466  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
author,  appear  very  clearly  from  the  recent  studies  of  Roininger.  Ou  the 
various  crystalline  terranes,  including  the  Taconian,  there  rests  in  this 
region  unconformably  an  enormous  series  of  sandstones  and  conglom- 
erates, with  contemporary  plutonic  rocks,  the  whole  being  remarkable 
for  its  content  of  metallic  copper.  This  series,  which  had  been  alter- 
nately confounded  with  the  Huronian  and  Taconian  terranes  on  the  one 
hand  and  with  the  trilobitic  sandstones  of  the  Cambrian  on  the  other, 
was  for  the  first  time  separated  by  the  author  in  1873  under  the  name 
of  Keweenaw  group,  a  term  which  he  in  1876  converted  into  that  of 
Keweenian  terrane.  It  still  remains  to  be  determined  whether  this 
series,  on  which  these  same  trilobitic  sandstones  rest  unconformably, 
should  form  part  of  the  Cambrian  terrane  or  whether  it  should  form  a 
distinct  terrane  between  the  Taconian  and  the  Cambrian. 
Bell,20  in  1889,  characterizes  the  Huronian  as  the  great  metallifer- 
ous series  of  Canada.  While  rocks  of  igneous  origin  constitute  a 
marked  feature  in  the  Huronian  system,  a  large  proportion  of  it  is  made 
up  of  those  of  an  undoubted  sedimentary  character.  On  the  other 
hand,  it  is  questionable  if  the  great  bulk  of  the  Lauren  tian  rocks  can 
be  proved  to  have  been  deposited  from  water.  It  is  supposed  by  many 
that  the  foliation  of  much  of  the  gneiss  may  have  been  produced  by 
pressure  and  some  kind  of  flowing  movement  in  an  igneous  mass. 
Whatever  view  we  may  take  of  the  origin  of  the  common  Laurentian 
gneiss,  which  forms  the  surface  of  the  country  over  such  a  vast  extent 
of  the  Canadian  half  of  North  America,  the  commencement  of  the  Hu- 
ronian period  marks  a  great  change  which  then  came  over  the  earth — 
a  change  characterized  by  widespread  volcanic  outbursts  and  by  evi- 
dence of  the  existence  of  water  (perhaps  the  first)  on  the  surface  of  the 
globe,  and  of  certain  progress  in  the  building  up  of  the  aqueous  de- 
posits which  lias  been  going  on  ever  since. 
Bell,21  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  classified, 
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  Huronian  are  usually 
intimately  associated,  but  their  lithological  features  and  internal  char- 
acters are  sufficiently  distinct  to  sex>arate  them.  The  Huronian  rocks 
are  less  contorted  or  corrugated  on  the  small  scale  than  the  Lauren- 
tian, 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 
