690  PRE- CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
rock,  hornblende,  and  clay  slates.  The  granite  passes  by  impercep- 
tible gradations  into  the  gneiss,  beds  of  the  two  sometimes  alternating, 
and  the  latter  also  passing  into  the  stratified  mica  slate.  Gneiss 
also  passes  into  the  stratified  mica  slate,  and  the  mica  slate  into  tal- 
cose  slate.     The  graywacke  is  the  beginning  of  the  transition. 
Little,04  in  1875,  mentions  crystalline  rock,  presumably  Primary, 
at  various  points. 
Campbell  (J.  L.)  and  Ruffner,95  in  1883,  divide  the  Archean,upon 
chemical,  lithological,  and  structural  grounds,  into  Laurentian  and 
Huronian.  In  the  metamorphosed  rocks  the  prevailing  dips  are 
toward  the  southeast.  It  is  believed  that  while  they  were  somewhat 
plastic  they  were  folded  and  overturned,  although  in  places  left  in  a 
vertical  position;  and  not  infrequently  they  are  found  in  a  nearly 
horizontal  position,  or  sometimes  resting  in  arches  and  depressions. 
In  the  Choccolocco  Valley  the  railroad  passes  abruptly  to  the  Lower 
Silurian  rocks.  This  relation  between  the  Silurian  and  the  Archean 
is  attributed  to  a  fault,  with  downthrow  of  the  former. 
Elliott,90  in  1883,  states  that  the  mica  schist  and  gneiss  at  Jasper 
dips  beneath  the  marble  and  is  therefore  metamorphosed  Knox  sand- 
stone. The  porphyritic  gneiss  of  the  West  Atlantic  Railway  is  iden- 
tical with  that  at  Talking  Rock,  and  is  a  metamorphosed  form  of  the 
Ocoee  sandstone. 
King,97  in  1894,  describes  the  geology  of  the  "  Crystalline  belt "  of 
Georgia  in  connection  with  the  occurrence  of  corundum.  The  Crys- 
talline belt  occupies  an  area  of  12,430  square  miles,  crossing  the  north- 
ern part  of  Georgia  from  the  northeast  to  the  southwest,  and  lying 
between  Paleozoic  strata  in  the  northwest  corner  of  the  State  and 
Mesozoic  and  Cenozoic  strata  in  the  southern  half  of  the  State. 
The  rocks  of  the  Crystalline  belt  are  divisible  into  two  petro- 
graphical  classes.  The  first  consists  of  a  series  of  mica  schists,  slates, 
shales,  conglomerate,  and  marble,  which,  though  more  or  less  crystal- 
line, show  evidence  of  clastic  character.  This  class  is  called  the  semi- 
crystalline  series.  The  semicrystalline  rocks  are  confined  to  an  area 
bordering  the  Paleozoic  to  the  northwest.  The  second  class  comprises 
eight  types  of  rock.  Three  of  them,  limestone,  quartzite,  and  slate, 
are  undoubtedly  clastic;  three  of  them,  granite,  gneiss,  and  mica 
schist,  are  completely  crystalline  and  show  no  trace  of  clastic  charac- 
ter ;  and  two,  peridotite  and  diorite,  are  presumably  of  eruptive  origin. 
Gneiss  and  mica  schist  are  the  prevailing  rocks.  This  second  class  is 
termed  the  holocrystalline  series. 
The  rocks  of  the  Crystalline  belt  are  separated  from  the  Paleozoics 
on  the  northwest  by  a  strong  unconformity.  Between  the  semicrystal- 
line and  holocrystalline  rocks  there  is  apparent  transition. 
Throughout  the  Crystalline  belt  there  is  a  uniform  dip  to  the  south- 
east, pointing  toward  a  moving  force  from  the  southeast,  but  in  the 
