o 
20  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY.    L905. 
cliffs  bordering  the  valley,  and  in  the  Long  Pine  Hills  in  the  vicinity  of  Camp  Crook,  S.  I) 
At  the  Mill  Iron  ranch,  on  Big  Box  Elder  Creek,  about   10  miles  above  the   boundary  li  j 
the  sandstone  is  well  shown  in  the  following  section: 
Si,-t ion  at  Mill  Iron  ranch,  on  Big  Box  Elder  Creek,  Montana. 
Ft.  in. 
s.  Sandstone  in  massive  ledges  alternating  with  soft  it,  thin-bedded  Layers; 
shows  cross  bedding  in  places L60 
7.  Ci ray  clay 6 
6.   Lignite 1      2 
5.  ("lay 10 
4.  Lignite 10 
3.  Clay 5 
2.   Lignite 1      G 
1.  Unexposi  Itocreek 45 
220      4 
This  thick  sandstone  was  found  to  extend  southward  for  30  miles  to  the  Long  P 
Hills.       In   these  hills  the  arenaceous  strata    have  a   thickness  of  from  250  to  275  feet,  1 
rock  being  gray  and  yellow  and  weathering  unevenly  into  a  variety  of  fantastic  fori 
Many  of  the  higher  buttes  of  North  and  South  Dakota  are  capped  with  sandstone,  h< 
belonging  to  the  Laramie  and  having  a  thickness  of  .">(>  to  75  feel  and  over. 
The  foregoing  sections  may  he  taken  as  representative  of  the  Laramie  formation.  Tl 
show  that  it  is  composed  of  alternating  beds  of  clay  and  sand,  together  with  seams 
lignite.  In  places  the  sand  layers  are  cemented  into  a  firm,  hard  sandstone,  but  conunoi 
they  form  incoherent  beds,  readily  crumbling  in  the  hand.  These  arenaceous  st  in 
however,  resist  the  action  of  weathering  agencies  better  than  the  clays,  and  their  oufen 
ping  vdiac*  form  vertical  ledges,  often  projecting  beyond  the  clays.  The  presence  of  t 
sandy  beds  can  thus  commonly  be  detected  at   a  distance  by  the  character  of  the  slope; 
Probabl}  two-thirds  of  the  thickness  of  the  Laramie  is  made  up  of  clays,  though  t 
varies  in  different  places.  These  <la\  -halo  range  in  composition  from  very  pure  el 
through  beds  containing  an  increasing  percentage  of  sand  to  those  with  only  a  sm 
proportion  of  day.  The  argillaceous  strata  thus  graduate  into  the  sandy  layers  throu 
every  intermediate  mixture  of  clay  and  sand,  and  we  have  sandy  clays  and  clayey  sar 
-tone-.  Many  of  the  so-called  clays  are,  strictly  speaking,  impure  sandstones.  In  soi 
cases  i  he  change  from  clay  shale  to  sand  is  abrupt  ;  in  others  it  is  gradual,  t  he  clay  becomf 
nioie  sandj   near  the  line  of  contact . 
One  of  the  most  marked  features  of  the  Laramie  is  this  rapid  alternation  of  sand  ai 
clay  s|  rata  and  t  he  l're<pient  occurrence  of  beds  of  lignite.  The  colors  of  t  he  (day  arc  win 
gray,  yellow,  brown,  and  red,  the  grays  and  yellows  predominating.  The  brown  layt 
are  rich  in  plant  remains,  and  it  is  to  the  presence  of  this  carbonaceous  matter  that  tl 
owe  their  color.  These  varied  colors  give  the  formation  its  banded  appearance  and  at 
greatly  to  the  beauty  and  variety  of  the  landscape. 
Crystallized  gypsum  or  selenite  is  common  in  the  clay.  It  is  particularly  liable  to  occ 
in  the  brown  layers  immediately  beneath  the  lignite  seams.  The  crystals  are  often  ve 
much  elongated,  some  being  found  8  and  10  inches  in  length,  and  are  seldom  perfect,  t 
faces  being  more  or  less  etched. 
Of  still  more  frequent  occurrence  are  ferruginous  nodules  and  concretions,  compos, 
either  of  iron  pyrites  or  of  liinonite.  They  usually  contain  more  or  less  siliceous  or  arg 
laceous  material,  and  after  exposure  to  the  weather  stain  the  clay  about  them  yello 
Many  of  the  clay  slopes  are  thickly  dotted  with  these  nodules  and  concretions  of  all  shap 
and  sizes  up  to  several  feet  in  diameter.  Thin,  impure  liinonite  or  clay  ironstone  bam 
yellow  in  color,  are  not  uncommon  in  the  clays. 
Another  feature  of  the  Laramie  clays  and  sands  which  can  not  fail  to  attract  the  noti 
of  even  the  casual  observer  is  the  greal  quantity  of  burnt  and  fused  clay  which  is  seldd 
