58        '  PRE-CAMBRTAN    GEOLOGY   OF    NORTH   AMERICA. 
the  oldest  system  is  prevailingly  gneissic  and  schistose,  that  it  is  dom- 
inantly  of  igneous  origin,  that  it  contains  also  sedimentary  rocks,  that 
between  this  system  and  the  Paleozoic  is  a  system  of  more  or  less  meta- 
morphosed sediments  and  volcanics  which  have  locally  been  divided 
into  series  and  formations,  and  that  this  system  essentially  lacks  fossil 
remains,  but  shows  traces  of  them.  Between  the  two  systems  in  va- 
rious countries  there  are  unconformities.  The  first  system  may  be 
called  Archean,  the  second  Algonkian.  Perhaps  the  most  signifi- 
cant feature  of  all  is  the  evidence  of  subordinate  amounts  of  sedi- 
ments in  the  Archean  gneissic  rocks,  just  as  is  now  being  found  in 
North  America. 
HISTORY  OF  NAMES  APPLIED  TO  PRE-CAMBRIAN  ROCKS. 
In  the  early  days  of  American  geology  the  name  "  Primary  "  or 
"  Primitive  "  was  more  widely  applied  to  the  ancient  rocks  than  any 
other.  Among  the  older  geologists  this  name,  applied  in  a  general 
way  to  the  pre-fossiliferous  or  metamorphic  rocks,  was  used  by 
Akerly,  Alexander,  Booth,  Dewey,  Ducatel,  Eaton,  Emmons,  Hitch- 
cock (Edward),  Jackson,  Mather,  Mitchell,  Percival,  Rogers  (H.  D.), 
Rogers  (W.  B.),  Silliman,  Tuomey,  Vanuxem,  and  others.  Its  use 
was  nearly  universal  in  1820  and  it  was  applied  as  late  as  in  the 
forties. 
The  term  "  Primitive  "  in  the  United  States  was  gradually  super- 
seded by  "Azoic."  Used  by  Adams  as  early  as  in  1840,  in  the  litera- 
ture of  the  fifties  and  sixties  it  very  widely  occurs.  Among  more 
prominent  geologists  in  whose  writings  it  may  be  found  are  Adams, 
Cook,  Crosby,  Emmons  (E.),  Frazer,  Hitchcock  (C.  H.),  Hitch- 
cock (E.),  Kerr,  Rogers  (H.  D.),  Safford,  Whitney,  Wadsworth,  and 
others.  In  its  earlier  use  "Azoic"  was  often  made  to  cover  all  rocks 
which  were  apparently  destitute  of  life,  without  reference  to  whether 
they  are  older  than  the  fossiliferous  rocks  or  not.  It  was  thus  applied 
by  Adams,  Emmons  (E.),  and  the  elder  Hitchcock.  With  Rogers 
the  "  Azoic  "  included  nonfossiliferous  rocks  which  are  younger  than 
the  "  Hypozoic  "  or  gneissic  series  proper.  Ordinarily,  however,  the 
term  was  used  to  cover  all  pre-Silurian  sedimentary  rocks,  the 
Silurian  being  then  regarded  as  the  base  of  the  fossiliferous  systems. 
It  was  thus  clearly  defined  by  Foster  and  Whitney  in  their  applica- 
tion of  it  to  the  Lake  Superior  rocks,  and  the  "  Azoic  "  was  held  by 
them  to  be  structurally  indivisible.  While  the  rocks  of  the  "  Prim- 
itive "  and  "  Azoic  "  were  early  subdivided  into  lithological  divisions, 
there  was  little  or  no  attempt  to  apply  stratigraphic  methods  to  them. 
Later  the  "  Azoic  "  was  subdivided  by  certain  geologists  into  "  Lauren- 
dan,"  "  Huronian,"  etc. 
The  work  of  Logan  and  Murray  marks  in  America  the  beginning 
of  a  truly  structural  study  of  the  ancient  rocks.     In  different  places 
