250  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    I. 
CHICKAMAUGA   ("TRENTON")   LIMESTONE. 
The  Chickamauga  limestone  is  of  Ordovician  or  lower  Silurian  age. 
[ts  stratigraphic  position  is  below  the  Rockwood  (Clinton)  formation, 
which  carries  the  red  ores  of  the  region,  and  above  the  Knox  chert. 
In  Birmingham  Valley  this  limestone  outcrops  along  the  west  escarp- 
ment of  Rod  Mountain  almost  the  entire  distance  from  a  point  south 
of  Bessemer  nearly  to  Springville  and  thence  southward  along  the 
east  side  of  Red  Mountain  nearly  to  Trussville.  'There  are  several 
pips  in  this  outcrop  that  are  probably  due  to  faults,  hut  perhaps  in 
souk4  places  to  nondeposit ion  of  the  limestone.  ( )ne  of  t  liese  is  on  the 
west  side  of  Red  Mountain,  1  mile  south  of  Saddlers  Gap;  another 
extends  northward  from  (May  for  2  miles,  and  anot  her  about  .'5  miles  in 
length  is  northeast  of  Ayres.  The  limestone  appears  to  be  absent 
along  the  wesl  side  of  the  Cahaba  trough  from  a  point  1  mile  south  of 
Argo  northeastward  to  the  boundary  of  the  Birmingham  quadrangle. 
It  outcrops  along  the  wesl  side  of  Birmingham  Valley  and  makes  the 
east  face  of  Wesl  Red  Mountain  from  the  vicinity  of  Cunningham  Gap 
to  Dale  and  thence,  along  the  same  belt,  extends  up  Murphree  Valley 
to  Chepultepec  and  beyond.  South  of  Cunningham  Gap  the  outcrop 
thins  gradually  and  finally  disappears  about  2  miles  to  the  south,  being 
cut  off  diagonally  by  a  fault.  Along  the  easl  side  of  Murphree  Valley 
it  is  cut  oul  by  a  fault  as  far  south  as  Village  Springs.  Around  the 
south  end  of  Blount  Mountain  it  outcrops  in  a  broad  belt  extending 
from  Village  Springs  to  Springville  and  thence  northward  parallel  to 
the  east  side  of  Blount  Mountain.  A  belt  of  Chickamauga  limestone 
also  extends  down  the  east  side  of  East  Red  Mountain  Prom  Canoe 
( "reek  nearly  to  Trussville. 
Mong  all  the  outcrop-  described  above  the  limestone  is  generally 
rather  thin  bedded  and  light  <>r  dark  gray  in  color.  In  places,  as  at 
Dale  and  west  of  Swansea,  there  is  a  t  hick-bedded  bull"  limestone  near 
the  base,  and  at  the  south  end  of  Blount  Mountain  purple-mottled 
layers  occur  through  the  lower  25  feel  or  more.  Crystalline  limestone 
also  occiii-  in  the  formation.  The  Chickamauga  limestone  appears  to 
run  from  :'»<)<)  to  7<)u  feel  in  thickness  except  where  affected  by  faults, 
as  described  above,  [ts  thickness  at  date  City  is  about  300  feet,  on 
Blackburn  Fork,  west  of  Swansea,  .".on  feet,  at  Chepultepec  000  to  800 
feet,  and  on  Butler  Mountain,  al  the  south  end  of  Blounl  Mountain,  700 
to  800  feet.  The  limestone  dips  into  the  hills  on  either  side  of  the 
valleys,  generally  at  angles  of  1.")°  to  20°,  though  higher  dips  may 
occur  locally.  In  some  localities,  as  in  the  region  of  Butler  and  Foster 
mountains,  it  lies  nearly  Hat. 
The  Chickamauga  limestone  outcrops  along  Cahaba  Valley  in  a  belt 
abou<  1  mile  wide.  In  fact  the  valley  has  been  eroded  in  the  lime- 
stone, which  is  less  resistant  than  the  rocks  on  either  side.     Leeds  is 
