92  PRE-CAMBRIAN    GEOLOGY   OF  ^NORTH   AMERICA. 
on  the  surface  of  the  globe,  and  of  certain  progress  in  the  building 
up  of  the  aqueous  deposits  which  has  been  going  on  ever  since. 
Bell,22  in  1890,  gives  a  general  account  of  the  Archean.  The  Azoic 
or  Archean  period  is  divided  into  the  Laurentian  and  Huronian  sys- 
tems, into  which  the  primitive  rocks  of  all  countries  may  be  classi- 
fied, and  which  everywhere  are  essentially  the  same  and  retain  the 
same  relative  positions.  In  some  instances  newer  rocks  have  been  so 
altered  locally  or  over  considerable  tracts  as  to  resemble  the  Azoic, 
but  there  is  generally  found  some  means  of  distinguishing  between 
them.  In  Canada  and  the  United  States  the  Laurentian  and  Hu- 
ronian are  usually  intimately  associated,  but  their  lithological  feat- 
ures and  internal  characters  are  sufficiently  distinct  to  separate  them. 
The  Huronian  rocks  are  less  contorted  or  corrugated  on  the  small 
scale  than  the  Laurentian,  but  on  the  large  scale  they  partake  of  the 
same  foldings  which  have  affected  the  latter.  The  Huronian  rocks 
seem  to  be  interwoven  with  the  Laurentian  as  basins  or  troughs  more 
or  less  elongated,  and  as  tracts  of  angular  and  other  forms  filling- 
spaces  between  great  nuclei  or  rounded  areas  of  Laurentian  rocks. 
The  Laurentian  system  is  divided  into  two  formations,  the  lower 
of  which  is  sometimes  called  the  Primitive  Gneiss  series.  It  consists 
essentially  of  obscurely  foliated  or  stratiform  granitic  or  syenitic 
gneiss.  The  prevailing  colors  of  the  Lower  Laurentian  gneiss  are 
grayish  and  reddish.  In  some  districts  the  Laurentian  rocks  are 
cut  by  dikes  of  greenstone  or  trap.  In  the  Upper  Laurentian  are 
placed  both  the  anorthosite  rocks  and  the  limestone-bearing  series  of 
eastern  Ontario.  The  anorthosites,  which  are  considered  by  some  as 
eruptives  and  by  others  as  bedded  rocks  interlaminated  with  the  lime- 
stones, may  be  in  part  of  both  origins.  Anorthosites,  after  spreading 
out  upon  the  surface  of  the  earth  or  the  bottom  of  the  sea,  may  have 
become  incorporated  in  a  conformable  manner  with  the  contem- 
poraneous deposits,  while  others  may  have  flowed  over  preexisting 
rocks  which  were  not  disturbed.  Between  the  Upper  and  Lower 
Laurentian  there  may  be  a  general  want  of  conformity.  The  Upper 
Laurentian  contains  metallic  ores  and  very  numerous  minerals,  which 
are  not  found  in  the  Lower  Laurentian.  The  gneisses  of  the  Upper 
and  Lower  Laurentian  often  have  a  close  resemblance.  As  the 
evidence  is  so  strongly  in  favor  of  the  aqueous  origin  of  a  part  of 
the  Upper  Laurentian  at  least,  this  lends  support  to  the  view  that 
even  the  primitive  gneisses  may  have  been  formed  by  the  action  of 
water  during  some  early  condition  of  the  earth,  of  which  we  can  form 
but  little  conception,  judging  by  the  later  stages  of  its  history. 
Eozoon  is  regarded  as  a  myth,  and  the  limestones,  iron  ores,  graphite, 
and  apatite  are  not  considered  as  evidence  of  the  existence  of  animals 
or  plants  in  Laurentian  times.  The  limestones  may  be  chemical  sedi- 
ments; the  graphite  and  apatite  occur  principally  as  vein  matter; 
