bbnneman.]  FLORENCE,   COLO.,   OIL    FIELD.  437 
tram  angle  of  the  mountain  border  caused  by  the  echelon  relations 
of  the  Front  and  Wet  ranges,  the  triangular  boundary  being  com- 
pleted by  a  fold  in  the  Cretaceous  and  ol  ier  strata  on  the  east. 
The  probable  depth  of  this  basin  as  defined  by  the  Dakota  sand- 
stone is  fully  8,000  feel  when  measured  from  the  tops  of  its  steeply 
dipping  crags  in  the  foothills  to  the  same  stratum  in  the  bottom  of 
the  basin.  As  a  generalized  statement  of  the  depth  of  the  basin 
below  it-  eastern  rim.  it  may  be  said  that  it  is  about  equal  to  the 
thickness  of  all  the  formations  represented  in  the  Florence  field 
above  the  Niobrara.  These  higher  formation-  aggregate  in  thick- 
aess  about  5,000  feet  and  are  eroded  from  the  eastern  rim  of  the 
basin. 
The  bottom  of  the  basin  is  not  reached  by  a  uniform  -lope.  On 
the  east  side  the  greater  part  of  the  descent  is  accomplished  by 
steep  dips — frequently  20°.  Approximately  along  the  meridian  of 
the  eastern  end  of  the  city  the  dips  fall  to  about  5°,  and  from  this 
line  westward  for  a  few  miles  the  average  dip  is  less  than  that 
amount.  It  is  in  these  strata  of  small  westward  dip  that  the  oil  is 
found.  The  deepest  part  of  the  basin  i>  a  few  miles  wot  of  the 
developed  oil  held.  The  latter,  therefore,  lies  upon  a  structural 
-lope  whose  local  dip  i-  relatively  small. 
Subordinati  folds  in  the  oil  field. — As  might  be  expected  from  the 
position  of  the  oil  held  within  the  basin,  the  dips  in  it>  northern  half 
are  south  of  west  and  those  in  its  southern  half  north  of  west,  leaving 
a  transverse  trough  near  it-  center,  pitching  gently  to  the  we>t.  A 
•  ■la—  of  smaller  and  very  gentle  folds  i-  revealed  mainly  by  local 
variation-  of  strike.  It  seems  probable  that  detailed  work  might 
'  reduce  these  minor  fold-  to  -y-tems.  One  such  fold  appears  to  be 
the  cause  of  a  complete  arrest  of  the  general  westward  dip,  giving 
ri>e  to  a  narrow  belt  of  horizontal  beds  trending  north  by  west  ami 
lying  a  little  east  of  the  village-  of  Coal  Creek  and  Williamsburg. 
A  detailed  study  of  these  smaller  fold-  might  show  them  to  be  inti- 
mately related  to  the  distribution  of  the  oil-. 
A  class  of  -till  -mailer  and  probably  superficial  fold-  also  exists. 
These  occur   frequently  in  the  freshly  exposed  shales  of  newly  cut 
stream  gorge-.      In  breadth  they  rarely  exceed  10  or  20  feet,  though 
their   folding  is  frequently   sharp.      Not   uncommonly   they   have   a 
trend   somewhat  north  of  east,  but  trend-  in  all  direction-  are  like- 
wise observed  and  it   i-  not  now  possible  to  generalize  on  this  point. 
:  Some  small  and  eminently  superficial  anticlinal  folds  of  this  char- 
'  aeter  appear  along  the  axes  of  newly  made  stream  gorges.     Their 
'appearance  i-  so  fresh  a-  strongly  to  suggest  that  they  are  younger 
than  the  gorges  themselves.     Tt  is  impossible  to  resist  the  suggestion 
that  the  rocks  have  been  under  compressive  strain  and  that  local 
