PRE-CAMBRIAN  GEOLOGY  OF  NORTH  AMERICA.0 
By  C.  K.  Van  Hise  and  C.  K.  Leith. 
INTRODUCTION. 
Our  purpose  is  to  give  an  account  of  the  present  state  of  knowl- 
edge of  the  general  structure  and  correlation  of  the  pre-Cambrian 
of  North  America.  This  book  is  not  primarily  a  bibliography  of 
pre-Cambrian  literature,  nor  a  petrography  of  the  pre-Cambrian 
rocks,  nor  a  treatment  of  metamorphism,  nor  an  account  of  economic 
facts.  Petrography,  metamorphism,  and  economic  geology  are/con- 
sidered only  so  far  as  they  have  a  direct  bearing  upon  structural 
results,  and  then  the  substance  of  the  established  conclusions  is 
given  rather  than  the  facts  upon  which  they  are  based  and  the  man- 
ner of  reaching  them. 
The  material  contained  in  the  volume  consists  of  a  general  account 
of  the  pre-Cambrian  rocks,  a  summary  of  all  articles  or  parts  of 
articles  which  have  contributed  knowledge  upon  the  subject  con- 
sidered, summaries  of  the  conclusions  which  appear  to  be  established, 
and  discussion  of  correlation  of  different  regions. 
In  the  summaries,  so  far  as  practicable,  the  original  language  of 
the  author  is  used,  although  a  single  sentence  of  the  summary  may  be 
taken  from  several  sentences  of  the  original.  If  the  ideas  can  be 
conveyed  in  a  briefer  or  clearer  manner  than  the  original  language 
conveys  them  other  words  are  used.  No  quotation  marks  are  em- 
ployed, for  the  ideas,  whether  in  the  original  language  or  not,  are 
wholly  the  ideas  of  the  author  whose  book  or  paper  is  summarized — 
the  whole  is  really  quoted.  It  may  be  thought  that  better  results 
would  have  been  reached  by  indicating  by  quotation  marks  what 
words  are  taken  from  the  original,  but  this  method  would  have 
necessitated  a  constant  and  unpleasant  alternation  from  quoted  to 
nonquoted  phrases,  and  would  have  made  it  much  more  difficult  to 
convey  briefly  the  thoughts  of  the  original,  for  language  adapted 
to  a  complete  exposition  is  often  not  the  best  for  a  resume. 
The  abstracts  have  the  defects  of  all  summaries— a  certain  amount 
of  inaccuracy  because  all  modifying  and  qualifying  acts  can  not  be 
"This  bulletin  is  a  revised  and  enlarged  edition  of  Bulletin  No.  8(5.  published  in   1892. 
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