CHAPTER  L 
GENERAL  ACCOUNT   OF   THE   PRE-CAMBRIAN   ROCKS 
OF  NORTH  AMERICA. 
DEFINITION  OF  PRE-CAMBRIAN. 
The  base  of  the  Cambrian  as  defined  by  Walcott  is  marked  by  the 
beds  known  to  contain  Olenellus  fauna  or  their  equivalents.  The 
pre-Cambrian  as  used  in  this  bulletin  includes  all  rocks  below  the 
Olenellus  beds.  For  much  of  the  continent  an  unconformity  sepa- 
rates the  Olenellus  beds  from  underlying  rocks ;  but  in  some  parts  of 
the  continent  the  Olenellus  beds  appear  to  be  continuous  and  con- 
formable with  underlying  beds,  some  of  which  contain  no  fossils  and 
some  of  which  contain  a  few  fragmentary  fossils  earlier  than  the 
Olenellus. 
The  discrimination  and  relations  of  the  Cambrian  and  pre-Cam- 
brian are  discussed  on  subsequent  pages. 
MAJOR  DISTRIBUTION  AND   STRUCTURE  OF  PRE-CAMBRIAN   ROCKS. 
It  is  convenient  to  think  of  the  distribution  of  the  pre-Cambrian 
rocks  of  North  America  as  indicating  the  basal  architectural  fea- 
tures in  the  ground  plan  of  the  continent.  The  largest  area  of  pre- 
Cambrian  rocks  of  North  America  occupies  the  northeastern  portion 
of  the  continent,  extending  northwest,  north,  and  northeast  from 
Great  Bear  Lake,  Great  Slave  Lake,  Athabaska  Lake,  Lake  Winni- 
peg, the  Great  Lakes,  and  the  St.  Lawrence  system  to  the  Arctic  and 
Atlantic  oceans.  This  has  been  called  the  "  pre-Cambrian  shield," 
the  "  Laurentian  continent,"  and  the  "Archean  protaxis  "  of  North 
America. 
On  the  southeast  the  pre-Cambrian  shield  is  separated  by  a  narrow 
area  of  younger  rocks  from  the  pre-Cambrian  rocks  of  the  northern 
end  of  the  Appalachian  system  or  its  northeastern  extension.  Begin- 
ning in  Newfoundland  and  extending  into  Nova  Scotia  and  New 
Brunswick  and  southern  Quebec,  through  New  England  and  thence 
down  the  eastern  side  of  the  Appalachian  system  in  the  mountains 
and  in  the  Piedmont  region  to  Alabama,  are  considerable  areas  of 
pre-Cambrian  rocks  which  in  part  are  not  yet  satisfactorily  discrim- 
inated from  crystalline  rocks  of  later  age.  Detailed  study  of  this 
region  has  continually  reduced  the  estimated  exposures  of  pre-Cam- 
brian rocks.  Also  in  eastern  Canada  and  in  the  Adirondacks  of  New 
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