INTRODUCTION.  13 
committee  of  the  United  States  Geological  Survey  on  nomenclature 
and  classification  for  the  Geologic  Atlas  of  the  United  States, 
published  in  the  Twenty-fourth  Annual  Report,  1903.  We  shall 
speak  of  the  Algonkian  system,  the  Animikie  group,  the  Xegaunee 
formation.  The  Keweenawan  series  and  the  Huronian  series  collect- 
ively will  be  referred  to  as  the  Algonkian  system. 
Great  difficulty  has  been  encountered  because  of  the  unequal  value 
of  statements  of  fact  by  different  men.  Oftentimes  we  have  found 
it  impossible  to  discriminate  surely  between  good  and  poor  work 
because  we  were  not  familiar  with  the  region  described.  In  certain 
cases  in  which  reports  have  read  plausibly  an  examination  of  the 
purported  facts  in  the  field  with  the  accounts  in  hand  has  shown  the 
descriptive  parts  to  be  so  inaccurate  as  to  raider  the  conclusions, 
while  apparently  well  founded,  wholly  valueless.  Facts  and  theories 
may  be  so  inextricably  mingled  that  no  independent  judgment  can 
be  reached  as  to  the  correctness  of  the  conclusions,  and  often  the  facts 
of  a  report  can  not  be  used  even  by  one  personally  familiar  with  the 
districts  of  which  the  report  treats.  The  conclusions  of  another  class 
of  geologists  are  a  series  of  guesses,  which  generally  serve  no  pur- 
pose except  that  when  any  one  of  the  numerous  guesses  has  been 
established  by  the  patient  work  of  an  investigator  the  conclusion  is 
at  once  claimed  as  a  prior  discovery  of  the  guesser.  Sometimes  the 
discoveries  announced  by  a  writer  almost  or  quite  simultaneously 
are  wholly  inconsistent  with  one  another  and  with  the  facts  which 
are  described ;  for  as  with  other  men,  so  with  geologists,  many  opin- 
ions are  held  at  the  same  time  which  logically  are  exclusive  of  one 
another.  Still  another  group  of  writers  early  reach  a  general  theory 
as  to  the  definite  order  of  the  evolution  of  the  world.  A  person  of 
this  group  year  after  year  repeats  the  old  statements  and  conclusions 
without  any  reference  to  the  establishment  of  their  falsity.  More 
often  than  not  he  is  one  who  has  done  little  or  no  systematic  detailed 
field  work  in  any  region.  All  facts  and  conclusions  which  bear  in 
his  direction  are  hailed  as  discoveries,  while  every  adverse  fact  or 
conclusion  is  explained  out  of  existence  or  dismissed  as  unworthy  of 
consideration. 
By  following  continuously  the  summaries  of  the  writings  of  a 
geologist  who  has  been  long  at  work  in  a  region  it  will  generally  not 
be  difficult  to  get  a  fairly  accurate  idea  of  the  value  of  the  work  (lone. 
In  many  instances  later  work  on  the  pre-Cambrian  areas  of  North 
America  has  naturally  been  so  much  more  full  and  accurate  than 
earlier  work  that  it  raises  the  question  whether  the  earlier  work 
should  be  summarized  in  a  book  of  this  sort.  Especially  is  this  true 
where  many  of  the  rocks,  early  included  in  Archean  or  ancient  meta- 
morphic  terranes,  have  subsequently  been  found  to  be  Cambrian  or 
later.     In  the  present  edition  some  of  the  summaries  printed  in  Bui 
