(SO  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,   1907,   PART   II. 
GEOLOGY. 
SEDIMENTARY   ROCKS. 
In  Ins  description  of  this  area  in  the  Little  Belt  Mountains,  Weed 
divides  the  Cretaceous  system  into  four  formations — the  Cascade, 
Yellowstone,  Laramie,  and  Livingston.  In  recent  years  the  Cre- 
taceous has  been  further  subdivided,  and  in  place  of  Weed's  four 
formations  the  writer  mapped  seven.  This  division  into  smaller 
units  is  based  on  lithologic  and  paleontologic  evidence  obtained  by 
various  geologists  and  paleontologists  in  the  last  few  years.  The 
distribution  of  the  Laramie  as  mapped  by  Weed  suggests  that  the 
structure  is  somewhat  irregular,  but  the  results  of  the  work  here 
described  show  that  the  structure  is  still  more  complex  than  was 
supposed.  The  geology  of  the  area  is  further  complicated  by  the 
intrusion  of  igneous  rocks  in  the  form  of  dikes,  sills,  and  laccoliths* 
These  are  most  numerous  in  the  vicinity  of  the  Crazy  Mountains. 
The  Cretaceous  and  Tertiary  rocks  of  this  area  are  divided  into 
nine  formations,  as  follows: 
Tertiary 
Cretaceous. 
(Fort  Union  formation. 
1  Livingston  formation. 
Laramie  formation. 
Montana  group: 
Bearpaw  shale. 
Judith  River  formation. 
Claggett  formation. 
Eagle  sandstone. 
Colorado  shale. 
Kootenai  formation. 
The  lowest  rocks  of  Cretaceous  age  belong  to  the  Kootenai  forma- 
tion, which  has  a  thickness  of  235  feet  and  is  composed  of  green  and 
red  shales  in  the  lower  half  and  brown,  gray,  and  pink  sandstones  in 
the  upper  half.  The  base  of  the  massive  sandstones  in  the  middle 
of  the  formation  is  coarse  grained  and  contains  thin  layers  of  quartz 
and  chert  pebbles.  In  some  places  also  the  debris  from  this  bed  is 
conspicuous  for  its  numerous  slickensided  faces  and  iron  staining. 
Below  this  conglomerate  is  a  black  shale,  slightly  bituminous,  which 
may  be  a  foot  or  more  thick.  It  is  at  this  horizon  that  the  workable 
coal  occurs  in  the  Lewistown  and  Great  Falls  fields. 
The  Colorado  shale,  lying  next  above  the  Kootenai,  has  according 
to  measurements  by  the  writer  a  thickness  of  approximately  1,300 
feet.  It  is  composed  largely  of  dark  shale.  The  prevailing  color  is 
green,  although  the  lower  part  in  places  is  nearly  black.  A  massive 
gray  sandstone  may  be  present  in  the  lower  third  of  the  formation; 
its  thickness  varies  greatly,  ranging  up  to  nearly  200  feet.  The 
upper  part  of  the  formation  shows  transitional  stages  of  deposition 
from  deep  to  shallow  water  by  the  occurrence  of  numerous  beds  of 
sandstone  in  the  shale. 
