PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.      691 
holocrystalline  area  the  dip  is  much  steeper  than  in  the  semicrystal- 
Jine  area.  Disturbances  and  alterations  are  more  extensive  in  the 
holocrystalline  rocks  than  in  the  seniicrystalline  rocks.  Corundum 
is  present  only  in  the  holocrystalline  rocks. 
From  these  facts  it  is  believed  that  the  holocrystalline  area  is  older 
than  the  seniicrystalline  area,  and  formed  the  continent  against  which 
washed  the  waters  of  the  sea  which  deposited  the  rocks  of  the  semi- 
crystalline  series.  While  a  portion  of  the  holocrystalline  series  may 
be  Archean,  because  of  the  presence  in  it  of  undoubted  elastics  the 
series  is  referred  to  the  Algonkian.  The  same  reference  is  made  for 
the  seniicrystalline  rocks. 
The  history,  varieties,  and  characters  of  corundum,  and  its  mode 
of  occurrence  in  the  holocrystalline  rocks,  are  fully  described. 
Jones,98 in  1901, in  connection  with  a  description  of  Tallulah  Gorge, 
in  northeastern  Georgia,  describes  the  crystalline  rocks  there  occur- 
ring and  gives  a  little  sketch  map  showing  their  relations.  They  are 
called  pre- Cambrian. 
Watson,90  in  11)01,  describes  the  granitic  rocks  of  the  Piedmont 
Plateau  of  Georgia.  Field  and  laboratory  studies  indicate  that  they 
are  not  all  contemporaneous  in  origin.  Some  of  them  are  pre-Cam- 
brian,  while  others  may  possibly  be  later  in  age.  The  composition 
of  the  gneisses  shows  them  to  be  of  igneous  origin. 
SECTION  9.     ALABAMA. 
SUMMARY   OF  LITERATURE. 
Tuomey,100  in  1850,  places  the  granites,  gneisses,  and  associated 
crystalline  rocks  as  Primary  and  metamorphic.  The  slates  sometimes 
carry  plumbago,  and  true  granite  is  found  only  at  Talladega. 
Tuomey,101  in  1858,  finds  in  various  sections  granite,  syenitic  gneiss, 
ordinary  gneiss,  hornblende  slate,  mica  slate,  talcose  slate,  and  soap- 
stone.  In  certain  localities  are  found  limestones,  and  also  occasion- 
ally interst ratified  quartz  rocks  occur.  Granite  is  found  about  Rock- 
ford  in  large  masses. 
Smith,102  in  1875,  states  that  the  counties  of  Chilton,  Talladega, 
Calhoun,  Cleburne,  Lee,  Tallapoosa,  and  Elmore  lie  partly,  and 
Coosa,  Clay,  Randolph,  and  Chambers  wholly,  within  the  Archean 
region  of  the  State.  On  account  of  the  absence  of  fossils,  it  is  difficult 
to  determine  the  relative  ages  of  the  subdivisions  of  the  crystalline 
rocks.  Pathologically  they  are  classified  into  Laurent  inn.  Hiironian, 
and  White  Mountain  series,  following  Hunt's  characterization  of 
these  terms.  The  rocks  here  included  are  granite,  gneiss,  mica  schist, 
mica  slate,  hydromica  slate,  clay  slate  or  argillite,  syenite,  syenitic 
gneiss,  hornblende  schist,  diorite,  norite,  talcose  slate,  soapstone  or 
steatite,  chlorite  schist,  quartzite,  siliceous  slate,  itacolumite.   itaba- 
