GENERAL   ACCOUNT    OF    THE    PRE-CAMBRIAN     ROCKS.  61 
referred  all  the  laminated  rock,  including  even  coarse-grained  lam- 
inated gneisses;  others  took  the  volcanics  associated  with  the  Hu- 
roniah  to  be  its  characteristic  feature  and  so  called  various  pre- 
Cambrian  volcanic  rocks  "  Huronian ;  "  others  regarded  metalliferous 
rocks  as  the  important  feature  of  the  Huronian. 
The  real  basis  of  all  this  work  is  the  assumption  that  rocks  of  a 
certain  kind  are  characteristic  of  a  definite  period  of  the  world's  his- 
tory, and  that  if  rocks  are  found  which  are  really  like  the  Huronian 
and  Laurentian  in  lithological  character  they  should  be  referred 
respectively  to  these  series. 
The  Lake  Superior  region  furnishes  a  rather  marked  exception,  as 
do  certain  others,  to  the  indiscriminate  and  unwarranted  use  of  the 
term  Huronian.  This  region  is  so  near  to  and  is  connected  in  such  a 
way  with  the  original  Huronian  of  Lake  Huron  that  it  was  possible 
to  make  a  strong  case  of  probability  in  favor  of  the  equivalence  of 
the  clastic  rocks  of  the  two  regions.  The  Lake  Superior  Huronian 
was  divided  into  formations  upon  the  same  principles  that  were  used 
in  mapping  the  original  Huronian.  While  the  term  "  Laurentian  " 
was  applied  to  the  pre-Huronian  rocks  on  the  north  shore  of  Lake 
Huron  and  about  Lake  Superior,  a  number  of  geologists  recognized 
the  fact  that  this  was  a  variation  from  its  application  in  the  original 
Laurentian  area. 
As  geological  knowledge  increased  and  as  the  theories  involved  in 
the  terms  "  Primitive  "  and  "Azoic  "  were  more  and  more  attacked, 
in  order  to  avoid  a  theory  of  origin  the  term  Archean  was  proposed 
for  the  ancient  rocks  by  Dana  in  1872.  This  term  rapidly  grew  in 
favor.  By  its  use  not  only  the  disadvantage  of  a  theory  of  origin  was 
avoided,  but  in  common  with  "  Primitive  "  and  "Azoic  "  it  was  nec- 
essary to  subdivide  the  ancient  rocks  into  "  Laurentian  "  and  "  Huron- 
ian," and  thus  imply  a  correlation  with  the  rocks  of  other  regions. 
In  the  early  rapid  work  of  the  Far  West,  detailed  observations  usu- 
ally stopped  at  the  base  of  the  fossiliferous  series,  and  it  was  conven- 
ient to  regard  all  the  remaining  rocks  as  a  unit,  and  to  cover  this 
unit  the  term  Archean  was  adopted.  After  a  more  detailed  study  of 
certain  regions  the  terms  "Laurentian"  and  "Huronian"  were  ap- 
plied to  subdivisions  of  the  Archean.  This  term  Archean  also  found 
early  favor  with  the  Canadian  Survey  to  include  these  two  divisions 
of  pre-Cambrian  rocks. 
"  Eozoic  "  was  another  term  suggested  to  replace  "Azoic  "  when 
many  had  come  to  the  conclusion  that  the  rocks  once  supposed  to  be 
destitute  of  life  are  not  really  so.  This  was  used  to  a  considerable 
extent  in  the  sixties  and  seventies.  This  term  implies  a  theory  just 
the  opposite  of  "Azoic." 
As  already  said,  the  theory  involved  in  referring  all  pro-Cambrian 
rocks  to  the  "  Laurentian  "  and  "  Huronian  "  is  that  there  was  in  pre- 
