20  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86, 
The  abstracts  given  have  the  defects  of  all  summaries — a  certain 
amount  of  inaccuracy,  because  all  modifying  and  qualifying  facts  can 
not  be  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  each  other. 
So  far  as  possible,  in  the  summaries,  the  original  language  of  the 
author  is  used,  although  a  single  sentence  of  the  summary  may  be  taken 
from  several  sentences  of  the  original.  Where  the  ideas  can  be  con- 
veyed in  a  briefer  manner  than  in  the  original  language  other  words 
are  used.  No  quotations  are  made;  for  the  ideas  contained,  whether 
in  the  original  language  or  not,  are  wholly  the  ideas  of  the  author — the 
whole  is  in  fact  really  quoted.  It  might  be  thought  that  better  results 
would  have  been  reached  by  indicating  through  quotations  what  words 
are  taken  from  the  original,  but  this  method  would  have  necessitated 
an  unpleasant  and  constant  alternation  from  quoted  to  nonquoted 
phrases.  It  would  have  made  it  much  more  difficult  to  convey  briefly 
the  thoughts  of  the  original ;  for  the  words  which  are  adapted  to  com- 
plete expositions  are  often  not  the  best  adapted  to  a  resume.  At  first 
the  plan  of  quoting  was  followed,  but  this  was  abandoned,  because  it 
was  seen  that  carrying  it  out  would  add  greatly  to  the  size  of  this 
volume  without  enhancing,  if  indeed  it  did  not  diminish,  the  accuracy 
and  comprehensiveness  of  the  review. 
Due  proportion  should  be  maintained  between  the  abstracts  of  the 
various  writings.  Frequently  a  short  article  contains  much  more  of 
structural  importance  than  one  of  far  greater  length,  although  the 
longer  article  may  contain  much  of  interest  which  does  not  come  within 
the  scope  of  the  paper.  Into  the  summaries  the  editor  enters  only  in 
so  far  that  he  must  of  necessity  take  what  appears  to  him  important 
and  omit  what  appears  unimportant.  Undoubtedly  in  this  respect  many 
mistakes  are  made;  future  investigations  will  show  that  omitted  facts 
and  conclusions  have  greater  importance  than  now  appears;  but  a  per- 
fectly rnoportioned  summary  could  be  made  only  by  perfect  knowledge. 
The  necessarily  brief  summaries  will  perhaps  serve  the  purposes  of 
those  who  are  interested  in  the  general  stratigraphy  of  the  pre-Oam- 
brian.  They  will  not  answer  for  those  who  wish  to  understand  in 
detail  the  structure  of  any  given  region.  For  this  local  details  are  nec- 
essary. 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  hypothesis.  But  even  for  these  special  purposes 
it  is  hoped  this  volume  may  be  found  sufficiently  full  to  be  useful,  and 
it  certainly  will  assist  in  directing  to  the  important  literature. 
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  conclu- 
