70  PRE-CAMBRIAN"    GEOLOGY   OF    NORTH    AMERICA. 
stetter,  after  an  exhaustive  study  of  the  old  gneiss  of  Bavaria  and 
Bohemia,  enunciated  its  identity  with  the  Laurentian  terrane  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  gneisses,  at  times  granitoid,  with  quartzites,  often 
conglomerates,  amphibolic  and  micaceous  schists,  pyroxenic  rocks, 
ophiolites,  and  limestones,  sometimes  magnesian.  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  thickness  of  this  series  on  the  Ottawa  exceeds  20,000 
feet,  which  is  probably  far  from  representing  the  total  volume  of  the 
system,  which  in  Bavaria  is  supposed  to  attain  not  less  than  00,000 
feet.  In  Hastings  County,  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  calcareous  schists,  and  terminating  in  a  heavy  mass 
of  dioritic  rocks.  Tt  seems  established  that  this  series,  which  differs 
sensibly  by  the  succession  of  the  beds  and  by  its  lithological  charac- 
ters from  that  described  above,  belongs  also  to  the  Lower  Laurentian, 
of  which  it  would  form  a  higher  member ;  and  thus  the  known  thick- 
ness of  this  system  in  Canada  would  rise  to  at  least  40,000  feet. 
The  Upper  Laurentian  or  Labradorian  terrane  is  found  resting, 
in  the  form  of  patches,  unconformably  on  the  Lower  Laurentian, 
both  on  the  Hastings  series  and  on  the  Ottawa  series,  where  it  often 
occupies  a  width  of  several  miles.  It  is  found  at  internals  from  Lake 
Huron  to  the  coasts  of  Labrador,  and  is  everywhere  recognized  by  its 
lithological  characters.  This  Labradorian  terrane  inclosed  gneiss 
with  orthose,  with  quartzites  and  crystalline  limestones,  but  its  pre- 
dominating element  is  an  anorthosite,  or  rock  composed  essentially  of 
a  feldspar  of  the  sixth  system,  with  a  mixture  of  pyroxene,  often  as- 
suming the  form  of  hypersthene.  This  anorthosite  is  sometimes, 
gneissoid,  and  even  fine  grained ;  but  it  assumes  rather  often  a  gran- 
itoid structure,  with  great  cleavable  forms  in  the  feldspar.  The  latter 
is  ordinarily  andesite  or  labradorite,  of-  which  it  sometimes  presents 
fine  opalescent  varieties  resembling  those  brought  from  Labrador. 
The  thickness  attained  by  the  Upper  Laurentian  terrane  is  not  certain, 
but  it  probably  exceeds  10,000  feet.  The  Lower  Laurentian  presents 
nothing  that  resembles  the  anorthosites  of  the  Upper  Laurentian, 
which  form  the  highest  summits  of  the  Adirondacks,  and  seems  to  be 
identical  with  the  hypersthenites  of  the  Hebrides  of  Scotland,  de- 
scribed by  MacCulloch.  The  limestones  of  the  Lower  Laurentian  of 
Canada  inclose  organic  remains,  principally  belonging  to  an  organism 
