LIMESTONE  AND  DOLOMITE,  BIRMINGHAM  DISTRICT,  ALABAMA.       253 
said  to  occur  in  the  rock  at  various  points.  At  the  top  it  begins  to 
show  more  or  less  abundant  chert  inclusions,  such  as  nodules,  string- 
ers,  and  thin  irregular  sheets.  This  transition  may  be  observed  in 
the  vicinity  of  the  abandoned  Dolcito  quarry,  where  the  overlying 
cherty  phase  of  the  Knox  is  fairly  well  exposed.  The  rock  is  in  most 
places  thick  bedded.  In  weathering  much  of  the  surface  becomes 
granular,  simulating  closely  the  appearance  of  a  coarse-grained  sand- 
stone. As  stated  above,  the  thickness  of  this  dolomite  seems  to  be 
about  500  feet.  It  was  measured  in  Opossum  Valley,  west  of  Birming- 
ham, and  along  the  section  from  the  Spencer  quarry  through  Lardona, 
where  the  top  and  bottom  can  be  determined  within  reasonably  close 
limits.  There  may  be  considerable  variation  from  this  thickness, 
however,  in  different  parts  of  the  region.  The  dip  along  the  western 
belt,  where  the  quarries  mentioned  above  are  located,  is  from  10°  to 
15°  E. 
From  this  dolomite  is  obtained  all  the  flux  quarried  in  the  Birming- 
ham quadrangle  except  that  obtained  from  the  Bangor  limestone. 
At  the  quarries  of  the  Tennessee  Republic  Company  at  Ketona,  and  of 
the  Sloss  Iron  Company  at  North  Birmingham,  this  rock  is  a  nearly 
pure  calcium  and  magnesian  carbonate,  as  shown  by  the  following 
analyses : 
Analyses  of  dolomite  from.  Ketona  and  North  Birmingham  quarries. 
Silica  (Si02) 1.31 
Alumina  ( A1203) I  .90 
Lime  carbonate  (CaC03) i  55.80 
Magnesium  carbonate  (MgC03) I  42.47 
0.70 
;63 
56.41 
43.00 
1.  Average  of  four  analyses  of  average  sample  from  Ketona  quarry,  August  to  October,  1903.    Analy- 
ses furnished  by  Tennessee  Republic  Company. 
2.  Average  of  ten  analyses  of  carload  lots  from  North  Birmingham  quarry,  August,  1903,  to  June, 
1905.    Analyses  furnished  by  Sloss  Iron  Company. 
These  analyses  indicate  that  the  lime  and  magnesia  in  this  rock  are 
nearly  in  the  proportions  of  the  mineral  dolomite  and  that  it  is  prop- 
erly called  dolomite.  W.  B.  Phillips a  has  made  a  number  of  silica 
determinations  from  the  dolomite  in  the  vicinity  of  Dolcito.  At  the 
south  end  of  the  Dolcito  quarry,  which  had  a  face  of  17  feet  at  the 
time  of  sampling,  samples  taken  from  every  foot  of  the  face  showed  a 
range  of  0.48  to  0.88  per  cent  of  silica,  with  an  average  of  0.64  per 
cent.  At  the  northeast  end  of  the  same  quarry,  presumably  from  the 
same  beds  as  at  the  southwest  end,  though  not  so  stated,  the  silica,  in 
samples  taken  in  the  same  way,  ranged  from  0.48  to  4.58  per  cent, 
with  an  average  of  1.69  per  cent.  This  shows  a  considerable  varia- 
tion within  a  short  distance.  Two  miles  northeast  of  Dolcito,  on 
Fivemile  Creek,  29  samples  taken  at  intervals  from  top  to  bottom  of 
aGeol.  Survey  Alabama,  Rept.  on  Valley  Regions,  pt.  2,  1897,  pp.  323-326, 
Bull.  315—07 17 
