386  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
western  and  smaller  of  the  two  flexures,  but  is  affected  both  in  degree 
of  flexure  and  in  the  direction  of  its  axis  by  the  stronger  fold  on  the 
cast. 
It  may  therefore  be  confidently  affirmed  that  whatever  may  be  the 
nature  of  the  occurrence  of  oils  elsewhere  in  the  Fort  Pierre,  there  is 
nothing  thus  far  known  in  the  field  at  Boulder  to  offer  encouragement 
to  prospecting,  except  in  intimate  relation  with  folds.  It  should  then 
be  determined  with  care  what  are  the  essential  features  of  this  par- 
ticular flexure.  Such  a  study  may  afford  a  basis  for  inference  as  to 
the  probable  oil-bearing  or  barren  character  of  other  similar  structures 
occurring  for  a  distance  of  50  miles  along  the  front  of  the  range. 
The  form  and  location  of  the  folds  with  which  we  are  here  concerned 
can  best  be  defined  after  a  description  of  the  sandstone  stratum  which 
made  their  discovery  possible. 
There  is  in  the  great  mass  of  Fort  Pierre  shales,  about  one-third 
way  up  from  their  base  or  a  little  higher,  a  strong  and  persistent 
sandstone  whose  outcrop  can  be  traced  with  little  interruption  from 
Boulder  for  many  miles  to  the  north.  Beyond  Fourmile  Gulch  it 
generally  forms  a  strong  ridge,  except  where  it  passes  under  the  broad 
mesa  known  as  Table  Mountain.  Just  south  of  Table  Mountain  it  has 
a  thickness  of  250  feet  An  isolated  outcrop  of  similar  sandstone  at 
Bear  Canyon,  3  miles  south  of  Boulder,  probably  represents  the  same 
horizon.     Its. thickness  here  is  150  feet. 
To  the  public  this  sandstone  has  become  well  known  on  the  Culver 
ranch,  6  miles  southwest  of  Berthoud,  where  it  crosses  Little  Thomp- 
son Creek  and  where  a  seepage  of  oil  has  long  been  known. 
The  sandstone  is  in  many  places  thick  bedded  and  frequently  cross 
bedded.  Much  of  it  has  a  dark-greenish  color  and  a  gritty  texture. 
The  remainder  is  of  a  lively  gray  color.  The  whole  is  calcareous 
where  fresh.  Much  of  its  surface  is  weathered,  having  lost  its  lime, 
taken  on  a  pale  greenish  tint,  and  become  friable.  It  is  fossiliferous 
and  frequently  contains  carbonaceous  matter,  suggesting  small  sticks 
of  wood  turned  to  coal. 
Near  Boulder  the  stratum  is  thin  and  weak  and  is  separated  from 
the  Niobrara  limestone  and  shales  on  the  west  by  not  more  than  1,000 
feet  of  shales.  Seven  miles  north  of  Boulder  it  is  250  feet  thick  and 
the  Pierre  shales  below  it  have  thickened  to  nearly  3,000  feet.  As  the 
dip  has  also  greatly  decreased  within  the  same  space,  the  sandstone 
outcrop  bears  strongly  to  the  east. 
Stated  more  definitely,  the  outcrop  forms  a  prominent  cuesta,  a 
ridge  of  long  and  gentle  dip  slope  and  a  steep  scarp,  running  from  the 
southwest  corner  to  the  northeast  corner  of  sec.  6,  T.  1  N.,  R.  70  W. 
This  northeastward  strike,  an  exception  to  the  prevailing  trend  which 
is  almost  north,  is  the  surface  expression  of  the  mild  echelon  folding 
