QUEBEC    NORTH    AND    WEST    OF    ST.    LAWRENCE    RTVER.  465 
Petrographically  the  rocks  of  the  district  are  found  to  fall  into  four 
classes : 
1.  Anorthosites  and  granites  of  igneous  origin.  All  gradations 
may  be  seen  between  the  ordinary  anorthosite  and  that  in  which  the 
whole  is  granulated  so  as  to  resemble  in  appearance  a  saccharoidal 
marble.  The  whole  rock  thus  moved  under  pressure  like  so  much 
dough,  its  continuity  being  perfectly  maintained.  This  is  Professor 
Heim's  "  Umformung  ohne  Bruch,"  millions  of  little  cracks  taking 
the  place  of  a  few  larger  ones,  and  it  is  by  this  process  that  granites 
and  many  gneisses  and  other  crystalline  rocks,  when  deeply  buried 
under  great  pressure  and  probably  very  hot,  move  and  accommodate 
themselves  to  stresses.  This,  it  will  be  observed,  is  quite  distinct  and 
different  from  the  shearing  accompanied  by  the  development  of  new 
materials,  which  takes  place  under  other  conditions  and  probably 
nearer  the  surface. 
2.  Augen  gneisses,  leaf  gneisses,  granulites,  and  foliated  anortho- 
sites, genetically  connected  with  the  last  group,  and  largely,  if  not 
exclusively,  o.f  igneous  origin  also.  The  structural  characteristic  of 
this  class  is  the  cataclastic  or  granulated  one,  formed  by  the  me- 
chanical breaking  down  of  the  web  of  the  rock  under  movements  in- 
duced by  great  pressure,  which  movements  produce  in  the  rock  a 
foliation  more  or  less  distinct,  according  to  their  intensity. 
3.  A  series  of  crystalline  limestones  and  quartzites,  together  with 
certain  gneisses  usually  found  associated  with  them.  In  these  rocks 
the  granulated  structure  is  very  subordinate  or  entirely  absent.  They 
are  characterized  by  very  extensive  recrystallization  with  the  devel- 
opment of  new  minerals.  These  minerals  have  crystallized  under  the 
influence  of  the  pressure  which  granulated  the  gneisses  of  the  second 
class,  and  are  not  in  any  marked  manner  deformed  by  it.  These 
gneisses  also  differ  from  the  granites  and  gneisses  of  classes  1  and  2 
in  chemical  composition,  giving  analyses  almost  identical  with  those 
of  slates.  Moreover,  the  rocks  of  this  class  are  very  frequently 
graphitic,  and  analyses  show  that  the  gneisses  correspond  in  chemical 
composition  more  closely  with  slates  than  with  granites. 
4.  Pyroxene  gneisses,  pyroxene  granulites,  and  allied  rocks  whose 
origin  is  as  yet  doubtful. 
With  regard  to  the  Grenville  series,  from  the  presence  of  numerous 
and  heavy  beds  of  limestone  and  quartzite,  their  prevalent  banded 
character,  the  widespread  occurrence  of  graphite,  and  the  fad  thai 
the  gneisses  associated  with  the  limestones  and  quartzites  have  (lie 
composition  of  sands  and  muds  and  not  of  igneous  rocks,  it  i^  con- 
cluded that  it  is  extremely  probable  that  this  is  an  altered  sedimen 
tary  series  which  has  been  deeply  buried,  invaded  by  great  masses 
of  igneous  rocks,  and  recrystallized.     In  places  the  Grenville  sedi 
55721— Bull.  360—09 30 
