12  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
given,  and  an  undue  amount  of  emphasis  in  the  conclusions.  In 
regions  in  which  much  work  has  been  done  these  defects  are  not  so 
serious  as  in  little  studied  regions,  for  in  the  former  the  observations 
of  independent  observers  confirm  or  neutralize  one  another.  Into 
the  summaries  the  editors  enter  only  in  so  far  that  they  must  of 
necessity  take  what  appears  to  them  important  and  omit  what  ap- 
pears unimportant.  Undoubtedly  in  this  respect  many  mistakes 
are  made,  and  future  investigations  will  show  that  omitted  facts 
and  conclusions  have  greater  importance  than  now  appears. 
When  the  same  writer  repeats  facts  and  conclusions  in  several 
articles  summary  is  made  of  the  most  comprehensive  article  and 
references  are  made  to  the  others  in  the  footnotes.  By  giving  the 
entire  summary  of  the  literature  of  one  district  before  taking  up 
another,  epitomes  of  parts  of  a  single  paper  are  necessarily  dis- 
sociated. By  this  method  something  of  correlation  is  lost;  but 
purely  general  work  is  summarized  in  the  general  chapter  (Chapter 
I),  and  the  subject  of  correlation  is  there  treated. 
References  to  literature  are  given  at  the  ends  of  the  chapters,  the 
reference  notes  having  continuous  numbers.  In  the  discussions 
closing  sections  or  chapters  and  in  the  general  chapter  citations  are 
not  repeated.  The  original  source  of  any  statement  attributed  to 
an  author  may  always  be  found  by  the  aid  of  the  index,  where  the 
name  of  each  author  is  followed  by  references  to  the  pages  where 
his  work  is  summarized. 
In  the  discussion  the  aim  has  been  not  to  call  attention  to  all  that 
seems  to  be  erroneous,  but  first  to  point  out  where  there  is  harmony 
between  the  different  authors,  often  veiled  because  terms  are  used 
with  different  significations,  and,  second,  to  note  the  important  con- 
clusions which  have  been  clearly  determined.  Statements  and  con- 
clusions with  which  the  writers  do  not  agree  are  in  general  not  criti- 
cised, nor  is  any  refutation  attempted,  unless  the  point  at  issue  is 
fundamental  and  can  not  be  overlooked. 
The  map,  like  the  summaries,  is  a  resume  of  the  present  imperfect 
knowledge. 
The  necessarily  brief  summaries  will  perhaps  serve  the  purposes 
of  those  who  are  interested  in  the  general  stratigraphy  of  the  pre- 
Cambrian.  They  will  not  answer  for  those  who  wish  to  understand 
in  detail  the  structure  of  any  given  region.  As  the  summaries  are 
not  made  with  reference  to  upholding  any  theory,  they  of  necessity 
will  fail  to  give  all  the  facts  which  bear  upon  any  particular  hy- 
pothesis. But  even  for  these  special  purposes  it  is  hoped  this  volume 
may  be  found  sufficiently  full  to  be  useful,  and  it  certainly  will  be 
serviceable  in  directing  students  to  the  important  literature. 
The  terms  "  system,"  "  series,"  and  "  formation  "  are  used  with 
the  stratigraphic  significance  given  them  in  the  report  of  the  special 
