420  PRE-CAMBRTAN    GEOLOGY   OF    NORTH    AMERICA. 
dikes  with  the  belts  of  rock  between  them.  The  greenstone  dikes  are 
never  found  to  traverse  the  overlying  Silurian,  and  it  is  supposed 
that  these  valleys  were  mostly  formed  before  the  deposition  of  the 
Paleozoic  strata.  It  is  thought  that  the  larger  part  of  this  Archean 
area  never  received  any  of  the  Paleozoic  rocks  upon  it,  and  that  the 
surface  of  the  Archean  had  been  reduced  to  something  like  its  present 
level  and  aspect  before  the  Paleozoic  deposits  were  laid  down.  As 
evidence  of  this  are  regarded  outliers  of  the  Potsdam  sandstone 
and  Black  River  limestone  filling  similar  narrow  valleys. 
Gibson,37  in  1895,  gives  a  summary,  from  the  reports  of  the  Ca- 
nadian Geological  Survey,  of  the  pre-Cambrian  geology  of  the  Hin- 
terland of  Ontario. 
Blue,38  in  1896,  sketches  the  geological  history  of  the  New  Ontario, 
which  includes  that  part  of  the  province  of  Ontario  lying  beyond 
Matawan  and  French  rivers  and  Nipissing,  Huron,  and  Superior 
lakes  to  the  north  and  west  boundaries  of  the  province.  Laurentian 
and  Huronian  rocks  form  highlands  which  in  Archean  time  were  the 
most  important  physical  feature  of  North  America,  sweeping  in  a 
curve  through  what  is  known  in  our  time  as  the  regions  of  Labrador, 
Quebec,  Ontario,  and  the  Northwest  Territories.  While  there  are 
large  areas  in  which  eruptive  masses  of  granite  and  gneiss  have  pene- 
trated the  Huronian  rocks  and  thrown  them  into  folds,  proving  their 
later  age,  in  general  the  reverse  is  the  case,  the  Huronian  resting  un- 
conformably  upon  the  Laurentian  and  being  of  later  origin.  The 
Huronian  is  overlain  unconformably  by  Cambrian  rocks,  under  the 
Cambrian  being  included  Animikie,  Nipigon,  and  Potsdam  rocks. 
Bell,39  in  1898,  reports  on  the  geology  of  the  area  of  the  French 
River  sheet,  which  represents  the  country  around  the  north  end  of 
Georgian  Bay.  Huronian  rocks  occupy  the  northwest  corner  of  the 
area  and  Laurentian  rocks  all  the  rest  to  the  southeast. 
The  Laurentian  rocks  in  general  resemble  the  Grenville  series, 
which  belongs  to  the  upper  division  of  the  Laurentian.  They  consist 
of  red  and  gray  mica  and  hornblende  gneisses,  in  beds  which  can  be 
traced  with  regularity  for  considerable  distances,  together  with  coarse 
hornblende  and  mica  schists  and  bands  of  quartz  rock  with  schistose 
partings.  No  limestones  have  yet  been  found  among  those  rocks 
within  the  boundaries  of  this  area,  but  in  the  Parry  Sound  district, 
to  the  east,  among  similar  strata,  the  writer  has  traced  five  bands  of 
crystalline  limestone  like  those  of  the  Grenville  series.  The  gneisses 
are  distinctively  stratified  and  regularly  arranged  in  anticlinal  and 
synclinal  forms,  according  to  the  structural  laws  governing  stratified 
rocks;  the  average  angles  of  clip  are  not  steep,  and  in  general,  so  far 
as  their  texture  is  concerned,  the  gneisses  have  the  characters  of 
altered  sedimentary  deposits.  Cutting  the  granites  are  greenstone 
dikes,  with  an  east-west  direction. 
