van  hisb.]  DISCUSSIONS    OF    PRINCIPLES.  115 
zones  of  limestone,  each  of  sufficient  volume  to  constitute  an  inde- 
pendent 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  conform  ably  followed  by  man  y  more.  Althou  g  1 1 
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  Laurentian 
limestone  met  with  in  other  parts  of  the  province  are  equivalent  to 
any  of  the  three  zones,  or  whether  they  overlie  or  underlie  them  all. 
As  evidence  of  life  in  the  Laurentian  limestone,  are  graphite,  great 
beds  of  iron  ore,  and  the  presence  of  recognizable  organic  forms  resem- 
bling Strom  atopora. 
Hunt,7  in  1867,  again  characterizes  the  Laurentian  and  Huronian 
rucks. 
Under  the  name  of  Laurentian  terrain,  the  Geologic  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  ancien  t 
gneissic  system  of  North  America,  the  Geologic  Commission  of  Canada 
gave  it  the  name  of  Laurentian  system,  taken  from  the  Laurentide 
mountains.  As  early  as  1855  the  conviction  was  expressed  that  it  is 
identical  with  the  primitive  gneiss  of  European  countries,  an  iden- 
tity which  afterwards  was  established  by  Murchison  for  Scotland. 
More  recently,  Gumbel  and  von  Hochstetter,  after  an  exhaustive 
study  of  the  old  gneiss  of  Bavaria  and  Bohemia,  enunciated  its  iden- 
tity with  the  Laurentian  terrain  of  Canada,  a  conclusion  which  the 
former  of  these  scientists,  moreover,  supported  by  a  comparison  of  the 
organic  remains  of  the  two  regions. 
The  Lower  Laurentian  is  composed  of  crystalline  schists,  a  large  part 
of  which  are  gneiss,  at  times  granitoid,  with  quartzites  often  conglom- 
erates, amphibolic  and  micaceous  schists,  pyroxenic  rocks,  ophiolites 
and  limestones  sometimes  magnesiau.  These  limestones,  ordinarily 
very  crystalline,  are  found  united  in  three  great  distinct  formations, 
each  having  a  mean  volume  of  1,000  to  1,500  feet,  and  separated  by  still 
more  considerable  masses  of  gneiss  and  quartzite.  The  measured  thick- 
ness of  this  series  on  the  Ottawa  exceeds  20,000  feet,  which  is  prob- 
ably far  from  representing  the  total  volume  of  the  system,  which,  in 
Bavaria,  is  supposed  to  attain  not  less  than  90,000  feet.  In  Hastings 
county,  to  the  north  of  lake  Ontario,  there  is  found  resting  conformably 
on  Laurentian  gneiss  a  series  of  at  least  20,000  feet  of  crystalline 
schists,  comprising  a  great  thickness  of  impure  limestones  and  calcare- 
ous schists^  and  terminating  in  a  heavy  mass  of  dioritic  rocks,     it 
