76  PRE-CAMBK1AN    GEOLOGY   OF    NORTH    AMERICA. 
fined  between  Lake  Matapedia  and  Cape  Maquereau,  in  Gaspe;  (5) 
the  Cape  Breton,  Nova  Scotia,  and  New  Brunswick  pre-Primordial 
subcrystalline  and  gneissoid  groups.  III.  Cambrian:  In  many  of 
the  areas,  especially  the  western  ones,  the  base  of  this  is  well  defined 
by  unconformity,  but  in  the  Eastern  Townships  and  in  some  parts  of 
Nova  Scotia  it  has  yet  to  be  determined.  The  limit  between  it  and 
Lower  Silurian  is  debatable  ground.  One  point  is  particularly  in- 
sisted on,  that  great  local  unconformities  and  lithological  differences 
may  exist  without  indicating  any  important  difference  in  age,  espe- 
cially in  regions  of  mixed  volcanic  and  sedimentary  strata,  and  that 
the  fact  of  crystalline  rocks  (greenstones,  diorites,  dolerites,  felsites, 
norites,  etc.)  a ppearing  as  stratified  masses  and  passing  into  schistose 
rocks  is  no  proof  of  their  not  being  of  eruptive  or  volcanic  origin; 
their  present  metamorphic  or  altered  character  is,  as  the  name  im- 
plies, a  secondary  phase  of  their  existence,  and  is  unconnected  with 
their  origin  or  original  formation  at  the  surface,  but  is  due  partly  to 
original  differences  of  composition  and  partly  to  the  varying  physical 
accidents  to  ivhich  they  have  since  their  formation  been  subjected. 
Selwyn,12  in  1881,  states  that  the  anorthosite  rocks  are  in  general 
conformity  with  the  crystalline  limestones,  but  are  occasionally  inter- 
fered with  and  disturbed  by  intrusions  (?)  of  the  more  massive  and 
granitoid  variety  of  labradorite.  This  is  proof  that  the  labradorite 
or  Norian  rocks  of  Hunt  do  not  constitute  an  unconformable  Upper 
Laurentian  formation,  but  occur  in  part  as  unstratified  intrusive 
masses  and  in  part  as  interstratifications  with  the  orthoclase  gneisses, 
quartz itos,  and  limestones  of  the  Laurentian  system,  as  developed  in 
the  Grenville  region  and  mapped  by  Logan. 
As  to  the  granites  which  have  been  regarded  as  intrusive  by  Logan, 
in  both  the  crystalline  and  the  fossiliferous  rock,  there  is  no  doubt 
that  they  are  of  later  origin  than  the  Silurian  rocks  which  surround 
them  and  which  are  everywhere,  on  approaching  the  granite,  consid- 
erably altered,  chiastolite,  anclalusite,  garnet,  mica,  and  other  minerals 
appearing  in  the  slates,  which  are  also  occasionally  changed  to  quartz- 
ose  or  feldspathic  mica  schists,  and  the  associated  fossiliferous  lime- 
stone to  crystalline  and  micaceous  dolomites,  with  the  fossils  still  per- 
fectly distinct.  It  has  been  customary  and  orthodox  to  regard  these 
granites  as"  intrusive,"  and  they  are  so  designated  by  Logan.  The 
author  holds  that  there  is  absolutely  no  proof  of  their  being  so,  either 
in  the  Eastern  Townships,  in  Nova  Scotia,  or  in  Australia,  and  that 
all  the  phenomena  connected  with  them  may  be  more  readily  ex- 
plained and  understood  if  we  regard  them  as  completely  metamor- 
phosed portions  of  the  strata  which  now  surround  them;  while  the 
mere  displacement  of  strata  involved  in  the  intrusive  theory  appears, 
in  view  of  the  enormous  area  now  occupied  by  the  granite,  wholly 
inexplicable,  as  does  also  the  manner  in  which  the  surrounding  strata 
