6  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
Alfred  R.  Schultz  was  appointed  to  consider  this  question  and  make 
recommendations  which  should  govern  the  practice  of  the  Geological 
Survey.  The  report  of  this  committee,  in  brief,  provides  for  the 
recognition  of  four  classes  of  areas,  as  follows: 
1.  Coal  province. — This  corresponds  in  a  general  way  with  a  geo- 
logic or  physiographic  province.  In  order  from  east  to  west  the  coal 
provinces  of  the  United  States  are  the  Eastern,  Interior,  Gulf, 
Northern  Great  Plains,  Rocky  Mountain,  and  Pacific  Coast. 
2.  Coal  region. — This  term  is  to  be  applied  to  a  part  of  a  province 
which  has  a  uniform  geologic  history  and  topographic  expression,  and 
which  is  manifestly  too  large  to  be  considered  as  a  coal  field.  In  the 
Eastern  coal  province  the  following  coal  regions  are  recognized: 
Atlantic  Coast,  Anthracite,  and  Appalachian.  In  the  Interior: 
Eastern,  Northern,  Western,  and  Southwestern.  In  the  Northern 
Great  Plains:  Fort  Union,  Assinniboine,  and  Black  Hills.  In  the 
Rocky  Mountain :  San  Juan  River,  Southwestern  Utah,  Raton  Moun- 
tain, Denver,  Uinta,  Green  River,  Bighorn  Basin,  and  Judith  Basin. 
3.  Coalfield. — A  coal  field  is  defined  as  (a)  an  isolated  area  that  is 
essentially  a  geologic  and  topographic  unit,  or  (b)  an  arbitrary  part 
of  an  extended  coal  region  that  for  economic  or  geologic  reasons  may 
be  considered  a  unit,  separate  and  apart  from  the  rest  of  the  region. 
The  Southern  Anthracite,  Cahaba,  Bull  Mountain,  Great  Falls, 
Hanna,  Canon  City,  Cerrillos,  Roslyn,  and  Coos  Bay  fields  are  exam- 
ples of  the  first  class;  and  the  Pittsburg,  New  River,  Pocahontas,  and 
Belleville  fields  are  examples  of  the  second  class. 
4.  Coal  district. — This  is  recognized  as  a  subdivision  of  a  field.  Its 
limits  are  generally  controlled  by  commercial  conditions,  and  it 
marks  a  center  of  development  and  production. 
A  novel  feature  of  the  new  coal  map  is  the  attempt  to  represent 
many  of  the  great  structural  basins  of  the  Rocky  Mountains  as  coal 
bearing,  although  the  coal  in  the  center  of  the  basin  is  in  many  places 
so  deeply  covered  by  later  barren  sediments  that  it  is  not  available  at 
the  present  time,  and  in  some  basins  it  is  even  doubtful  if  it  can  ever 
be  mined.  The  main  difference,  however,  in  the  outline  of  the  coal 
fields  here  shown,  as  compared  with  those  represented  on  any  pre- 
vious map,  is  due  to  the  increased  knowledge  already  referred  to, 
which  is  more  fully  presented  in  the  various  papers  constituting  this 
volume. 
GEOLOGIC  WORK. 
GENERAL    OUTLINE. 
Early  in  June,  1907,  sixteen  geologic  parties  were  outfitted  and 
placed  in  the  field,  in  the  States  of  North  Dakota,  Montana,  Wyoming, 
Colorado,  Utah,  and  New  Mexico,  to  classify  as  much  of  the  land 
which  had  previously  been  withdrawn  by  departmental  orders  as 
time  and  means  would  permit.     In  all  about  20,000  square  miles  were 
