322         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   1907,   PART   II. 
The  determination  of  the  geologic  age  of  the  formations  in  the 
Grand  Mesa  coal  field  is  based  on  paleontologic  and  stratigraphic 
evidence.  The  generalized  section  embodies  the  conclusions  of  the 
paleontologists  regarding  the  geologic  age  and  also  the  results  of  the 
field  studies  with  respect  to  the  character  and  relations  of  the  various 
formations. 
The  Gunnison  formation  consists  of  variegated  shale  and  sand- 
stone, and  is  exposed  in  the  Grand  Mesa  field  only  in  the  canyon  of 
Gunnison  River.  It  underlies  the  Dakota  sandstone,  a  more  resistant 
formation,  which  by  its  superior  hardness  maintains  the  steep  cliffs 
and  inclined  mesas  that  border  the  river.  The  oldest  coal  of  the 
Grand  Mesa  field  occurs  between  the  conglomeratic  sandstone  that 
is  here  referred  to  the  Dakota  and  the  overlying  Mancos  shale.  It 
is  locally  known  as  the  " Dakota  coal,"  on  the  supposition  that  it 
occurs  in  the  Dakota  sandstone,  but  there  are  reasons  which  will  be 
discussed  in  the  more  detailed  description  to  be  published  later  for 
referring  it  to  the  Mancos  rather  than  to  the  Dakota.  The  coal  out- 
crops between  Grand  Junction  and  Delta,  as  indicated  on  PI.  XXI, 
and  again  near  the  mouth  of  Gunnison  Canyon  east  of  Delta,  but 
so  far  as  is  now  known  it  is  not  of  commercial  value  and  is  not  here 
included  among  the  coals  of  economic  importance. 
The  Mancos  is  a  marine  formation  and  contains  only  the  so-called 
"Dakota  coal.".  The  lower  part  corresponds  to  the  Benton  forma- 
tion of  other  fields  and  the  top  to  the  lower  part  of  the  Pierre,  as 
shown  by  fossil  evidence. 
The  Mesaverde  formation  lies  conformably  upon  the  Mancos  shale 
and  is  the  only  important  coal-bearing  formation  in  this  field.  The 
lower  part  of  it  was  called  "Fox  Hills  "  and  the  upper  part  "Laramie  " 
by  the  geologists  of  the  Hayden  Survey,"  and  other  geologists  have 
followed  them  in  referring  the  coal  to  the  Laramie.  However,  a 
study  of  the  fossils,  in  connection  with  the  stratigraphy  of  western 
Colorado,  proves  that  it  is  much  older  than  the  Laramie,  and  the 
entire  series  of  coal-bearing  rocks  in  this  field  is  now  referred  to  the 
Mesaverde  formation,  originally  named  by  Holmes/'  in  southwestern 
Colorado.  Where  the  full  section  is  exposed  the  Mesaverde  is  sepa- 
rated from  the  Laramie  by  a  marine  formation  known  as  the  Lewis 
shale,  but  in  the  Grand  Mesa  field  there  is  no  representative  of  the 
Lewis  or  of  the  Laramie  beds.  The  sedimentary  rocks  of  Tertiary 
age  rest  unconformably  upon  the  Mesaverde. 
In  this  field  the  upper  part  of  the  Mesaverde  does  not  contain  coal 
in  beds  thick  enough  to  be  of  commercial  value  and  need  not  be 
described.     The  lower  part  contains  all  the  workable  coal  so  far  as 
a  Hayden,  F.  V.,  Atlas  of  Colorado,  1877. 
b  Holmes,  W.  H.,  Ninth  Ann.  Rept.  U.  S.  Geol.  and  Geog.  Survey  Terr.,  1875,  pi.  35 
