214  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906. 
BROOK  WOOD    AND    GWIN    GROUPS. 
The  Brookwood  and  Gwin  groups  are  so  high  in  the  series  that  they  do  not  occur  in  the 
Birmingham  quadrangle,  though  the  sandstone  ridge  south  of  Camp  Branch  reaches  up 
to  the  horizon  of  the  Gwin  seam.  No  traces  of  that  seam  were  seen,  and  if  it  is  present  it 
occurs  only  in  small  areas  under  thin  cover  in  the  southwest  corner  of  the  quadrangle, 
and  is  of  no  commercial  importance. 
COBB    GROUP. 
The  Cobh  group  of  coals  occurs  only  in  the  southwest  corner  of  the  quadrangle  and  is 
not  economically  important,  so  will  not  be  discussed  here. 
PRATT    GROUP. 
The  Pratt  group  includes  the  Piatt,  Nickel  Plate1,  Curry,  and  Gillespie  scams.  Only  the 
Pratt  and  Nickel  Plate  are  important.  Generally  the  Curry  and  Gillesp;e  seams  are  not 
minable. 
Pratt  seam. — This  has  been  the  most  important  mining  scam  in  the  quadrangle,  but  its 
area  is  less  than  that  of  any  of  the  other  prominent  seams,  as  shown  by  its  outcrop  on 
the  map  (PI.  V).  Its  thickness,  character,  and  quality  are  fairly  constant.  In  an  area 
lying,  in  a  general  way,  south  of  Fivemile  Creek,  and  including  Republic,  Pinknev,  and 
Blossburg,  the  seam  has  two  or  three  partings.  The  following  sections  are  the  average  of 
three  measurements  each  of  the  seam  in  this  area.  Section  No.  1  represents  its  condition 
in  that  part  of  the  area  in  which  there  are  three  partings  and  section  No.  2  where  there1  are 
only  two  partings. 
Averagt  sections  of  Pratt  seam  in  tin  Republic-Blossburg  region. 
No.  i. 
Inches. 
Coal 52 
Parting f 
Coal 4 
Parting 3 
Coal 1H 
Parting | 
Coal [51 
38| 
No.  2. 
Inches. 
Coal 6 
farting 2!. 
Coal 26f 
Parting 1] 
Coal 2i 
38| 
Outside  of  the  area  discrlbed  above,  on  the  northern  and  eastern  margins  of  the  field, 
the  Pratt  seam  has  almost  universally  one  paring,  as  shown  in  the  following  section, 
which  is  the  average  of  26  measurements: 
Average  section  of  Pratt  seam  around  northern  and  eastern  margins  of  'fi eld. 
Inches. 
Coal b\ 
1  'a  it  ing 2i 
Coal 34 
41| 
The  above  sections  indicate  a  thinning  as  well  as  a  deterioration  in  the  character  of  the 
seam  to  the  northwest.     The  Pratt  seam  has  been  more  extensively  mined  than  any  other 
