294         CONTRIBUTIONS   TO    ECONOMIC   GEOLOGY,    1907,    PART    IT. 
formations  outcrop  continuously  westward  from  the  main  part  of  the 
Yampa  field  at  least  as  far  as  Lay.  In  view  of  the  recent  completion 
of  the  township  resurveys  west  of  the  line  between  Rs.  91  and  92  W., 
that  part  of  the  field  was  examined  in  more  detail  during  the  last 
season,  and  the  map  forming  PL  XVI  embraces  the  results  obtained 
during  this  examination. 
MESA  VERDE  COAL. 
Distribution  and  structure  of  Mesaverde  rocks. — Along  the  valley  of 
Yampa  River  west  of  the  mouth  of  Williams  Fork  the  Mesaverde 
rocks  are  tilted  in  several  broad  folds.  The  principal  structure  in 
this  part  of  the  field  is  the  Axial  Basin  anticline,  which  forms  the 
southern  limit  of  the  Green  River  Basin.  The  strata  rise  toward  the 
axis  of  that  uplift,  from  the  summit  of  which  the  Mesaverde  forma- 
tion has  been  eroded  so  that  the  coal  field  terminates  with  the  south- 
ward-facing escarpments  of  lies  and  Duffy  mountains.  Northeast  of 
the  Axial  Basin  uplift  a  corresponding  syncline  passes  through  Round 
Bottom,  on  Yampa  River,  with  an  axis  approximately  parallel  to  the 
major  anticlinal  fold.  Northeast  of  the  Round  Bottom  syncline  a 
secondary  anticline,  with  axis  parallel  to  the  other  two  folds,  extends 
northwestward  from  the  mouth  of  Williams  Fork.  This  is  exposed 
as  far  as  Sand  Springs,  near  the  Craig-Lay  wagon  road,  and  doubtless 
influences  the  underlying  structure  much  beyond  that  point,  although 
concealed  by  the  overlying  unconformable  deposits.  The  prevailing 
dips  in  all  these  structures  are  light,  for  the  most  part  not  exceeding 
10°  to  15°. 
Typical  sections  and  coal  groups. — The  complete  section  of  the  Mesa- 
verde formation  exposed  in  the  western  part  of  the  Yampa  field  is 
about  3,500  feet  thick.  This  resembles  the  section  of  the  same  for- 
mation exposed  in  the  Danforth  Hills,  although  in  the  Yampa  field 
it  appears  to  be  somewhat  thinner.  The  evidence  on  which  the  lower 
members  of  the  formation  in  the  two  fields  are  correlated  consists  of 
(1)  the  structural  relations  in  Axial  Basin,  (2)  the  striking  corre- 
spondence in  arrangement  in  the  basal  group  of  sandstones  and  shales 
on  either  side  of  the  erosional  break,  and  (3)  the  correspondence  of 
fauna  and  flora  obtained  from  the  two  sections,  of  which  some  repre- 
sentative collections  have  been  made  and  studied  by  T.  W.  Stanton. 
The  arrangement  and  grouping  of  the  coal  beds  in  the  Yampa  field 
correspond  well  with  the  sections  exposed  in  the  Danforth  Hills. 
Above  the  lower  and  relatively  barren  zone  no  distinction  can  be 
made  between  the  lower  and  middle  coal  groups  as  these  were  de- 
scribed in  the  report  on  the  Yampa  field,  and  they  thus  merge  into  a 
single  large  coal  group.  The  lower  coal  group  of  this  part  of  the 
field,  as  in  the  Danforth  Hills,  is  recognized  as  a  group  of  relatively 
inconspicuous  beds  occurring  lower  than  any  distinguished  in  the 
groups  that  were  described  in  the  report  on  the  main  Yampa  field. 
