vanhisr.]  DISCUSSIONS   OF   PRINCIPLES.  473 
Lying  at  the  root  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  respect- 
ively to  these  series. 
As  to  the  relations  between  the  Laurentian  and  Huronian,  it  was 
plainly  believed  by  Logan,  Murray,  and  the  other  early  geologists  that 
where  the  two  come  in  contact  they  are  unconformable,  although 
oftentimes  the  structural  relations  which  obtain  are  admittedly  ob- 
scure. That  was  the  position  of  the  present  director  of  the  Canadian 
survey  as  late  as  1879,  Avho  used  the  term  Huronian  not  only  to  include 
rocks  that  had  been  theretofore  placed,  but  to  cover  all  of  the  Upper 
Laurentian  and  the  Upper  Copper-Bearing  series,  thus  greatly  expand- 
ing the  system.  In  recent  years  he  has  held  that  the  Huronian  and 
Laurentian  are  always  conformable,  and  that  often  the  former  grades 
downward  into  the  latter,  and  this  is  the  position  which  has  been 
taken  by  many  geologists  of  the  United  States,  both  in  the  East  and  in 
the  West, 
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  used  in  mapping  the  original 
Huronian.  While  the  term  Laurentian  was  applied  to  the  pre-Huro- 
nian  rocks  on  the  north  shore  of  lake  Huron  and  about  lake  Superior, 
it  was  recognized  by  a  number  of  geologists  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  an- 
cient rocks  by  Dana  in  1872.  This  term  rapidly  grew  in  favor.  By  its 
use  not  only  the  advantage  of  a  theory  of  origin  was  avoided,  but  in 
common  with  Primitive  and  Azoic  it  was  not  necessary  to  subdivide  the 
ancient  rocks  into  Laurentian  and  Huronian,  and  thus  imply  a  corre- 
lation with  the  rocks  of  other  regions.  In  the  early  rapid  work  of  the 
Far  West,  detailed  observations  usually  stopped  at  the  base  of  the  fos- 
siliferous  series,  and  it  was  convenient  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  applied  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  it  was 
thought  by  many  that  the  rocks  once  supposed  to  be  destitute  of  life 
