PREFACE.  1 7 
The  labor  involved  iu  abstracting-  the  pre-Cambrian  literature  of 
North  America  has  been  great.  This  is  shown  by  the  large  number  of 
books  and  articles  actually  summarized.  These,  however,  give  an  im- 
perfect idea  of  the  volume  of  literature  covered;  for  very  numerous 
articles  have  been  examined  which  repeat  what  had  already  been  sum- 
marized from  other  papers.  Also  a  vast  amount  of  material  from  which 
nothing  is  taken  has  been  looked  through,  in  order  to  ascertain  whether 
it  contained  anything  which  ought  to  be  considered.  In  all  cases 
summaries  are  made  from  the  original  articles,  with  the  exception  of 
a  part  of  the  section  upon  northern  Canada.  Nearly  the  entire  litera- 
ture of  the  geology  of  this  region  has  recently  been  clearly  compiled  by 
Br.  George  M.  Dawson,  and  from  this  compilation  the  major  part  of  the 
summary  for  this  region  is  taken. 
To  a  card  catalogue  of  the  Appalachians  by  N.  H.  Darton,  to  Azoic 
Eocks  and  other  works  by  Hunt,  to  the  Minnesota  reports,  to  the  Azoic 
System  by  Whitney  and  Wads  worth,  to  living's  Copper-bearing  Eocks, 
and  also  to  many  other  works  the  writer  is  much  indebted  as  furnishing 
guides  to  the  pre-Cambrian  literature.  In  this  way  Barton's  catalogue 
and  the  Azoic  System  have  been  by  far  the  most  valuable.  After  in- 
dependently preparing  abstracts  of  papers  and  reports  I  have  compared 
them  with  the  abstracts  contained  in  the  Azoic  System  in  order  to  dis- 
cover omissions,  and  this  book  has  thus  enabled  me  to  make  the  sur- 
vey of  literature  more  nearly  complete  than  it  otherwise  would  have 
been.  To  a  certain  extent  other  books  have  been  used  in  the  same 
fashion. 
Mr.  W.  N.  Merriam  has  drawn  all  the  maps  for  the  illustration  of  the 
volume.  To  Mr.  George  E.  Luther  the  writer  is  indebted  for  most  effi- 
cient clerical  assistance  from  the  outset,  without  which  it  would  have 
been  impossible  to  complete  this  volume  within  the  time  allotted. 
To  Sir  Archibald  Geikie,  Br.  Hans  Eeusch,  Br.  K.  A.  Lossen,  Michel- 
Levy,  and  Br.  Johannes  Lehmann  I  am  indebted  for  summary  statements 
of  the  condition  of  knowledge  with  reference  to  the  pre-Cambrian  of 
Great  Britain,  France,  Germany  and  Scandinavia. 
With  Messrs.  C.  B.  Walcottj  G.  H.  Williams,  Bailey  Willis,  and  E,  W. 
Ells  I  have  been  in  the  field  and  in  consultation,  and  from  them  have 
received  much  useful  information. 
Mr.  G.  K.  Gilbert  has  kindly  read  the  manuscript,  and  has  made  many 
suggestions  which  have  been  of  value. 
To  the  late  Prof.  Roland,  B.  Irving,  and  to  Prof.  Raphael  Pumpelly  I 
am  indebted  in  a  peculiar  manner.  With  the  former  I  was  associated 
in  work  from  my  earliest  studies  in  geology  until  his  death.  With 
the  latter  I  have  been  much  in  the  field  for  the  last  two  seasons  and 
have  received  many  pregnant  ideas.  What  part  of  the  thoughts  con- 
tained come  from  these  two  friends  I  am  unable  to  specify  in  detail,  but 
I  am  conscious  that  the  debt  is  a  heavy  one. 
Bull.  86 2 
