40  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1907,   PAKT   TT. 
parts  of  Montana,  Wyoming,  North  Dakota,  and  South  Dakota.  It  is 
named  after  Fort  Union,  an  old  military  post  at  the  mouth  of  Yellow- 
stone River.  The  total  thickness  of  the  formation  can  not  be  deter- 
mined in  this  field,  as  neither  top  nor  base  is  represented.  The 
rocks  consist  of  comparatively  fine  material,  mainly  clay  shale  and 
sandstone,  including  lignite  and  thin  beds  of  impure  limestone. 
They  are  all  fresh-water  deposits  and  contain  an  extensive  fossil 
flora  of  about  400  species  of  plants  which  resemble  those  of  modern 
times,  and  a  fauna  including  fishes,  fresh-water  mollusks,  and  rep- 
tiles. Evidence  now  in  hand  indicates  that  the  waters  in  which 
these  sediments  were  deposited  extended  from  east-central  Wyoming 
to  northern  Montana,  and  from  Livingston,  Mont.,  eastward  to  the 
center  of  North  Dakota.  Within  these  limits  areas  not  occupied  by 
Fort  Union  rocks  are  those  in  which,  owing  to  deformations  of  the 
earth's  crust,  they  have  been  removed  by  erosion. 
The  total  thickness  of  Fort  Union  rocks  exposed  in  the  Miles  City 
field  is  about  900  feet,  which  may  be  increased  by  data  obtained 
from  drill  holes  at  Miles  City  to  a  total  of  1,400  feet.  From  a  com- 
parison of  the  section  at  Miles  City  with  that  exposed  about  70 
miles  to  the  northeast  near  Glendive  it  is  inferred  that  the  base  of 
the  Fort  Union  formation  is  not  very  deeply  buried,  and  it  is  prob- 
able that  some  of  the  drill  holes  at  Miles  City  have  penetrated  to 
underlying  Cretaceous  rocks. 
The  Fort  Union  rocks  exposed  are  readily  separated  into  two 
members  by  a  marked  difference  in  lithologic  character.  The 
lower  member,  about  500  feet  thick,  consists  mainly  of  alternating 
beds  of  clay  shale  and  sandstone,  having  a  general  dark-gray  or  som- 
ber hue.  In  detail  the  individual  layers  of  this  part  of  the  formation, 
especially  the  sandstones,  show  numerous  irregularities  of  deposition. 
Thin  layers  of  ferruginous  limestone  occur  at  short  intervals  through- 
out the  section,  and  concretion-like  masses  of  the  same  material, 
more  or  less  lenticular  in  form,  are  included  in  the  shales  and  sand- 
stones. The  thicker  sandstone  beds  are  usually  characterized  by 
more  or  less  irregular  cross-bedding.  Coal  is  found  at  numerous 
horizons,  but  the  beds  are  extremely  variable  in  thickness  and 
horizontal  extent.  The  many  alternations  of  soft  rocks  with  thin 
hard  layers  give  rise  during  erosion  to  marked  badland  topography. 
Altogether  sections  of  the  rocks  vary  greatly  in  their  detail  from 
place  to  place,  certain  general  features,  such  as  thicker  coal  beds 
at  or  near  certain  horizons  and  prevalent  sandstones  or  abundant 
concretions  at  others,  can  be  recognized  in  a  general  way  throughout 
the  field.  The  following  section,  measured  in  detail  near  Miles  City, 
is  inserted  to  illustrate  the  rapid  alternation  of  various  materials 
comprising  this  part  of  the  Fort  Union  formation: 
