470  PRE-CAMBRIAN   ROCKS    OF    NORTH   AMERICA.  [btttl.  8fi. 
2.  The  era    of  the  first   Animal    life;   the   Protozoans,    and 
forms  related  to  the  embryos  of  higher  invertebrate  species, 
commencing  possibly  with  the  mean  surface  temperature  of 
the  waters  about  120°  F.,  and  ending-  with  00°  F.  or  below. 
While  these  divisions  mark  off  great  steps  in  the  progress  of  the  de- 
veloping earth,  the  rocks  bear  no  marks  of  them  that  can  be  distin- 
guished. 
The  Huronian  period  covered  probably  much  of  Archaeozoic  time, 
and  this  is  all  in  the  way  of  correlation  that  can  be  said.  It  is  well  to 
note  here  that  if  the  Eozoon  is  really  animal  in  origin,  the  "Laurentian" 
rocks  of  Canada  in  which  it  occurs  must  be  Huronian  or  the  later  of 
Archaean  terranes. 
SECTION  II.      GENERAL  DISCUSSION. 
NAMES   APPLIED   TO   PRE-CAMBRIAN  ROCKS. 
In  the  early  days  of  American  geology  the  name  Primary  or  Primi- 
tive was  more  widely  applied  to  the  ancient  rocks  than  any  other. 
Among  the  older  geologists  this  name,  including  under  it  in  a  general 
way  the  pre-fossiliferous  or  metamorphic  rocks7  was  used  by  Akerly, 
Alexander,  Booth,  Dewey,  Ducatel,  Eaton,  Emmons,  Hitchcock  (Ed- 
ward), Jackson,  Mather,  Mitchell,  Percival,  Sogers  (H.  D.),  Eogers  (W. 
B.),  Silliman,  Tuomey,  Vanuxem,  and  others.  It  was  nearly  universal 
in  1820  and  was  applied  as  late  as  in  the  forties. 
The  term  Primitive  in  the  United  States  was  gradually  superseded 
by  Azoic.  Used  by  Adams  as  early  as  in  1846,  in  the  literature  of  the 
fifties  and  sixties  it  very  widely  occurs,  and  has  not  yet  disappeared. 
Among  more  prominent  geologists  in  whose  writings  it  may  be  found 
are  Adams,  Cook,  Crosby,  Emmons  (E.),  Frazer,  Hitchcock  (C.  H.), 
Hitchcock  (E.),  Kerr,  Kogers,  (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  ap- 
plied by  Adams,  Emmons  (E.)  and  the  elder  Hitchcock.  With  Eogers 
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 
definitely  defined  by  Foster  and  Whitney  in  their  application  of  it  to 
the  lake  Superior  rocks  and  the  Azoic  was  held  by  them  to  be  struc- 
turally indivisible.  While  the  rocks  of  the  Primitive  and  Azoic  were 
early  subdivided  into  lithological  divisions  there  was  little  or  no  at- 
tempt to  apply  stratigraphical  methods  to  them;  Later  the  Azoic  was 
subdivided  by  certain  geologists  into  Laurentian,  Huronian,  etc. 
The  work  of  Logan  and  Murray  marks  in  America  the  beginning  of 
a  truly  structural  study  of  the  ancient  rocks.    They  found  in  different 
