462 
PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA. 
[bull.  86. 
The  following  table  shows  alternative  arrangement  suggested  by  the 
above : 
Systems. 
Paleozoic 
Agnotozoic  or  Eparchean. 
Archean 
(rl'OUpS. 
C  Carboniferous 
)  Devonian 
i  Silurian 
[  Cambrian 
(  Keweenawan 
<  Huron ian 
(  (Otber  groups ?) 
Laurentian  (including  Upper  Laurentian). 
Systems. 
>  Paleozoic. 
J        . 
Archean. 
Hunt,19  in  1888,  summarizes  the  results  of  his  work  on  the  arrange- 
ment, subdivision,  and  nomenclature  of  the  pre-Cambrian  terranes,  as 
follows : 
(1)  Laurentian.  Under  this  name,  proposed  and  adopted  by  the 
author  in  1854,  is  comprised  the  old  gneissic  terrane  foAnd  especially  in 
the  Laurentide  and  Adirondack  mountains,  as  well  as  in  the  great 
Atlantic  chain  and  in  the  Eocky  mountains  of  the  center  of  North 
America.  To  this  same  series  the  author  has  also  annexed  the  similar 
gneisses  oY  Great  Britain  and  Scandinavia,  as  well  as  the  old  or  centf  al 
gneiss  of  the  Alps.  From  the  time  of  our  first  studies  in  Canada,  in 
1847,  we  had  pointed  out  the  existence,  in  this  gneissic  terrane,  of  two 
subdivisions,  one  lower,  of  granitoid  gneiss  which  blends  with  the  fun- 
damental granite,  to  which  succeeds  with  unconformable  stratification 
a  series  of  gneisses  also  granitoid,  frequently  amphibolic,  interspersed 
with  quartzites  and  crystalline  limestones,  with  serpentine.  These  two 
subdivisions,  which  we  may  provisionally  name  Lower  and  Upper  Lau- 
rentian, have  been  called  respectively  the  Ottawa  gneiss  and  the  Gien 
ville  series.  In  order  to  avoid  all  error  it  is  necessary  to  note  that  the 
title  of  Upper  Laurentian  was  for  some  time  given  by  Logan  to  the  ter 
rane  designated  afterward  as  Labradorian  and  Norian.  It  is  therefore 
through  misunderstanding  that  some  have  wished  to  retain  as  a  desig- 
nation of  the  upper  division  of  the  Laurentian  terrane  the  term  Middle 
Laurentian. 
(2)  Norian.  The  terrane  thus  designated  by  the  author  in  1870  is  in 
large  part  composed  of  those  stratified  rocks  with  an  anorthic  feldspar 
base,  to  which  the  name  norite  has  been  given.  This  terrane,  however, 
includes  intercalated  beds  of  gneiss,  quartzite,  and  crystalline  lime- 
stone, all  being  rather  similar  to  those  of  the  Upper  Laurentian  terrane. 
These  norites,  which  have  sometimes  been  designated  by  the  name 
gabbro,  must  not  be  confounded  with  the  very  distinct  gabbros  of  the 
Huronian  terrane,  nor  with  certain  plutonic  rocks,  to  which  they  bear 
mineralogic  resemblances.  The  facies  of  the  norites  serves  to  distin- 
guish them. 
(3)  Arvonian.  This  terrane  is  composed  in  large  part  of  petrosiliceous 
