154        CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,   PART  II. 
MONTANA,  OR   LOWEST    COAL-BEARING   FORMATION. 
The  subjoined  generalized  section  of  the  Montana  formation  is  rep- 
resentative of  its  character  in  the  Glenrock  field. 
Generalized  section  of  Montana  formation. 
Feet. 
12.  Sandstone,  with  numerous  hard  layers  and  some  beds  of 
shale 200 
11.  Shale,  sandy,  locally  carbonaceous 125 
10.  Sandstone,  white,  marly  or  argillaceous 50 
9.  Sandstone,  brown,  lower  part  massive,  upper  part  more 
thinly  bedded,  medium  grained;  has  numerous  worm  or 
lamellibranch  borings,  rather  resistant  to  weathering 575 
8.  Shale,  dark,  uniform  in  color  and  texture 475 
7.  Sandstone,  white,  massive,  the  most  resistant  member  of  the 
Montana;  includes  one  or  two  beds  of  coal 120 
6.  Shale  and  sandstone  alternating 510- 
5.  Shale,  sandy 280 
4.  Coal,  one  or  two  beds,  total  thickness 4± 
3.  Sandstone,  white,  argillaceous,  with  two  layers  of  very  re- 
sistant dark-brown  sandstone  forming  hogback  with  char- 
acteristically irregular  crest 110 
2.  Shale,  sandy,  with  several  layers  of  sandstone 800 
1.  Shale,  sandy 1,  800 
5,  049± 
The  character  of  any  particular  member  is  fairly  constant,  though 
the  color,  composition,  and  thickness  of  the  beds  composing  it  change 
from  place  to  place.  Exposures  50  feet  apart  on  the  outcrops  of  a 
single  layer  2  or  3  feet  thick  are  commonly  so  different  in  appear- 
ance that  it  is  difficult  to  recognize  them  as  the  same  bed. 
The  lowest  member  of  the  Montana  may  be  recognized  by  its  posi- 
tion just  below  the  coal-bearing  rocks  and  above  older  noncoal- 
bearing  formations,  also  by  its  physical  character.  It  is  composed  of 
uniformly  dark  shale  1,800  feet  thick,  containing  numerous  more  or 
less  round  concretions.  The  outcrop  of  the  shale  forms  a  belt  along 
the  south  side  and  west  end  of  the  coal  field,  including  the  area  in 
which  the  town  of  Casper  is  located. 
The  next  member  (No.  2  in  the  section  given  above)  contains  sev- 
eral beds  of  sandstone.  Otherwise  it  resembles  the  member  just 
described. 
The  third  member  commonly  forms  a  hogback  with  a  very  ragged 
crest  of  dark-brown  sandstone.  The  white  parts  of  the  member, 
being  less  resistant,  are  not  so  prominent. 
Just  above  this  sandstone  is  a  bed  or  two  of  coal  (bed  B  on  the 
map,  PL  IX).  The  overlying  shale  (No.  5)  is  sandy,  and  is  much  like 
member  No.  l,but  its  outcrop  is  generally  covered  with  soil,  and  the 
details  of  its  composition  are  unknown.  There  are  local  exposures  of 
parts  of  the  sixth  member,  but  the  seventh  is  the  most  resistant  and 
