SUMMARY   OF   GENERAL   LITERATURE.  69 
The  united  thickness  of  these  three  great  series  may  possibly  far 
surpass  that  of  all  the  succeeding  rocks,  from  the  base  of  the  Paleozoic 
series  to  the  present  time.  We  arc  thus  carried  back  to  a  period  so 
remote  that  the  appearance  of  the  so-called  Primordial  fauna  may  be 
considered  a  comparatively  modern  event.  We,  however,  find  that, 
even  during  the  Laurentian  period,  the  same  chemical  and  mechanical 
processes  which  have  ever  since  been  at  work  disintegrating  and 
reconstructing  the  earth's  crust  were  in  operation.  In  the  conglom- 
erates of  the  Huronian  series  there  are  inclosed  bowdders  derived  from 
the  Laurentian,  which  seem  to  show  that  the  parent  rock  was  altered 
to  its  present  crystalline  condition  before  the  deposition  of  the  newer 
formation,  while  interstratified  with  the  Laurentian  limestones  are 
beds  of  conglomerate,  the  pebbles  of  which  are  themselves  rolled  frag- 
ments of  still  older  laminated  sand  rock,  and  the  formation  of  these 
beds  leads  us  still  further  into  the  past. 
In  both  the  Upper  and  the  Lower  Laurentian  series  there  are  sev- 
eral zones  of  limestone,  each  of  sufficient  volume  to  constitute  an  in- 
dependent formation.  Of  these  calcareous  masses  it  has  been  ascer- 
tained that  three,  at  least,  belong  to  the  Lower  Laurentian.  But  as 
we  do  not  as  yet  know  with  certainty  either  the  base  or  the  summit  of 
this  series,  these  three  may  be  conformably  followed  by  many  more. 
Although  the  Lower  and  Upper  Laurentian  rocks  spread  over  more 
than  200,000  square  miles  in  Canada,  only  about  1,500  square  miles 
have  yet  been  fully  and  connectedly  examined  in  any  one  district,  and 
it  is  still  impossible  to  say  whether  the  numerous  exposures  of  Lau- 
rentian limestone  met  with  in  other  parts  of  the  province  are  equiva- 
lent to  any  of  the  three  zones,  or  whether  they  overlie  or  underlie 
them  all.  As  evidence  of  life  in  the  Laurentian  limestone  are  graph- 
ite, great  beds  of  iron  ore,  and  recognizable  organic  forms  resembling 
Stromatopora. 
Hunt,7  in  1867,  again  characterizes  the  Laurentian  and  Huronian 
rocks. 
Under  the  name  Laurentian  terrane,  the  Geological  Commission  of 
Canada  at  first  comprehended  two  distinct  series  of  rocks,  one  resting 
unconformably  on  the  other,  which  it  afterward  distinguished  as 
Lower  Laurentian  and  Upper  Laurentian  or  Labradorian.  The  first 
of  these  two  series  corresponds  to  the  Primitive  gneiss  (Urgneiss)  of 
Scandinavia  and  of  the  west  coast  of  Scotland.  After  carefully 
studying  this  ancient  gneissic  system  of  North  America,  the  Geolog- 
ical Commission  of  Canada  gave  it  the  name  of  Laurentian  system, 
taken  from  the  Laurentide  Mountains.  As  early  as  1855  the  convic- 
tion was  expressed  that  it  is  identical  with  the  Primitive  gneiss  of 
European  countries,  an  identity  which  afterward  was  established  by 
Murchison  for  Scotland.     More  recently  Cumbel   and   Yon    Hoch- 
