SUMMARY    OF    GENERAL    LITERATURE.  77 
often  dip  down  against  and  onto  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  and1  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  im- 
mediately 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  1888,  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  pro- 
duced in  large  areas  of  Paleozoic  and  perhaps  pre-Paleozoic  rocks. 
When  these  altered  Paleozoic  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  crys- 
talline rocks,  it  is  possible  to  distinguish  them  or  to  define  their  re- 
spective limits  only  by  the  most  careful  and  minute  stratigraphical 
work,  such  as  the  nature  of  the  regions  in  New  England  and  in  the 
adjacent  provinces  of  Canada,  where  these  rocks  are  chiefly  developed, 
renders  almost  impossible — at  any  rate,  it  has  never  yet  been  at- 
tempted. Hence  the  maps  hitherto  published,  representing  the  geo- 
logical structure  of  these  regions,  have  necessarily  been  based  almost 
entirely,  so  far  as  the  crystalline  groups  are  concerned,  on  lithological 
and  mineralogical  comparisons  and  considerations,  producing  peno- 
logical rather  than  geological  maps,  and  as  a  consequence,  though  im- 
portant and  valuable  aids  to  future  investigation,  they  afford  a  very 
incorrect  and  imperfect  idea  of  the  true  geological  structure  and  the 
sequence  and  distribution  of  the  several  formations.  Unfortunately, 
while  careful,  patient,  and  minute  observation  in  the  field  has  beei 
