468  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
series,  characterized  by  the  presence  of  crystalline  limestones,  quartz- 
ites,  and  gneisses  of  sedimentary  origin  with  a  banded  structure, 
called  the  Grenville  series;  and  a  lower  series  of  gneisses  in  which 
no  limestone,  etc.,  occur  and  which  possess  a  foliated  rather  than  a 
banded  structure,  known  as  the  Fundamental  gneiss.  Grenville 
rocks  are  recognized  south  of  Rawdon  and  in  the  westerly  portion  of 
the  St.  Sauveur  district.  The  Fundamental  gneiss  apparently  occu- 
pies much  of  the  St.  Jerome  district.  However,  it  has  been  found 
impossible  to  separate  the  two  series  and  delimit  them  on  the  map. 
The  composition  of  most  if  not  all  of  the  gneisses  belonging  to 
the  Fundamental  gneiss  can  be  paralleled  among  the  igneous  rocks, 
and  it  is  concluded  that  many  of  these  gneisses,  at  least,  were  of 
igneous,  probably  of  intrusive,  origin.  In  the  Grenville  also  some 
of  the  gneisses  are  of  igneous  origin.  However,  many  are  believed 
to  be  of  sedimentary  origin,  for  the  following  reasons:  (1)  They 
are  associated  with  numerous  and  heavy  beds  of  limestones  and  quart- 
zite;  (2)  they  have  a  prevailing  banded  character,  accompanied  by 
very  extensive  recrystallization ;  (3)  graphite  is  of  frequent  occur- 
rence in  them;  (4)  chemical  analyses  show  that  they  have  the  com- 
position, not  of  igneous  rocks,  but  of  sedimentary  sands  and  muds. 
The  quartzite  is  sometimes  pure,  but  frequently  holds  garnet,  silli- 
manite,  or  other  minerals.  The  limestones  are  coarsely  crystalline 
marbles,  sometimes  pure,  but  at  other  times  including  grains  of 
quartz,  pyroxene,  phlogopite,  graphite,  and  other  minerals. 
The  anorthosite  belongs  to  the  gabbros,  but  is  characterized  by  a 
great  preponderance  of  plagioclase  feldspar,  which  is  often  so  abun- 
dant as  to  make  up  the  entire  rock.  At  its  contact  with  the  gneisses 
are  many  contact  phases.  The  anorthosite  has  been  squeezed  and 
foliated,  together  with  the  gneisses  which  it  cuts,  and  it  is  concluded 
that  its  intrusion  antedated  at  least  the  termination  of  the  great 
earth  movements  which  affected  the  Laurentian  in  pre-Potsdam  time. 
In  proportion  as  the  anorthosites  exhibit  granulation  they  become 
light  colored,  some  of  the  most  metamorphosed  ones  resembling 
marble  in  appearance,  although  chemically  they  do  not  differ  from 
the  less  modified  anorthosites. 
On  the  upturned  edges  of  the  Archean  rocks,  both  gneiss  and 
anorthosite,  the  Potsdam  sandstone -and  other  Cambro-Silurian  rocks 
repose  in  flat  and  undisturbed  beds. 
Adams,41  in  1897,  describes  the  Laurentian  granitoid  gneiss  and 
granite  of  the  Admiralty  group  of  the  Thousand  Islands,  Ontario. 
The  granitoid  gneiss  is  presumably  derived  by  metamorphism  from 
the  granite.  A  large  exposure  of  crystalline  limestone  on  Island  No. 
18  resembles  in  all  respects  that  of  the  Grenville  series  of  the  main- 
land adjacent. 
