YAMPA    COAL    FIELD,    COLORADO.  239 
The  outcrop  of  the  coal-bearing  series  continues,  in  part,  at  least,  from  the  "rim  rock" 
horthward  along  the  east  side  of  California  Park,  lapping  against  the  slopes  of  Sand  Moun- 
tain. Little  is  known  concerning  the  coal  in  this  extremity  of  the  field,  for  much  of  the 
[ground  is  covered  by  debris  of  igneous  rocks. 
The  field  within  which  the  three  coal  groups  are  known  to  have  been  affected  in  places 
ipy  the  heat  from  the  intrusion  of  igneous  rocks  is  probably  limited  to  about  25  square  miles. 
[This  estimate  does  not  include  the  Quaker  Mountain  district,  whose  coal  seams  are  con- 
sidered to  be  of  later  or  Laramie  age  and  in  most  cases  of  inferior  quality  to  the  older  seams 
iof  the  field,  although,  as  explained  in  a  later  paragraph,  some  of  these  Laramie  coals  have 
[been  altered  to  anthracite  coal.  Of  this  area  of  25  square  miles  a  considerable  portion  is 
jjoccupied  by  the  basalt,  both  as  rock  in  place  and  in  the  form  of  heavy  talus  cover.  The 
;  actual  area  in  which  anthracite  coal  is  now  known  to  be  present  or  in  which  conditions 
iseem  favorable  for  its  occurrence  probably  does  not  exceed  10  square  miles. 
LOWER    ELKIIEAD    AND    FORTIFICATION    CREEKS. 
The  center  of  the  main  Yampa  field,  as  mapped,  is  occupied  by  a  thick  series  of  slightly 
consolidated  beds  of  light-colored  sands  and  clays,  containing  many  coal  seams.  This 
[formation  appe.ars  to  lie  conformably  over  the  lower  beds  of  Montana  age,  and,  as  a  few 
marine  shells  were  found  in  it  north  of  Hayden,  it  seems  probable  that  it  is  of  Laramie  age. 
The  coal  that  it  contains  has  been  prospected  somewhat  and  in  one  case  at  least  has  been 
| mined  for  local  use. 
The  Seymour  opening  (Z),  on  Dry  Fork  of  Little  Bear  Creek,  a  tributary  of  Fortification 
Creek  northeast  of  Craig,  is  on  a  thick  seam  of  this  coal.  The  roof  is  a  compact  blue  clay, 
well  exposed  for  a  thickness  of  6  feet.  This  seam  of  solid,  black,  bright  coal,  at  least  17  feet 
thick,  shows  for  150  feet  in  natural  exposure  in  the  creek  bank  near  by.  The  coal  itself, 
however,  seems  to  be  of  comparatively  light  weight. 
The  coals  exposed  at  the  "  potholes  "  of  Elkhead  Creek,  although  in  some  cases  anthracized 
by  the  intrusion  of  ignecus  sheets,  belong  to  the  unconsolidated  beds  of  the  Laramie.  To 
this  formation  also  belong  most  of  the  coals  on  Quaker  Mountain  and  some  seams  on  the 
north  side  of  California  Park.  At  the  extreme  east  end  of  Quaker  Mountain,  however, 
several  prospects  and  tunnels  of  the  Egeria  Coal  Company  (V)  have  opened  seams  of  coal 
whose  proper  classification  is  not  certain.  From  the  character  of  the  associated  shales 
and  sandstones  it  is  thought  that  they  probably  belong  to  the  Laramie,  but  owing  to  the 
fact  that  the  middle  and  upper  groups  of  the  Montana  coals  have  been  lost  sight  of  in  this 
vicinity  under  the  basalt  cover,  this  opening  may  be  on  one  of  these  older  coal  seams. 
Bull.  285—06 16 
