306  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  26 
The  upper  member  of  this  section  consists  of  both  limestone  and 
shale,  commonly  in  alternating  beds,  although  there  is  comparatively 
little  uniformity  in  this  respect  in  different  parts  of  the  region.  In 
general  it  can  be  said  that  the  shale  layers  are  better  developed  in  the 
west,  while  the  limestone  is  more  important  in  the  east.  The  shale 
beds  are  usually  green  or  blue,  though  some  of  them  are  white,  yellow, 
chocolate  colored,  and  even  black.  The  chocolate-colored  and  black 
shales  are  highly  carbonaceous  and  are  locally  termed  "  oil  rock," 
though  the  main  bed  of  chocolate-colored  shale  or  the  oil  rock  proper 
lies  just  at  the  base  of  the  Galena.  The  limestone  is  commonly  a 
thin-bedded,  fine-grained  blue  rock,  which  is  occasionally  subcrysta 
line  and  usually  nonmagnesian,  although  it  becomes  magnesia 
toward  the  east.  The  most  marked  part  of  this  member  of 
Platteville  is  the  "  glass  rock,"  which  is  a  dense,  very  fine-graine 
hard,  conchoidally  breaking  limestone.  It  is  light  chocolate  colorec 
when  fresh,  but  quickly  weathers  to  light  gray.  The  shales,  and  es*- 
pecially  the  limestones,  of  this  member  are  rich  in  fossils,  and  among! 
these  a  brachiopod — Dalmanella  subcequata  Conrad — is  common 
The  Galena  limestone  is  essentially  a  dolomite,  most  of  which  iiii 
coarse  grained  and  porous,  and  occurs  in  beds  from  1  foot  to  4  fee 
in  thickness.     The  top  and  sometimes  the  bottom  of  this  format i or 
is  thin  bedded,     Four  divisions  are  recognizable,  as  follows: 
General  section  of  Galena  limestone  at  Dubuque,  Iowa. 
Fee 
4.  Thin-bedded,  nonflinty,  earthy  dolomite 3 
3.  Thick-bedded,  nonflinty,  coarse  dolomite 6 
2.  Thick-bedded,  flinty,  coarse  dolomite 9 
1.  Thick  to  thin  bedded,  nonflinty  dolomite,  with  the  main  oil-rock  horizon 
at   the   base 6»i 
V  a 
tal-i 
ian 
'"; 
ied 
In  the  Wisconsin  region  No.  1  averages  40  feet  in  thicknesi 
No.  1  of  this  section  is  not  always  entirely  dolomitized,  some  of  th* 
lower  part  in  certain  localities  consisting  of  thin-bedded,  subcry*] 
talline  limestone,  but  the  dolomitic  character  is  the  common  featui ) 
in  the  immediate  vicinity  of  ore  deposits.  At  the  extreme  base  c  f 
the  Galena  limestone  is  a  chocolate-colored  carbonaceous  shale,  ( I 
shale  mixed  with  earthy  limestone,  to  which  the  name  oil  rock  In  I 
been  applied.  This  varies  in  thickness  from  1  inch  to  5  feet  and  i 
a  very  characteristic  and  easily  recognized  horizon.  It  occurs  j  l| 
the  bottom  of  many  of  the  mine  shafts.  The  bottom  of  this  oil  roc  i 
is  regarded  as  the  base  of  the  Galena  limestone.  It  is  usually  unde  I 
lain  by  a  thin  bed  of  yellow  or  white  clay  or  soft  shale,  which  is  fr  ••! 
quently  spoken  of  by  the  miners  as  "  the  pipe  clay  "  or  "  the  els  j] 
bed." 
