452  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  fBuix.Sfi. 
tered;  chiastolite,  andalusite,  garnet,  mica,  and  other  minerals  ap- 
pearing 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  explained  and 
understood  if  we  regard  them  as  completely  metamorphosed  portions 
of  the  strata  which  now  surround  them;  while  the  mere  displacement 
of  strata  involved  in  the  intrusive  theory  appears,  in  view  of  the  enor- 
mous area  now  occupied  by  the  granite,  wholly  inexplicable,  as  does 
also  the  manner  in  which  the  surrounding  strata  often  dip  down  against 
and  on  to  the  granite  and  show  no  signs  of  having  been  deflected  or 
otherwise  affected  as  regards  strike  and  dip  by  the  supposed  intrusions. 
There  is,  however,  often  seen  along  the  contact  lines  of  the  granite 
and  the  slates  a  considerable  breaking  up  and  crushing  of  the  latter, 
and  this  has  been  held  to  indicate  and  be  the  result  of  the  intrusion 
of  the  granite.  It  appears  to  be  mainly  due  to  the  unequal  resistance 
that  the  two  rock  masses  have  offered  to  the  disturbing  forces  of  up- 
heaval, depression,  and  consequent  pressure  which  have  repeatedly 
affected  them  long  after  the  formation  of  the  granite.  The  effect  thus 
produced  is  analogous  to  that  which  occurs  where  the  forces  producing 
slaty  cleavage  encounter  interstratified  hard  layers  of  sandstone,  when 
the  elsewhere  perfectly  regular  and  parallel  cleavage  planes  are  imme- 
diately crushed,  crumpled,  and  deflected. 
In  regions  where  the  granite  or  other  hard  crystalline  rock  is  older 
than  the  adjacent  or  alternating  softer  strata,  perfectly  similar  contact 
lines  may  be  seen,  but  unaccompanied  by  any  change  in  the  mineralog 
ical  character  of  the  adjacent  strata,  such  as  occurs  when  the  crystal- 
line rock  is  the  youngest;  and  therefore  this  phenomenon  can  not  be 
taken  as  conclusive  evidence  of  the  intrusive  origin  of  granite  or  other 
crystalline  rock. 
Selwyn,13  in  1883,  remarks  that  the  Devonian  granite-forming  epoch 
has  had  immense  influence  in  the  pre-Carboniferous  rocks  of  the  region  to 
the  southeast  of  the  great  St.  Lawrence,  Champlain,  and  Hudson  river 
break.  This  is  certainly  deserving  of  more  careful  consideration  and 
study  than  it  has  yet  received,  and  more  especially  so  in  connection  with 
the  alteration  and  metamorphism  it  has  i>roduced  in  large  areas  of 
Paleozoic,  and  perhaps  pre-Paleozoic  rocks.  When  these  altered  Paleo- 
zoic strata  come  in  contact,  as  they  often  do  in  eastern  Canada  and  in 
New  England,  with  the  more  ancient  Huronian  and  Laurentian  gneiss, 
granite,  mica-schist  and  other  crystalline  rocks,  it  is  only  possible  to 
distinguish  them  or  to  define  their  respective  limits  by  the  most  careful 
and  minute  strati  graphical  work,  such  as  the  nature  of  the  regions  in 
