458  PRE-CAMBR1AN    GEOLOGY   OP   NORTH   AMERICA. 
Adams,29  in  1889,  finds  that  the  massive  and  stratified  varieties  of 
the  anorthosite  in  that  part  of  the  Laurentian  area  lying  to  the  north 
of  the  Island  of  Montreal  are  really  only  different  portions  of  one  and 
the  same  mass.  It  is  concluded  that  most,  if  not  all,  of  the  areas  are 
of  eruptive  origin,  since  they  are  frequently  found  cutting  the 
gneisses.     Bands  of  crystalline  limestone  are  found  in  the  region. 
Low,30  in  1892,  describes  the  Archean  of  Portneuf,  Quebec,  and 
Montmorency  counties  in  Quebec.  These  rocks  cover  about  980  square 
miles,  and  are  covered  on  the  south  by  Cambro- Silurian  limestones 
and  shales.  A  rough  section  from  west  to  east  across  the  northern 
portion,  through  Lake  Simon,  at  right  angles  to  the  strike,  is  as 
follows : 
Section  through  Lake  Simon,  Quebec. 
Miles. 
1.  Dark    schistose    mica    gneiss,    interbanded    with    coarser 
red  and  gray  mica  gneisses 10 
2.  Fine-banded  gray,  pink,  and  red  mica  gneisses,  and  mica 
hornblende   gneisses 10 
3.  Dark-gray  garnetiferons  hornblende  gneiss 2 
4.  Fine-banded  gray,  pink,  and  red  mica  gneisses  and  mica 
hornblende   gneisses Ti 
5.  Dark-green,  basic,  crushed  granitic  gneiss lh 
6.  Coarse  red  and  gray  augen  gneiss 2\ 
7.  Fine-banded  gneiss  (2)  and   (4) 6 
8.  Coarse  red  and  gray  augen  gneiss G 
9.  Fine-banded  gray  and  pink  mica  gneiss 14 
10.  Anorthosite    2 
11.  Fine-banded  gray  and  pink  gneisses : . 12 
In  this  section  the  rocks  are  grouped  in  accordance  with  the  pre- 
dominating kind,  although  bands  of  other  varieties  are  included  in  all 
of  the  rough  divisions.  Divisions  1,  2,  3,  4,  9,  and  11  appear  to  have 
been  originally  clastic  rocks,  subsequently  completely  metamorphosed 
into  schists  and  gneisses  and  subjected  to  great  pressures,  which  have 
folded  and  twisted  them  so  that  their  original  horizontal  succession  is 
greatly  obscured.  The  different  bands  are  conformable  and  appear  to 
grade  into  one  another.  Division  5  embraces  rocks  probably  of  igne- 
ous origin,  which  have  been  injected  along  a  line  of  weakness  between 
the  banded  gneisses  and  the  coarser  grained  rocks  of  division  6. 
Divisions  6  and  8  are  usually  gneissic,  but  in  many  places  are  granitic. 
They  appear  to  underlie  the  banded  gneisses,  and  are  either  the  re- 
mains of  older  beds  that  have  been  re-fused  or  original  molten  mat- 
ter which  has  dissolved  and  floated  portions  of  the  banded  beds, 
since  fragments  of  them  are  inclosed  in  the  coarser  gneisses.  The 
anorthosite  is  also  igneous,  having  apparently  been  intruded  in  its 
present  position  after  the  formation  of  the  banded  gneiss  with  which 
it  is  in  contact.  This  contact  is  not  sharp  in  places,  as  the  gneisses 
usually  seem  to  have  been  infiltrated  by  basic  feldspar  material  from 
the  anorthosite,  causing  a  gradual  passage  from  one  to  the  other.    At 
