112  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
and  an  appreciable  thickness,  but  too  minute  to  give  interference 
figures.  In  this  respect  it  probably  varies  in  character  from  the  Mont- 
rose County  carnotite,  which  is  reported  to-be  composed  of  " exceed- 
ingly fine,  dust-like  particles  without  crystal  outlines  and  acting  so 
faintly  on  polarized  light  as  to  at  first  seem  almost  amorphous/ ; 
although  a  few  exceedingly  minute  crystals  are  also  mentioned  in  the 
description  of  that  material. 
The  Montrose  County  carnotite  is  reported  to  be  found  in  the  La 
Plata  sandstone,  which  is  of  Jurassic  age,  and  the  mineral  therefore 
occurs  in  an  older  geologic  formation  than  the  carnotite  of  Rio  Blanco 
County,  which  is  found  in  the  Dakota  sandstone.  Since,  however,  in 
both  localities  the  carnotite  was  evidently  deposited  in  these  rocks 
long  after  the  time  of  their  formation  as  such,  this  difference  has  no 
especial  significance  relating  to  a  differing  mode  or  time  of  formation 
of  the  mineral  itself. 
GEOLOGY  OF  THE  COAL  CREEK  DISTRICT. 
The  summit  of  Sleepy  Cat  Mountain  is  a  peak  of  about  10,800  feet 
elevation,  capped  by  basaltic  lava  representing  a  succession  of  surface 
flows  that  probably  took  place  in  late  Tertiary  time.     The  bowlders 
of  this  basalt  strew  the  spurs  and  ridges  surrounding  the  peak  and 
obscure  much  of  the  structure  of  the  underlying  sediments.     Coal 
Creek  rises  on  the  west  flank  of  Sleepy  Cat  Mountain  and  flows  nearly 
due  west  for  6  miles  in  a  rather  deep,  narrow  valley.     About  midway 
in  this  portion  of  its  course  the  creek  has  cat  its  valley  into  the  crest  t 
of  an  anticlinal  fold  or  dome  in  the  rock  strata,  exposing  formations 
that   are   doubtless   of  Triassic   age.     (See   geologic  map,   PL    III.) 
Along  the  upper  half  of  this  stretch  of  Coal  Creek  the  rocks  dip  to  the  • 
east  or  northeast ,  toward  Sleepy  Cat  Mountain.     Down  the  creek  bed  i 
west  of  the  fold  these  same  rock  beds  may  be  recognized  dipping  west- 
ward, in  the  direction  the  creek  is  (lowing,  and  as  they  pass  beneath 
water  level  they  are  covered  by  successively  younger  formations.     At 
the  lower  end  of  its  narrow  valley  the  creek  passes  through  a  gateway 
in  large  ledges  of  white  quartzite  and  sandstone,  west  of  which  it 
emerges  into  a  long,  straight,  northwest-southeast  valley.     The  white 
sandstone  ledges  represent,  in  part  at  least,  the  formation  so  widely   | 
known  as  the  Dakota  sandstone.     The  depression  immediately  east 
of  this  sandstone  and  quartzite  ridge  is  formed  upon  shaly  strata 
showing  in  outcrops  of  various  colors,  in  the  main  shades  of  pink  and 
red.     It  is  suggested  that  these  underlying  strata  may  be  of  the  same 
age  as  the  Gunnison  formation  of  the  Anthracite-Crested  Butte  areaj 
The  fact  that,  so  far  as  the  writer  knows,  no  identifiable  fossils  have 
as  yet  been  collected  from  the  formations  including  and  below  th( 
Dakota  in  this  locality  or  near  vicinity  causes  some  doubt  as  to  tin 
aEldridge,  G.  H.,  Geologic  Atlas  U.  S.,  folio  9,  U.  S.  Geol.  Survey,  1894. 
