460  ERE-CAMBRIAN   GEOLOGY   OE    NORTH   AMERICA. 
by  the  following  facts:  It  is  a  plagioclase  gabbro;  it  cuts  across  the 
Laurentian  schists;  it  holds  as  inclusions  blocks  of  gneiss;  about  its 
masses,  forming  girdles,  are  many  characteristic  contact  belts.  The 
areas  of  anorthosite  are  isolated  and  lie  along  the  border  of  the  Ar- 
chean  continent  of  that  time,  exactly  as  the  volcanoes  of  to-day  are 
along  the  continental  borders.  In  the  great  interior  area  of  Lauren- 
tian no  anorthosite  has  been  found.  The  formation  is  all  pre-Cam- 
brian,  as  shown  by  the  fact  that  it  lies  unconf ormably  below  the 
Cambrian ;  and  it  received  its  metamorphism  and  was  deeply  eroded 
also  before  the  Cambrian  was  deposited.  Its  relations  to  the  Huronian 
have  not  been  determined,  but  it  probably  does  not  belong  to  the  Huro- 
nian period,  but  rather  to  the  closing  part  of  the  Laurentian. 
The  several  regions  of  anorthosite  are  described  separately,  that  of 
Morin  and  Saguenay  being  most  fully  considered.  The  Morin  area 
is  surrounded  by  the  Grenville  series.  In  the  Grenville  series  are 
interlaminated  limestones,  bands  of  which  can  be  traced  many  miles. 
In  many  places  there  are  thin  layers  of  the  gneiss  within  the  lime- 
stone. The  limestone  is  less  resistant  and  more  plastic  than  the 
gneiss.  As  a  result  of  folding,  the  bands  of  gneiss  have  been  broken 
up,  producing  irregular  banded  blocks,  which  are  isolated  in  the 
limestone  in  such  a  manner  as  to  give  rise  to  extraordinary  pseudo- 
conglomerates. 
The  Saguenay  region  is  of  great  size — 5,800  square  miles.  It  is 
surrounded  on  all  sides  by  the  orthoclase  gneiss,  or  Ottawa  gneiss. 
The  anorthosite  of  this  district  is  more  basic  than  that  of  the  Morin 
district,  the  plagioclase  being  in  many  places  labradorite  or  bytown- 
ite.  That  it  is  an  intrusive  is  shown  by  the  same  facts  as  in  the  Morin 
area. 
Ells,33  in  1893,  gives  a  description  of  the  Laurentian  of  the  Ot- 
tawa district.  A  reexamination  of  the  Trembling  Mountain  section 
shows  that,  instead  of  its  being  a  continuous  ascending  series,  there 
are  no  fewer  than  three  anticlines  and  their  corresponding  synclines, 
and  the  section  is  still  further  complicated  by  faults  of  very  con- 
siderable extent.  But  one  limestone  was  found — that  of  Trembling 
Lake,  and  this,  instead  of  being  interstratified  with  the  orthoclase 
gneiss,  is  in  the  form  of  a  syncline  overlying  this  gneiss.  This  lime- 
stone at  no  point  was  observed  to  be  more  than  50  feet  in  vertical 
thickness. 
In  the  region  between  the  anorthosite  area  and  Gatineau  River  the 
limestone  in  nearly  every  case  occupies  well-defined  synclines  sepa- 
rated by  anticlines  of  the  underlying  gneiss.  In  this  area  it  has  been 
found  impossible  to  trace  any  bands  of  limestone  to  any  considerable 
distance  continuously,  the  limestones  being  often  local  in  development 
and  lenticular  in  form. 
