290          CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,    1907,    PART   II. 
coals  are  largely  of  marine  origin,  as  shown  by  the  fossils  they  contain. 
This  lower  marine  portion  of  the  Mesaverde  formation  is  distinguished 
by  a  separate  pattern  on  the  geologic  maps. 
The  middle  group  comprises  most  of  the  workable  coal  beds  found 
in  the  Mesaverde  formation.  The  strata  associated  with  these  coals 
are  in  many  places  rather  sharply  delimited  from  the  lower  marine 
part  of  the  Mesaverde  formation,  both  by  the  marked  change  in  the 
character  of  the  strata  from  barren  below  to  coal  bearing  above,  and 
in  many  parts  of  the  fields  by  the  occurrence  of  a  large  and  conspicu- 
ous white  sandstone  bed  just  below  the  base  of  the  coals.  This  may 
very  properly  be  included  with  the  lower  or  relatively  barren  part  of 
the  formation,  as  in  numerous  places  it  contains  marine  fossils.  Where 
it  can  be  identified  it  furnishes  a  very  convenient  key  rock  from  which 
to  measure  the  stratigraphic  positions  of  the  various  beds  that  overlie 
it.  As  noted  in  previous  reports  on  this  general  region,  it  is  commonly 
referred  to  as  the  " white  rock." 
Above  the  " white  rock,"  coal  beds  of  the  middle  coal  group  show 
almost  everywhere,  either  in  weathered  outcrops  or  more  generally  as 
bands  of  reddened  and  baked  rock  or  slag,  where  the  coal  has  been 
burned  along  its  outcrop.  Higher  in  the  formation  the  coal  beds  are 
scattered  irregularly  and  vary  from  place  to  place,  so  that  individual 
sections  do  not  correspond  closely  in  the  number  and  grouping  of  the 
coal  beds.  The  massive  sandstone  ledges  are  by  far  the  most  con- 
spicuous features,  in  many  places  masking  by  their  debris  the  outcrop 
of  the  intervening  strata  of  weaker  rocks. 
An  upper  coal  group  near  the  top  of  the  Mesaverde  section  is  com- 
monly separated  from  the  middle  group  by  a  considerable  interval  of 
relatively  barren  strata.  There  is  some  evidence  that  the  coal  of  this 
part  of  the  section  is  of  lighter  weight  than  the  average  of  the  older 
coals  and  that  it  usually  slacks  more  rapidly,  in  this  way  resembling 
the  subbituminous  coals  of  the  Laramie  and  later  formations.  Though 
this  difference  may  not  hold  constantly  throughout  the  fields,  it  seems 
nevertheless  to  be  general. 
The  total  thickness  of  workable  coal  in  the  whole  formation  of 
course  varies  largely.  It  is  perhaps  at  or  near  a  maximum  at  New- 
castle, Colo.,  where  108  feet  of  coal  is  found  distributed  in  seven  work- 
able beds.  At  Meeker  the  section  is  known  to  contain  at  least  75  feet 
of  coal  distributed  in  ten  beds,  with  possibly  others  not  shown  by 
present  development.  Unfortunately  the  sections  of  this  formation 
in  the  fields  here  described  are  too  incompletely  known  to  give  any 
reliable  estimates  of  the  total  amount  of  coal  they  contain. 
The  three  coal  groups  denned  in  the  report  on  the  Yampa  coal 
field  do  not  correspond  to  the  limits  assigned  to  these  groups  in  the 
western  part  of  the  same  field  or  in  the  fields  to  the  south.  The  lower 
and  middle  groups  of  the  eastern  Yampa  field  are  not  distinguished  as 
