152         CONTRIBUTIONS    TO    ECONOMIC   GEOLOGY,    1907,   PART   II. 
The  field  lies  at  the  foot  of  the  front  range  of  the  Rocky  Moun- 
tains, at  the  point  where  it  changes  in  trend  from  the  long  north- 
south  course  across  Colorado  and  the  southern  part  of  Wyoming  to 
an  east-west  course  of  50  miles  parallel  with  and  on  the  south  side  of 
North  Platte  River.  At  the  west  end  of  this  50-mile  stretch  the 
range  as  such  disappears,  but  farther  north  it  is  represented  by  the 
Bighorn  Mountains,  which  are  similarly  situated  with  reference  to 
the  coal  fields  on  the  east.  About  three-fourths  of  the  Glenrock  field 
lies  on  the  north  side  of  North  Platte  River. 
The  work  of  which  this  paper  is  a  report  was  of  a  reconnaissance 
character,  ai.  1  on  the  west  and  south  it  was  carried  to  the  limits  of 
the  coal-bearing  rocks.  The  eastern  and  northern  boundaries  of 
the  field  as  mapped  are  arbitrary,  depending  principally  on  the  amount 
of  time  at  the  disposal  of  the  party.  East  of  the  field  the  coal- 
bearing  formations  are  partly  covered  with  barren  Tertiary  rocks, 
but  toward  the  north  they  are  found  at  the  surface  almost  without 
interruption  for  hundreds  of  miles. 
The  writer  was  assisted  by  Charles  T.  Lupton,  E.  H.  Sirich,  R.  A. 
Branson,  and  A.  S.  Ogle.     As  the  work  was  primarily  one  of  land 
classification,  it  was  necessary  to  retrace  the  lines  of  the  old  land 
survey  and  locate  the  outcrops  of  the  coal  beds  with  reference  to 
the  corner  stones  established  by  that  survey.     The   other  features 
of  the  country  were  located  also  bj  their  distance  and  direction  from 
the  land  corners.     The  exact  astronomic  location  of  the  land  lines  is 
not  known. 
TOPOGRAPHY. 
The  surface  of  the  country  is  hilly,  but  not  mountainous.  It  is 
generally  possible  to  drive  wherever  one  desires,  whether  there  is  a 
road  or  not.  Near  the  mountains  the  country  is  essentially  a  plain 
that  has  been  deeply  dissected  by  the  streams,  leaving  mesas  sloping 
gently  toward  the  north  or  away  from  the  mountains.  Farther  away 
from  the  mountains  the  country  is  gently  rolling,  with  a  few  flat- 
topped  hills,  but  there  are  a  few  small  areas  of  rough  country  where 
the  mantle  of  Tertiary  and  Quaternary  material  is  removed,  revealing 
the  presence  of  the  lower  and  more  indurated  rocks.  A  rather  unusual 
feature  is  that  much  of  the  field  is  covered  with  sand  dunes. 
North  Platte  River  flows  eastward  across  the  field,  and  there  are 
five  bridges  across  it.  It  is  seldom  that  the  stream  can  be  forded 
except  late  in  summer,  but  at  that  time  it  may  be  crossed  at  a  great 
many  places.  There  are  very  few  other  perennial  streams,  and  most 
of  these  are  on  the  south  side  of  the  river  and  owe  their  existence  to 
the  mountains.  North  of  the  river  water  is  scarce  and  the  quality 
is  very  poor.  There  are  some  "alkali  seeps"  where  water  may  be 
had  at  any  time  of  the  year  by  digging  a  shallow  well,  but  to  get  good 
water  it  would  generally  be  necessary  to  drill  a  thousand  feet  or  more. 
Casper,  Douglas,  and  all  the  smaller  towns  and  post-offices  are  situa- 
