94  PKE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
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  Town- 
ships and  the  provinces  of  New  Brunswick,  Nova  Scotia,  Cape 
Breton,  and  Newfoundland.  In  the  Cambrian  system  are  placed  in 
ascending  order  the  Animikie,  Nipigon,  and  Potsdam  formations. 
Between  the  Huronian  and  the  Cambrian  is  a  great  unconformity. 
Between  the  Animikie  and  the  Nipigon  and  between  the  Nipigon  and 
the  Potsdam  are  probable  unconformities. 
Walcott,23  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;  be- 
neath that  horizon  the  strata  are  referred  to  some  of  the  pre-Cam- 
brian  groups  of  rocks.  In  some  cases  the  underlying  rocks  are  in 
layers,  conformably  beneath  the  Cambrian,  and  no  physical  separa- 
tion 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  the  pre-Cam- 
brian  may  be  considered  (1)  at  the  base  of  the  Olenellus  zone,  in  con- 
tinuous sections;  (2)  at  the  line  of  an  unconformable  contact  between 
any  member  of  the  Cambrian  group  and  the  subjacent  Algonkian  or 
Archean;  (3)  at  the  line  of  unconformable  contact  which  is  the  base 
of  the  Olenellus  zone. 
Placed  in  the  Algonkian  under  this  definition  are  11,000  feet  of 
quartzites  conformably  below  the  Olenellus  in  the  Wasatch;  10,000 
feet  of  argillites,  sandstones,  quartzites,  and  conglomerates  conform- 
ably beneath  the  Olenellus  in  British  Columbia ;  12,000  feet  of  sand- 
stones, shales,  and  limestones  unconformably  beneath  the  lowest 
known  Cambrian  in  the  Grand  Canyon  of  Colorado;  a  similar  series 
of  rocks  unconformably  beneath  the  Cambrian  in  Llano  County, 
Tex.;  a  series  unconformably  below  the  Upper  Cambrian  in  the 
Adirondacks;  and  the  rocks  of  St.  Marys  and  Placentia  Bay,  New- 
foundland, which  are  unconformably  below  Lower  Cambrian  strata. 
In  the  Grand  Canyon,  in  a  bed  of  dark  argillaceous  shale  3,550  feet 
from  the  summit  of  the  section,  was  found  a  small  patelloid  or  dis- 
cinoid  shell  and  a  fragment  of  what  appears  to  be  the  pleural  lobe  of 
a  segment  of  a  trilobite,  also  an  obscure,  small  Hyolithes  in  a  layer 
of  bituminous  limestone.  In  layers  of  limestone  lower  in  the  section 
an  obscure  stromatoporoid  form  occurs  in  abundance.  These  fossils 
indicate  a  fauna,  but  do  not  tell  what  it  is. 
