390  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull.  225. 
This  hypothesis  has  been  put  to  test  in  one  instance.  A  fine  fold 
of  this  kind  north  of  Lyons  flattens  out  in  the  vicinity  of  Hygiene. 
Here  in  a  line  transverse  to  the  axis  three  wells  were  drilled  to 
depths  approximating  3,000feet.  They  were  not  without  showings  of 
oil  but  a  better  showing  was  obtained  4  miles  farther  down  its  axis  to 
the  southeast,  where  the  fold  was  probably  never  considered  a  factor. 
Oil  from  a  depth  of  about  600  feet  has  also  accumulated  in  an  aban- 
doned water  well  a  mile  north  of  Hygiene.  This  is  also  on  or  near 
the  axis  of  the  fold. 
If  the  larger  folds  fail  to  yield  oil,  the  one  remaining  hope,  probably 
more  promising  from  the  start,  lies  in  the  smaller  folds.  For  reasons 
given  above,  when  these  folds  do  not  affect  the  sandstone  bed  they  are 
betrayed  only  by  the  most  obscure  outcrops  in  the  Fort  Pierre  shales. 
Yet  it  is  in  this  same  formation,  at  a  distance  of  a  few  miles  from  the 
harder  rocks  of  the  foothills,  that  they  must  be  found. 
Herein  lies  the  large  significance  of  the  similar,  often  very  small 
flexures,  which  affect  the  stronger  and  well-exposed  strata  of  the  foot- 
hills, from  Jurassic  to  Niobrara,  inclusive.  In  these  stronger  rocks 
the  folds  may  be  very  small,  the  looping  back  generalty  being  less  than 
a  quarter  of  a  mile;  but  the  same  stress  will  be  found  to  have  produced 
larger  results  in  the  weaker  Fort  Pierre. 
If  the  Pierre  shales  are  to  be  prospected  in  this  region  of  echelon 
folding,  much  time  will  be  saved  by  making  a  preliminary  examination 
of  the  stronger  strata.  Where  the  sandstone  bed  is  exposed,  it  is  the 
natural  index  of  structure  and  the  best.  Where  it  does  not  outcrop, 
the  basal  Niobrara  limestone  is  the  nearest  criterion.  In  the  area  to 
be  reported  on  beautiful  echelon  folding,  correlated  with  the  struc- 
ture of  the  oil  field,  is  found  as  low  as  the  Morrison  (Jurassic). 
In  these  suggestions  no  account  is  taken  of  the  possibility  of  folds 
running  strictly  parallel  to  the  foothill  outcrops.  This  is,  of  course, 
possible,  especially  south  of  Boulder,  where  the  echelon  type  is  not 
prevalent.  The  suggestions  for  procedure  here  laid  down  are  those 
only  which  have  been  derived  from  the  known  structure  of  the  already 
developed  field.  They  are  not  intended  to  exclude  all  consideration  of 
structures  not  yet  exemplified. 
The  Maxwell  gas  well  may  seem  to  be  an  exception  to  all  that  has 
been  said  above  regarding  the  location  of  oil  (and,  by  inference,  of  the 
closely  associated  gas);  and  it  might  therefore  be  thought  of  as  putting 
the  generalization  to  a  severe  test.  It  is  less  than  2  miles  north  of 
Boulder  and  little  more  than  half  a  mile  from  the  Pierre-Niobrara 
contact.  Nearby  outcrops  are  too  few  and  unsatisfactory  to  fix  the 
structure  with  any  definiteness,  but  it  is  a  remarkable  fact  that  this 
well  lies  south-southeast  from  an  incipient  echelon  fold  which  affects 
the  boundaries  of  all  outcrops  from  the  upper  Wyoming  to  the  Fort 
