vanhise.]  INTRODUCTION.  21 
sions  which  have  been  clearly  determined.  Statements  and  conclu- 
sions with  which  the  writer  does  not  agree  are  in  general  not  criti- 
cised nor  is  any  refutation  attempted,  unless  the  point  at  issue  is  one  of 
such  a  fundamental  character  that  it  can  not  be  overlooked. 
The  maps  are  in  all  cases  compiled  from  the  original  sources,  and,  like 
the  summaries,  have  whatever  excellence  or  defects  the  original  work 
has.  In  many  cases  the  editor  feels  assured  that  the  maps  will  need  to 
be  materially  modified,  but  he  has  not  the  detailed  knowledge  necessary 
to  make  the  modifications.  The  generalized  character  of  the  bounda- 
ries is  often  indicated  by  the  fact  that  they  are  straight  lines.  It  need 
not  be  said  that  true  geological  boundaries  are  not  of  this  nature  in 
much  disturbed  regions.  In  the  Appalachians  the  maps  merely 
outline  the  crystalline  rocks.  Much  of  the  area  included  is  known  to 
belong  to  the  Paleozoic,  but  it  is  impossible  either  accurately  to  separate 
these  areas  or  to  subdivide  the  pre-Cambrian.  All  that  can  be  done 
in  many  areas  is  to  indicate  that  the  rocks  are  pre- Cambrian,  although 
it  is  oftentimes  certain  that  Algonkian  and  Archean  rocks  both  occur, 
which  have  not  been  separated  in  mapping.  In  a  few  regions,  not  only 
can  the  Archean  and  Algonkian  be  discriminated,  but  the  latter  has  been 
subdivided  into  series  and  these  into  formations.  There  are  then  on 
the  maps  all  grades  of  knowledge,  from  the  eastern  United  States, 
where  the  pre-Cambrian  is  not  outlined,  to  areas  in  which  pre-Cam- 
brian series  are  divided  into  formations.  The  maps,  as  the  summaries, 
are  a  resume  of  the  present  imperfect  knowledge. 
The  order  in  which  the  districts  are  taken  up  is  not  consistently 
geographical  or  geological,  but  rather  the  order  of  the  development 
of  exact  knowledge  of  the  pre-Cambrian  rocks.  The  great  Canadian 
pre-Cambrian  area  in  all  its  parts  is  a  geological  unit,  yet  the  de- 
tailed study  of  it  has  been  confined  somewhat  closely  to  particular 
regions;  and  of  necessity  the  regions  which  have  been  units  of  work 
have  to  be  followed  rather  than  a  strictly  logical  order.  Chapter  I  is 
devoted  to  the  original  Laurentian  and  original  Huroni an  areas.  Chap- 
ter II  is  given  to  the  lake  Superior  region.  These  regions  are  taken 
first  because  the  exact  knowledge  of  the  pre-Cambrian  rocks  is  here 
greater  than  in  other  areas.  The  Appalachian  region,  although  tin' 
earliest  to  be  studied,  is  reviewed  last,  for  it  is  the  area  in  which  I  lie 
conditions  for  obtaining  exact  knowledge  are  the  least  favorable,  and 
about  which  comparatively  little  structural  knowledge  of  the  pre-Cam- 
brian has  been  acquired. 
In  the  summaries  of  the  individual  districts  the  order  is  that  of  ap- 
pearance of  the  papers.  By  giving  the  entire  summary  of  the  literati  ire 
of  one  district  before  taking  up  another,  epitomes  of  parts  of  a  single 
paper  are  necessarily  dissociated.  By  this  method  something  of  corre- 
lation is  lost;  but  purely  general  work  is  summarized  in  the  general 
chapter  and  the  subject  of  correlation  is  bere  treated. 
