288         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,    1907,    PART   II. 
Rock  formations  of  the  northwestern  Colorado  coalfields — Continued. 
Geo- 
logic 
ase. 
Forma- 
tion. 
Description  of  strata. 
Topographic  feat  ures. 
Thickness. 
Economic  value. 
u5 
o 
w 
a 
w 
"m 
w 
a 
a 
o 
a  a. 
tJ0& 
w 
ra 
O 
o 
fl 
oj 
3 
A  thick  mass  of  dark 
drab  or  gray  shale, 
containing  lenticu- 
lar    members     of 
sandstone  or  sandy 
strata,  and  near  its 
base  very  constant- 
ly one  or  more  beds 
of  fine-grained  dark 
sandstone         and 
some       limestone. 
In  Utah  the  lower 
sandstone  is  lighter 
colored,  coarse,  and 
more     prominent. 
In  that  part  of  the 
field   coal    is    also 
found     associated 
with    these    lower 
sandstone      mem- 
bers.   At  the  base 
is  a  compact  dark 
slaty  shale. 
Forms    open   valleys 
and    broad    parks, 
many  of  which  are 
extensive.       These 
are  commonly  trav- 
ersed   by    some   of 
the  larger  streams, 
and  in  places  broad 
bottom    lands    arc 
developed.      The 
sides  of  the  valleys 
generally  show  ter- 
races or  "mesas "  at 
various    elevations 
above    the     valley 
bottoms.     All  these 
features     are     well 
represented     in 
Agency  Park,  Axial 
Basin,' Raven  Park, 
and  elsewhere. 
5,000  feet. 
Oil  in  the  Rangely 
district.    Agri- 
cultural    lands 
where  favorably 
situated.  Work- 
able   coal    near 
the  base  in  the 
Vernal  district. 
a; 
Ch 
o 
a 
m 
o 
A 
Commonly    consists 
of    two    or    more 
members   of   mas- 
sive sandstone  or 
quartzite,  in  many 
places    conglomer- 
atic, with  sandy  or 
clay  shale  interven- 
ing.   In  places  car- 
bonaceous shale  is 
found  in  this  for- 
mation.   It  is  lim- 
ited at  the  base  by 
varicolored    clays, 
clay  shales,  and  as- 
sociated strata  now 
supposed  to  belong 
to  the  Jurassic. 
Is  developed  as  an  in- 
dependent hogbaek 
where    tilted,    and 
differentiated  by  in- 
termediate      shale 
bodies  from  the  un- 
derlying     Jurassic 
sandstones.      Very 
commonly        com- 
bined with  the  lat- 
ter in  high,  sharp 
hogbacks  with  ele- 
vated    broad     dip 
slopes  usually  cov- 
ered with  brush  or 
timber. 
From  100   to  300 
feet  or  possibly 
more. 
Below  the  formations  described  in  the  foregoing  table  a  great  thick- 
ness of  older  rock  strata  occur  which  are  not  described  here,  as  they 
are  not  directly  connected  with  the  discussion  of  the  commercially 
important  coal  fields. 
AGE    OF    THE    COAL-BEARING    ROCKS. 
As  indicated  in  the  table,  workable  coal  is  found  in  a  number  of 
formations,  including  the  Mancos,  Mesaverde,  Laramie,  and  an  over- 
lying formation  supposed  to  represent  the  Fort  Union  or  earlier  rocks. 
Beds  of  coal  and  carbonaceous  or  lignitic  material  are  even  more 
widely  distributed  in  the  geologic  section.  Two  localities  were 
observed  where  coal  occurs  in  beds  older  than  the  Cretaceous  for- 
mations described  in  the  table.  Of  these  one  of  possible  commercial 
value  in  the  Henrys  Fork  field,  in  Utah,  is  described  on  page  311. 
A  thin  bed  of  coal  of  doubtful  quality  has  also  been  noted  in  Jurassic 
strata  on  upper  Red  Wash,  on  the  south  side  of  Blue  Mountain,  Routt 
County,  Colo. 
