428  PRE-CAMBRIAN   ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
granite  and  granitoid  gneiss  region.  Not  only  tins,  but,  if  lithologieal 
characters  are  sufficient  evidence,  parts  areArchean;  that  is,  belong 
with  the  fundamental  rocks  of  Canada  and  the  West;  for  areas  are 
found  which  consist  of  an  intricate  complex  of  foliated  and  contorted 
granite  and  gneiss  cut  by  or  containing  masses  of  basic  eruptives  of 
various  kinds.  In  their  intensely  implicated  structure  and  lack  ot 
thoroughly  massive  characters  they  give  evidence  of  vast  antiquity. 
How  far  the  rocks  which  have  been  denominated  Huronian  belong 
with  the  pre-Cambrian  there  is  no  means  even  of  guessing.  It  is  very 
certain  that  large  parts  of  the  rocks  called  Taconic  and  Huronian  are 
Cambrian,  Silurian,  or  later.  When  the  Cambrian  and  Silurian  have 
been  definitely  outlined,  whether  any  of  the  unmistakable  elastics  will 
remain  to  be  correlated  with  the  Huronian,  or,  more  accurately,  to  be 
placed  in  the  Algonkian,  is  uncertain. 
The  disturbances  in  the  southern  Appalachians  are  of  a  different  type, 
and,  upon  the  whole,  have  been  less  intense  than  in  the  central  and 
northern  regions,  as  a  consequence  of  which  it  will  be  easier  to  reach 
definite  results  here  than  it  has  been  farther  north. 
The  only  really  systematic  work  which  has  been  done  is  that  in  Vir- 
ginia by  Rogers ;  that  of  Emmons  in  North  Carolina;  that  of  Safford 
in  Tennessee,  Avhose  work  barely  reaches  the  pre-Cambrian  rocks;  and 
that  of  Lieber.  The  last  in  his  earlier  reports  carefully  refrained  from 
generalizations  based  upon  insufficient  evidence,  but  patiently  mapped 
the  rocks  lithologically  in  several  counties,  and  thus  gives  serviceable 
information  unmingled  with  theories  of  no  value. 
In  the  south,  as  in  New  England,  occur  formations  which  by  con- 
tained belts  of  conglomerates  are  definitely  proved  to  be  of  clastic 
origin,  and  these  gradually  pass  into  unmistakable  crystalline  schists. 
These  transitions  were  clearly  described  and  their  meaning  definitely 
pointed  out  by  Emmons  and  Lieber  respectively  in  1856  and  1858. 
More  remarkable  than  this  is  the  discovery  of  Emmons  and  Lieber 
that  hornblende-schists  and  other  schists  are  metamorphosed  erup- 
tives. These  rocks  are  said  sometimes  to  be  as  thinly  laminated  as 
paper,  and  are  compared  with  slates;  but  their  occurrence  in  dikes 
within  the  granites,  and  their  gradations  into  the  ordinary  massive 
forms,  demonstrate  them  to  be  later  igneous  rocks.  These'  conclusions 
were  not  based  upon  petrographical  work,  but  upon  careful  field  study. 
The  microscope  in  recent  years  has  shown  accurately  the  method  of 
change;  but  that  the  change  does  occur  from  a  massive  eruptive  rock 
to  a  thoroughly  schistose  one  was  proved  beyond  doubt  by  these  men 
before  1860. 
Emmons  and  Lieber  further  appreciated  that  the  granite-gneisses  in 
their  lithologieal  affinities  as  well  as  by  actual  transitions  belong  with 
igneous  granitic  rocks  rather  than  with  the  sedimentaries.  Emmons 
also  reached  the  same  conclusion  for  the  South  which  Emerson  demon- 
strated many  years  later  for  the  New  England  states,  that  there  are 
