14  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
letin  No.  86,  which  do  not  refer  to  pre-Cambrian  rocks,  have  been 
condensed  or  omitted.  They  have  been  retained,  however,  if  the 
rocks  described  are  partly  pre-Cambrian  or,  in  a  few  cases,  if  the 
summary  serves  to  bring  out  clearly  the  development  of  views  of 
terranes  early  thought  to  be  pre-Cambrian  but  later  found  to  be 
wholly  or  in  part  post-Algonkian.  In  this  way  there  is  retained 
some  that  may  seem  useless,  irrelevant,  and  overemphasized  to  any- 
one looking  for  the  latest  and  best  information  on  a  pre-Cambrian 
area,  and  some  that  may  seem  useless  even  from  a  historical  stand- 
point. We  may  have  gone  to  an  extreme  in  emphasizing  the  histor- 
ical importance  of  early  work,  and  the  space  given  to  the  earlier 
work  may  be  out  of  proportion  to  later  and  more  accurate  studies. 
In  this  we  have  followed  our  best  judgment.  In  future  editions  of 
the  pre-Cambrian  correlation  papers  early  summaries  will  necessarily 
be  condensed  to  a  greater  extent  in  order  to  leave  room  for  proper 
treatment  of  later  work. 
In  the  preparation  of  the  first  edition  (Bulletin  No.  86),  the  senior 
author  visited  most  of  the  important  areas  of  the  pre-Cambrian  of 
North  America,  and  since  its  publication,  in  1892,  he  has  given  con- 
siderable time  to  the  study  of  the  ancient  crystalline  rocks  of  the 
southern  Appalachians  and  Piedmont,  of  Pennsylvania  and  Mary- 
land, of  New  England,  of  the  original  Huronian  and  Lake  Superior 
regions  and  their  northward  extensions,  and  of  the  Grand  Canyon  of 
Arizona  and  other  districts  of  the  Cordilleras.  The  junior  author 
has  studied  principally  the  Lake  Superior  and  Lake  Huron  regions 
and  their  extensions  to  the  north,  east,  and  west  in  Ontario.  He  has 
visited  also  pre-Cambrian  areas  in  Nova  Scotia,  Newfoundland,  New 
Brunswick,  Arizona,  Utah,  Colorado,  Wyoming,  and  Montana. 
The  present  edition  includes,  in  addition  to  the  historical  treat- 
ment, general  summaries  of  the  present  state  of  knowledge  of  the 
geology  of  each  of  the  principal  pre-Cambrian  areas,  without  special 
regard  to  the  history  of  the  development  of  this  knowledge  or  of 
nomenclature.  In  case  the  published  literature  would  not  yield  a 
satisfactory  connected  summary  of  the  present  state  of  knowledge 
regarding  an  area,  this  has  been  supplied  by  the  authors,  if  they  were 
sufficiently  familiar  with  the  ground,  or  by  others  who  had  especially 
studied  these  areas.  Our  acknowledgment  and  thanks  are  due  to 
Messrs.  Whitman  Cross  for  a  summary  of  the  pre-Cambrian  geology 
of  Colorado,  J.  E.  Spurr  and  Sydney  H.  Ball  for  the  Georgetown 
(Colorado)  quadrangle,  Walter  Harvey  Weed  for  Montana,  Walde- 
mar  Lindgren  for  Idaho,  J.  S.  Diller  for  California,  Arthur  Keith 
and  Thomas  L.  Watson  for  the  southern  Appalachians  and  Pied- 
mont, Florence  Bascom  and  E.  B.  Mathews  for  Pennsylvania  and 
Maryland,  George  Otis  Smith  and  William  H.  Hobbs  for  Massachu- 
