710  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
Baltimore  gneiss  is  pre- Cambrian,  but  there  seems  to  be  a  difference 
of  opinion  in  reference  to  the  Wissahickon  mica  gneiss,  Bascom 
holding  that  this  formation  is  probably  pre-Cambrian,  while  Mathews 
holds  it  to  be  Ordovician.  The  problem  is  a  complicated  one,  but 
the  weight  of  evidence  at  the  present  time  seems  to  favor  Bascom's 
view. 
Bascom  thinks  that  both  the  Wissahickon  mica  gneiss  and  the 
Baltimore  gneiss  are  sedimentary  in  origin.  If  this  conclusion  be 
correct,  it  has  a  bearing  upon  the  age  of  the  metamorphic  sedimen- 
tary rocks  of  the  southern  Piedmont.  It  has  been  intimated  that 
these  may  be  the  equivalents  of  the  Ocoee,  and  hence  very  likely  of 
late  Algonkian  or  Cambrian  age ;  but  if,  in  the  northern  Appalachian 
region,  there  are  sedimentary  rocks  of  pre-Cambrian  age,  this  pos- 
sibility can  not  be  excluded  for  the  metamorphic  sediments  of  the 
southern  Appalachians  which  have  not  been  connected  structurally 
with  either  the  Cambrian  or  the  pre-Cambrian  rocks.  On  the  other 
hand,  the  oldest  Archean  gneiss  (Carolina)  is  shown  by  Keith  to  be 
in  considerable  part  of  sedimentary  origin. 
While  in  this  summary  the  emphasis  has  been  placed  upon  the 
results  of  the  later  workers,  it  would  not  be  fair  to  pass  by  the  pioneer 
work  of  the  early  men,  and  in  this  connection  attention  is  particu- 
larly called  to  the  early  work  done  in  Virginia  by  Rogers;  that  of 
Emmons  in  North  Carolina;  that  of  Safford  in  Tennessee,  whose 
work  barely  reaches  the  pre-Cambrian  rocks,  and  that  of  Lieber. 
The  last  in  his  earlier  reports  carefully  refrained  from  generaliza- 
tions based  upon  insufficient  evidence,  but  patiently  mapped  the  rocks 
lithologically  in  several  counties,  and  thus  gives  serviceable  informa- 
tion 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  185G  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  lithological  affinities  as  well  as  by  actual  transitions  belong 
