546  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
plex  is  in  all  essential  respects  like  that  described  as  occurring  in  the 
original  Huronian,  original  Laurentian,  and  Lake  Superior  areas. 
All  the  varieties  of  rocks  found  in  these  areas  occur  here.  It  appears 
probable  that  the  orthoclase  gneiss,  which  is  the  predominant  rock 
in  the  original  Laurentian  district,  also  predominates  in  the  vast 
northern  region. 
At  many  places  within  the  expanse  of  Archean  are  relatively  small 
belts  mapped  as  "  Huronian  "  on  the  Canadian  maps.  These  consist  of 
much-altered  sedimentary  rocks,  such  as  quartzite,  graywacke,  con- 
glomerate, limestone,  etc.,  and  of  igneous  greenstones  and  granites 
and  a  variety  of  crystalline  schists,  hornblendic,  micaceous,  and 
chloritic,  most  of  them  undoubtedly  derived  from  the  alteration  of 
sediments  and  igneous  rocks.  Similar  assemblages  of  rocks  in  the 
Lake  Superior  region  have  been  found  to  belong  to  four  great  divisions 
separated  by  unconformities  and  referred  to  the  Keewatin,  lower 
Huronian,  middle  Huronian,  and  upper  Huronian.  The  so-called 
"  Laurentian  "  granites  and  gneisses  are  in  large  part  intrusive  into 
the  "  Huronian,"  but  it  is  apparent  that  the  term  "  Laurentian  "  covers 
granites  of  different  ages,  varying  from  as  old  as  the  Archean  to  late 
Huronian.  Indeed,  so  far  as  the  pre-Keweenawan  series  are  con- 
cerned, the  past  classifications  of  the  rocks  of  the  Great  Northern 
region  into  "  Laurentian  "  and  "  Huronian  "  means  simply  a  litho- 
logical  division  between  light-colored  rocks  such  as  granites  and 
gneisses  on  the  one  hand  and  gray  and  green  sediments  and  green- 
stones on  the  other  hand.  This  classification  does  not  signify  struc- 
tural or  age  relations  beyond  the  fact  that  the  rocks  are  older  than  the 
Keweenawan  and  usually  the  Animikie. 
Overlying  the  rocks  mapped  as  "  Laurentian  "  and  "  Huronian  "  in 
several  areas  are  so-called  "  Cambrian  "  sediments,  distinguished  from 
the  rocks  called  "  Huronian  "  by  their  less  altered  and  less  folded  con- 
dition and  by  association  with  fewer  intrusives.  Some  of  these  series 
of  rocks  are  similar  to  the  upper  Huronian,  while  others  are  very  like 
the  Keweenawan  rocks  of  the  Lake  Superior  region. 
For  the  most  part  it  is  impossible  to  give  any  definite  lithological 
or  structural  account  of  the  numerous  districts  in  which  "  Lauren- 
tian," "  Huronian,"  and  "  Cambrian  "  rocks  have  been  mentioned  as 
occurring,  but  a  few  of  the  principal  features  may  be  noted.  In  so 
doing  it  will  be  necessary  to  use  the  terms  "  Laurentian,"  "  Huronian," 
and  "  Cambrian  "  with  the  broad  significance  given  them  on  the 
Canadian  maps. 
Bell  emphasizes  as  the  distinguishing  feature  of  the  geology  in 
the  southern  part  of  Baffin  Land  the  great  abundance,  thickness,  and 
regularity  of  limestones  associated  with  gneisses,  which  he  correlates 
with  the  Grenville  series. 
