410         CONTRIBUTIONS   TO    ECONOMIC   CEOLOGY,    1907,    PART    II. 
has  been  opened  at  a  number  of  places,  but,  like  all  beds  above  No.  6, 
it  has  not  been  mined  in  the  Pocket  district.  A  section  of  this  bed 
is  given  below: 
Section  of  bed  12  at  the  head  of  the  left  fork  of  Gin  Creek. 
Sandstone  (Harlan?).  Ft.     in. 
Coal G 
Shale,  carbonaceous A\ 
Coal 4 
Clay. 
Total  coal 4       6 
CHARACTER   OF   COAL. 
PHYSICAL  PROPERTIES. 
During  the  examination  of  the  coal  of  this  district  special  attention 
was  given  to  a  study  of  the  physical  properties,  and  a  number  of  sam- 
ples were  collected  for  comparative  study.  The  result  of  a  system- 
atic study  shows  that  the  coals  of  the  different  horizons,  14  in 
number,  do  not  differ  widely  in  physical  properties,  but,  on  the  other 
hand,  bear  a  regional  resemblance  to  each  other.  Certain  excep- 
tions were  noted,  however,  which  will  be  described. 
The  coals  of  this  district  are  usually  of  a  pitch-black  color,  with  a 
dark-brown  streak  and  brown  to  black  powder.  They  have  a  sub- 
dued vitreous  luster,  owing  to  the  fact  that  they  are  composed  of 
indistinct  bands  of  bright  and  dull  coal.  The  coal  has  a  distinctly 
bedded  structure,  which  on  close  examination  is  found  to  be  lami- 
nated. It  has  cubic  jointing  or  cleavage,  the  major  joints  occurring 
at  intervals  of  about  4  inches  and  the  minor  at  one-half  inch.  The 
coal  breaks  with  an  irregular  fracture  and  is  moderately  brittle, 
although  in  some  beds  in  the  district  it  is  tough.  In  no  place  was  the 
coal  observed  to  be  crumbly.  It  has  a  metallic  impact  and  is  mod- 
erately heavy.  The  coal  burns  with  a  long,  yellow  flame,  emitting  a 
bituminous  odor.  In  burning  it  leaves  a  relatively  small  amount  of 
ash,  which  is  reddish-yellow  in  color  and  moderately  fine.  There  is 
very  little  sulphur  or  resin  in  the  coal,  but  mineral  charcoal  is  fairly 
abundant.  These  general  statements  concerning  the  physical  prop- 
erties of  the  coals  of  this  district  do  not  wholly  apply  to  beds  7  and 
12.  The  former  contains  coal  which,  although  bedded,  is  decidedly 
more  massive  than  the  average  coal  of  the  district.  It  is  also  firmer 
and  more  resistant  to  breakage.  On  the  whole,  the  coal  of  bed  7, 
from  a  physical  point  of  view,  is  regarded  by  the  writer  as  a  superior 
variety.  The  coal  of  bed  12  is  also  a  very  firm,  finely  laminated  coal, 
and  in  some  respects  resembles  a  splint  coal.  It  breaks  in  large 
blocks  and  appears  to  resist  weathering  more  successfully  than  some 
of  the  other  coals. 
