304         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    1. 
Atlantic  Coasta]  region  have  falls  at  the  points  where  they  issue  from 
the  hard  crystalline  rock  into  the  soft  Coastal  Plain  formations,  and  a 
line  connecting  these  points  has  therefore  been  called  the  fall  line. 
The  clays  to  be  described  are  confined  to  a  narrow  belt  of  Cretaceous 
strata  along  the  fall  line  and  do  not  at  any  point  extend  more  than  30 
miles  south  of  it.  The  eastern  belt  of  clays,  between  Macon  and 
Augusta,  ranges  in  width  only  from  3  to  15  miles.  Between  Macon 
and  Columbus  the  clays  are  also  confined  to  a  narrow  strip,  only  a  few 
miles  wide.  Just  west  of  Macon,  however,  and  near  Fori  Valley 
valuable  clays  are  found  throughoul  an  area  having  a  north-south 
width  of  about  30  miles. 
GEOLOGY. 
Formations  containing  clays.  As  above  stated  the  clays  occur  in 
strata  of  Cretaceous  age.  The  Cretaceous  deposits  in  Georgia  lie  in 
contact  with  the  metamorphic  and  crystalline  rocks  of  the  Piedmont 
Plateau  and  extend  entirely  across  the  State.  The  greatest  develop- 
ment of  the  Cretaceous  is  in  west-central  Georgia,  between  Ocmulgee 
and  Chattahoochee  rivers,  where  it  aggregates,  perhaps,  2,500  feet  in 
thickness.  Easl  of  Ocmulgee  River  and  between  Macon  and  Augusta 
only  w  hat  is  probably  t  he  lower  part  of  the  Cretaceous  is  exposed.  It 
is  confined  to  a  narrow  strip  and  is  in  many  places  overlapped  by 
tongues  of  Eocene  strata  and  obscured  1>\  superficial  deposits  of 
Lafayette  and  Columbia  sands.  The  detailed  stratigraphy  of  the 
Cretaceous  in  Georgia  has  not  been  worked  out.  It  may,  however,  be 
divided  into  Lower  Cretaceous  (Tuscaloosa  or  Potomac)  and  Upper 
( !retaceous. 
Tuscaloosa  formation. — The  Tuscaloosa  formation,  which  contains 
the  valuable  day-  of  the  State,  probably  occupies  the  same  geologic 
position  as  the  Potomac  of  Virginia  and  Maryland  and  as  the  Raritan 
of  New  Jersey,  which  contains  the  valuable  days  of  that  State.  It 
can  be  traced  directly  into  the  Tuscaloosa  formation  in  Alabama, 
where  ii  also  contains  valuable  deposit-  of  white  clay.  In  South 
Carolina0  the  formation  has  been  named  Hamburg  and  has  been  sub- 
divided into  Lowei-  Hamburg  and  Upper  Hamburg.  On  account  of 
the  variable  character  of  the  formation  in  Georgia  it  will  probably  be 
difficult  to  subdivide  and  map  1  he  formation  accurately.  Recent  field 
work  done  by  the  writer  has  made  it  possible  roughly  to  subdivide 
that  part  of  the  formation  lying  between  Macon  and  Augusta  into  a. 
lower  sand  member  and  an  upper  clay  member,  which  probably  cor- 
respond to  the  Lower  Hamburg  and  the  Upper  Hamburg  of  South 
Carolina,  the  upper  member  in  both  States  containing  the  most  val- 
uable clays.  For  convenience  of  description  the  Tuscaloosa  will  be 
divided  into  an  eastern  belt  and  a  western  belt ,  Ocmulgee  River  form- 
ing the  dividing  line.     The  Tuscaloosa  is  composed  chiefly  of  Cross- 
es Bull.  South  Carolina  Geological  Survey  No.  1. 
