476  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
the  greatest  limestone  developments  of  any  age.  A  representative 
section  shows  that  53.3  per  cent  of  the  series  is  limestone.  The  be- 
lief is  reaffirmed  that  the  Hastings  series  passes  over  imperceptibly 
into  the  more  altered  Grenville  series.  Within  the  Hasting  series,  as 
also  probably  in  the  "  Grenville  "  phase  of  the  same  rocks,  there  are 
two  series  which  are  petrographically  identical.  These  series,  when 
worked  out,  should  be  distinguished  by  specific  names,  as  upper  and 
lower  Grenville,  or  one  may  be  termed  the  "  Madoc  series."  It  is  con- 
jectured that  the  series  should  be  correlated  with  the  Huronian  rather 
than  the  Keewatin,  and  the  increase  in  the  amount  of  limestone  to- 
ward the  east  side  of  the  Lake  Superior  pre-Cambrian  is  cited  as 
bearing  on  this  question,  but  it  is  concluded  that  the  Grenville  series 
may  be  a  distinct  entity,  separate  from  either  and  differing  in  age 
from  both. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
The  districts  of  the  region  to  be  considered  are  the  original  Lau- 
rentian,  the  Hastings,  and  outlying  districts. 
ORIGINAL    LAURENTIAN    DISTRICT. 
The  original  Laurentian  district  may  be  considered  to  cover  the 
broad  area  of  pre-Cambrian  rocks  which  has  been  closely  studied 
north  of  Ottawa  and  St.  Lawrence  rivers,  between  the  cities  of  Ottawa 
and  Montreal.  Identical  in  lithology  and  closely  related  in  struc- 
ture is  the  area  north  and  east  to  beyond  St.  Maurice  River,  included 
in  the  area  covered  by  the  two  western  sheets  of  the  Eastern  Town- 
ships map  of  the  Canadian  Geological  Survey. 
A  granitoid  gneiss,  called  the  Ottawa  gneiss  or  "  Fundamental 
gneiss,"  occupies  the  larger  part  of  the  area.  It  is  red  or  gray  in 
color.  It  varies  in  coarseness,  in  relative  proportion  of  constituent 
minerals,  and  in  distinctness  of  foliation.  It  is  poor  in  the  iron-bear- 
ing minerals,  but  bands  of  hornblende  gneiss  and  amphibolite  are  not 
uncommon.  Other  schistose  rocks  are  occasionally  found.  Contained 
in  this  gneiss  are  considerable  areas  of  intrusive  material,  of  which 
granite  and  syenite  are  two  of  the  most  important  kinds.  The  gneiss 
contains  no  material  which  is  demonstrably  of  sedimentary  origin, 
or  even  of  surface  origin,  or  any  material  which  suggests  meta- 
morphosed sedimentary  or  surface  volcanic  material.  Its  lithological 
characteristics  are  those  of  modified  plutonic  rocks.  It  shows,  both 
in  the  field  and  under  the  microscope,  evidence  of  profound  orogenic 
movements.  It  has  no  structure  but  foliation,  or,  when  more  than 
one  variety  of  rock  is  present,  a  banded  structure.  The  vast  thick- 
nesses which  have  been  given  for  the  gneiss  are  based  upon  the  sup- 
position that  the  foliation  is  bedding. 
