has  a  westward-dipping 
limb.  The  axis  of  this  fold  runs  north  by  a  little  west  through  sees. 
4,  33,  28,  etc.,  and  has  not  been  tested  by  the  drill. 
Hayden\s  large-scale  map  shows  a  series  of  large  echelon  folds 
with  considerable  detail,  but  those  described  above  are  in  general 
too  small  to  appear  on  that  map,  though  the  salient  curve  in  the 
Dakota  north  of  Lefthand  Creek  must  be  correlated  with  the  long 
J  loop  in  the  sandstone  bed  above  described.  If  that  formation  were 
drawn  on  the  Hayden  map,  the  loop  in  question  would  be  three- 
fourths  of  an  inch  long;  but  a  fair  idea  of  relative  size  may  be  had  by 
comparing  the  effects  of  each  on  the  same  formation,  as,  for  example, 
the  Dakota. 
The  question  of  stores  of  oil  in  these  larger  folds  naturally  suggests 
itself  in  view  of  the  known  relation  of  oil  to  the  smaller  folds.  It  is 
to  be  remembered,  however,  that  in  the  Appalachian  region  it  is  not 
the  great  anticlines  which  have  conserved  the  hydrocarbons.  Here, 
as  there,  it  may  prove  that  the  greater  deformations  have  made  poor 
reservoirs,  probably  by  the  opening  of  cracks.  It  has  not  yet  been 
shown  that  the  larger  of  the  small  folds  in  the  Boulder  field  is  an  oil 
reservoir.  If  the  future  proves  it  otherwise  there  will  be  a  strong 
presumption  in  favor  of  restricting  the  search  to  folds  of  the  smaller 
order.  If,  then,  the  large  folds  shown  on  the  Hayden  map  are  to  prove 
oil  bearing  it  will  probably  be  in  their  remote  ends,  where  the  fold  is 
dying  out  on  the  plains. 
a  Bull.  U.  S.  Geol.  Survey  No.  213. 
