308         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,    PART   II. 
Beyond  that  point  no  coal  was  observed  in  the  Mesaverde  rocks,  and 
farther  west  the  formation  may  be  either  cut  out  entirely  by  the 
unconformity  between  the  Cretaceous  and  Tertiary  rocks  or  con- 
cealed by  some  of  the  Tertiary  and  later  deposits. 
MANCOS    COAL. 
Age. — The  older  coal  of  the  Vernal  field  is  associated  with  fossil- 
iferous  strata  both  above  and  below  the  workable  bed,  and  collections 
from  these  horizons  are  reported  to  be  distinctive  of  the  Benton  shale 
of  other  fields.  Where  a  group  of  sandstones  and  the  contained  coal 
beds  form  a  conspicuous  lithologic  unit,  as  they  do  in  most  of  this 
Utah  field,  their  outcrop  is  indicated  on  the  map  as  far  as  it  has  been 
traced,  and  it  is  referred  to  in  the  legend  of  the  map  and  elsewhere 
in  the  report  as  the  Mancos  coal  group. 
Position  of  the  outcrop. — The  coal  group  in  the  lower  part  of  the 
Mancos  shale  presents  a  sinuous  outcrop  in  the  vicinity  of  the  Green 
River  valley  near  Vernal.  These  beds. follow  the  sharp  folds  of  the 
older  strata  of  the  Uinta  Mountain  axes  as  continuous  curved  ridges, 
tracing  around  the  ends  of  the  pitching  anticlines  and  into  the  deep 
reentrant  valleys  of  the  synclines.  Only  a  small  portion,  considerably 
less  than  one-half  of  the  length  of  outcrop  exposed,  is  known  to  con- 
tain valuable  coal.  Thus  the  sandstone  beds  of  this  horizon  are 
exposed  in  the  banks  of  Green  River  at  Jensen,  but  are  not  known  to 
contain  coal.  They  are  also  exposed  west  of  Green  River  around 
the  foot  of  Split  Mountain.  On  Brush  Creek  the  coal  is  of  worka- 
ble thickness  at  a  few  places.  Very  little  is  known  concerning  the 
coal  around  the  northeast  end  of  the  Island  Park  syncline,  but  it  is 
said  to  be  of  small  practical  value.  Along  the  cropping  of  this  bed 
north  and  northeast  of  Vernal  it  is  or  has  been  mined  at  a  ^ew  places 
along  the  valley  of  Brush  Creek,  but  the  principal  developments  are 
grouped  in  a  locality  7  to  8  miles  northwest  of  Vernal,  on  the  southeast 
side  of  Little  Mountain. 
Character  of  the  bed  mined. — The  coal  bed  presents  the  same  or  a 
similar  character  in  the  number  and  thickness  of  its  partings  and  is 
doubtless  at  the  same  horizon  throughout  the  field.  One  or  two 
other  small  beds  of  coal  are  reported  at  various  places,  but  only  the 
large  bed  has  ever  been  shown  to  be  of  value.  Most  of  the  accessible 
mines  and  prospects  were  visited  and  measurements  taken  of  the  coal, 
as  well  as  a  set  of  representative  samples  for  chemical  analysis.  The 
results  of  the  analyses  are  included  in  the  table  on  pages  314-315. 
The  thickness  of  the  bed  and  the  nature  of  its  partings  are  well  shown 
in  tbfe  following  table  of  measurements,  which  are  arranged  in  the 
order  of  their  position  along  the  outcrop  from  northwest  to  southeast. 
