468  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  I  pull.  86. 
sharp  folds,  or  stand  at  high,  angles,  they  are,  as  a  rule,  less  bent  about 
or  contorted  than  the  Laurentian.  The  total  volume  of  the  system  is 
very  great,  probably  not  tar  from  40,000  or  50,000  feet,  or  perhaps  even 
more. 
In  Canada,  as  far  as  our  investigations  have  gone,  the  two  systems 
appear  to  be  everywhere  conformable  to  each  other,  but  in  rocks  of 
such  ancient  date  and  which  have  undergone  such  profound  structural 
changes,  owing  to  pressure,  etc.,  affecting  alike  the  stratified  and 
unstratified  portions,  this  appearance  may  not  everywhere  indicate  a 
truly  conformable  sequence.  Both  sets  of  rocks  having  been  thrown 
by  lateral  pressure  into  sharp  folds,  standing*  at  high  angles  to  the 
horizon,  the  Huronian  often  appears  to  dip  under  the  older  Laurentian, 
but  this  is  merely  the  effect  of  overturning,  and  does  not  show  that  a 
part  of  the  Laurentian  is  newer  than  the  locally  underlying  Huronian. 
Notwithstanding  the  geographical  relations  of  the  two  sets  of  rocks, 
their  general  difference  in  character  and  composition  would  indicate 
that  some  great  change  in  terrestrial  conditions  had  occurred  when  the 
formation  of  the  one  system  ended  and  that  of  the  other  began.  In  the 
Laurentian  an  "acid"  or  siliceous  composition  prevails,  whereas  the 
Huronian  rocks  as  a  whole  are  more  basic,  chemically  speaking.  The 
latter  can  be  shown  to  be  very  largely  of  volcanic  origin,  although  this 
may  not  always  be  obvious  at  first  sight. 
The  term  Huronian  is  made  to  include  all  the  rocks  lying  between 
the  Laurentian  below  it  and  the  Cambrian  or  earliest  fossiliferous  rocks 
above.  Among  the  areas  placed  with  the  Huronian  are  the  Keewatin 
and  similar  rocks.  In  the  Huronian  are  numerous  areas  of  northern 
Canada,  and  perhaps  certain  of  the  rocks  of  Hastings  and  Lanark 
counties,  some  of  the  crystalline  rocks  of  the  Eastern  Townships  and 
the  provinces  of  New  Brunswick,  Nova  Scotia,  cape  Breton,  and  New- 
foundland. In  the  Cambrian  system  arc  placed  in  ascending  order 
the  Animikie,  Nipigon,  and  Potsdam  formations.  Between  the  Huro- 
nian and  Cambrian  is  a  great  unconformity.  Between  the  Animikie 
and  Nipigon  and  the  Nipigon  and  Potsdam  are  probable  unconformi- 
ties. 
Walcott,22  in  1890,  gives  a  full  account  of  the  Lower  Cambrian  or 
Olenellus  zone. 
The  base  of  the  Olenellus  zone  is  considered  to  be  where  the  genus 
Olenellus,  or  the  fauna  usually  accompanying  it,  first  appears;  beneath 
that  horizon  the  strata  are  referred  to  some  of  the  pre-Cambrian  groups 
of  rocks.  In  some  cases  the  underlying  rocks  are  in  layers,  conformably 
beneath  the  Cambrian,  and  no  physical  separation  of  the  two  groups  is 
possible.  In  other  instances  the  subjacent  rocks  are  the  remains  .of  the 
old  Archean  continent,  near  the  shores  of  which  much  of  the  life  of  this 
portion  of  the  Cambrian  period  existed. 
The  line  of  demarcation  between  the  Cambrian  and  pre-Cambrian  may 
be  considered  (1),  at  the  base  of  the  Olenellus  zone,  in  continuous  sec- 
