QUEBEC    NORTH    AND    WEST    OF    ST.    LAWRENCE    RIVER.  477 
Within  this  great  Ottawa  gneiss  complex  are  comparatively  small 
areas,  with  prevailing  north-south  trend,  of  a  series  predominantly 
sedimentary  called  the  Grenville  series.  In  this  series  gneisses  are 
still  the  predominant  rocks,  but  they  present  a  greater  variety  of 
mineral  composition,  are  more  frequently  strongly  foliated,  and  often 
occur  as  well-defined  bands  or  layers,  like  strata  of  sedimentary  for- 
mations. Chemical  analyses  of  these  gneisses  by  Adams  show  that 
they  have  the  composition  of  shales  and  slates  rather  than  that  of  the 
Ottawa  gneiss;  in  particular,  the  former  contain  less  alkalies  than 
the  latter.  Adams  estimates  that  in  the  area  nearly  50  per  cent  of  the 
Grenville  consists  of  limestone.  From  this  fact  it  may  be  inferred 
that  a  considerable  part  of  the  adjacent  gneisses  are  sedimentary, 
because  no  known  normal  sedimentary  succession  would  contain 
limestones  in  such  amounts  over  a  large  area,  the  average  proportion 
of  limestone  to  other  sediments  derived  from  the  alteration  of 
average  igneous  rocks  being  much  less.  Interstratified  with  these 
gneisses  in  the  Grenville  are  hornblende  gneisses  and  schists,  beds  of 
quartz  schists,  and  thick  beds  of  crystalline  limestone  or  marble.  The 
gneisses  are  often  strongly  garnetiferous.  The  limestones,  and  some- 
times the  gneisses,  are  graphitic,  and  in  certain  places  are  so  rich 
in  magnetite  as  to  furnish  iron  ores.  The  folding  of  the  series  is 
complex,  the  contortions  of  the  gneisses  and  their  infoldings  with  the 
marbles  being  remarkably  intricate.  At  places  bands  of  gneiss  asso- 
ciated with  the  marble  have  been  disrupted  and  autoclastic  rocks 
produced.  In  many  places  these  breccias  closely  resemble  a  conglom- 
erate, especially  when  bands  of  gneiss  have  been  broken  within  a  mar- 
ble. Even  when  the  interstratified  gneiss  is  brecciated  the  marble  is 
often  perfectly  massive.  Since  in  certain  cases  thick  bands  of  gneiss 
have  resisted  pressure  without  fracturing,  while  other  belts  of  gneiss 
immediately  above,  interstratified  with  the  marble,  have  been  broken 
into  autoclastic  rocks,  it  has  been  suggested  that  the  breccias  are  basal 
conglomerates  and  that  there  is  a  structural  break  between  the  part 
of  the  series  bearing  the  marble  and  the  gneiss  below.  However,  in 
all  cases  in  which  close  study  has  been  made  these  pseudo-conglom- 
erates have  been  traced  step  by  step  into  the  ordinary  interlaminated 
set  of  gneisses,  schists,  and  marbles. 
According  to  Adams,  the  gneisses  of  the  Grenville  series  are  un- 
like the  Ottawa  gneiss  in  showing  less  granulated  structure,  but  are 
characterized  by  extensive  recrystallization,  with  the  development  of 
new  minerals. 
The  Grenville  series,  consisting  as  it  docs  of  regular  alternations 
of  marbles,  gneisses,  and  subordinate  quartz  schists,  bearing  graphite 
and  iron  ores,  is  believed  to  be  a  profoundly  metamorphosed  sedimen- 
tary series,  although  there  has  not  been  discovered  anywhere  in  the 
region  any  other  evidence  of  clastic  characters.     So  profound  have 
