CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903,  [bull.  225. 
limestone  strata  at  455  feet.  It  is  possible  that  the  Carboniferous 
rocks  which  are  so  extensively  developed  in  Iowa  and  Nebraska  to  the 
southeast  of  this  region  once  extended  as  far  as  the  southeast  corner 
of  South  Dakota,  but  probably  for  the  most  part  they  were  removed 
by  erosion  before  the  deposition  of  Cretaceous  strata.  Some  limited 
outliers  of  these  rocks  may  remain,  deeply  buried,  but,  if  so,  it  has 
not  been  demonstrated  that  they  are  coal  bearing-,  and  the  cost  of  the 
extensive  drilling  that  would  be  necessar}^  to  shed  light  on  this  point 
would  doubtless  far  exceed  the  returns  to  be  expected  in  the  event  of 
finding  a  remnant  of  a  coal  bed. 
Conclusions. — Several  factors  enter  into  the  consideration  of  the 
economic  value  of  this  lignite.  A  steady  demand  for  a  suitable  fuel 
is  assured  in  this  region.  This  narrows  the  question  to  (1)  character 
of  fuel;  (2)  its  extent;  (3)  practicability  of  mining  it. 
The  lignite  is  of  equal  fuel  value  with  the  widely  used  brown  coals 
of  Europe,  and  with  those  of  Texas  and  Arkansas,  which  are  locally 
in  use.  The  physical  character  of  the  lignite,  however,  offers  a  fatal 
objection  to  its  use  in  more  than  a  very  restricted  local  area.  Its 
tendency  to  disintegrate  or  slack  while  drying  precludes  its  being 
handled  or  transported  without  great  loss.  Even  if  it  could  be  satis- 
factorily delivered  in  neighboring  cities  at  an  advantageous  price, 
especially  constructed  grates  would  be  necessary  to  its  economical 
combustion,  whether  it  were  used  for  steam  or  domestic  purposes. 
This  would  be  a  handicap  to  it  in  competition  with  better  coals.  Its 
immediate  value  in  the  restricted  area  in  which  it  occurs  depends  upon 
the  cost  of  production,  for  this  region  along  the  river  escarpment  is 
still  well  wooded  and  so  provided  with  cheap  fuel.  Therefore,  at 
present,  the  value  of  the  lignite  as  a  fuel  is  extremely  doubtful. 
As  to  extent,  the  bed  at  the  second  horizon  may  be  said  to  have  a 
satisfactory  areal  extent,  but  its  average  thickness  (20  inches)  is  not 
sufficient.  Beds  of  good  bituminous  coal  of  this  thickness  are  some- 
times profitably  worked,  but  it  is  clear  that  the  Dakota  County  lignite 
does  not  occur  in  paying  quantity,  when  it  is  compared  with  the  lig- 
nites of  southwestern  Arkansas,  which  are  from  3  to  6  feet  thick; 
with  those  of  Texas,  which  are  from  I  to  6  feet  thick,  occasionally 
reaching  over  15  feet,  and  usually  occur  as  outcrops;  and  with  those  of 
Wyoming  and  North  Dakota,  which  are  20  feet  or  more  in  thickness. 
The  practicability  of  mining  the  lignite  depends  on  its  character  and 
extent,  in  conjunction  with  such  local  conditions  as  affect  actual  mining 
operations.  In  this  region  it  is  certain  that  only  such  beds  can  be 
worked  as  are  above  the  ground-water  horizon  or  above  the  level 
of  the  river  flood  plain,  so  that  they  may  be  drained  without  pumping. 
The  nature  of  the  roof  over  the  bed  is  of  the  greatest  importance.  In 
this  region,  where  lumber  is  comparatively  expensive,  if  the  roof  of 
i  tunnel   must    be  timbered  the  cost  of  material  and  labor   would 
