446  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
seems  established  that  this  series,  which  differs  sensibly  by  the  suc- 
cession of  the  beds  and  by  its  lithological  characters  from  that  described 
above,  belongs  also  to  the  Lower  Laurentian,  of  which  it  would  form  a 
higher  member;  and  thus  the  known  thickness  of  this  system  in  Canada 
would  rise  to  at  least  40,000  feet. 
The  Upper  Laurentian  or  Labradorian  terrain  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  intervals  from  lake 
Huron  to  the  coasts  of  Labrador,  and  is  everywhere  recognized  by  its 
lithological  characters.  This  Labradorian  terrain  inclosed  gneiss  with 
orthose,  with  quartzites  and  crystalline  limestones,  but  its  predomi- 
nating element  is  an  anorthosite,  or  rock  composed  essentially  of  a 
feldspar  of  the  sixth  system,  with  a  mixture  of  pyroxene,  often  assum- 
ing the  form  of  hyper sthene.  This  anorthosite  is  sometimes  gneissoid, 
and  even  fine  grained;  but  it  assumes  rather  often  a  granitoid  structure, 
with  great  cleavable  forms  in  the  feldspar.  The  latter  is  ordinarily  ancle- 
site  or  labradorite,  of  which  it  sometimes  presents  fine  opalescent 
varieties  resembling  those  brought  from  Labrador.  The  thickness 
attained  by  the  Upper  Laurentian  terrain  is  not  certain,  but  it  prob- 
ably exceeds  10,000  feet.  The  LoAver  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,  described  by  MacGulloch. 
The  limestones  of  the  Lower  Laurentian  of  Canada  inclose  organic 
remains,  principally  belonging  to  an  organism  studied  and  described 
by  Dawson,  who  has   given  it  the  name  ofEozoon  canadense. 
The  Lower  Laurentian  terrain  is  affected  by  many  undulations  that 
have  upraised  the  beds,  rendering  them  at  times  almost  vertical.  The 
mean  direction  of  these  foldings  is  about  north  and  south,  but  sec- 
ondary undulations  from  east  to  west  appear  in  the  region  north  of  the 
Ottawa,  the  only  one  where  thus  far  it  has  been  possible  to  study  the 
intimate  structure  of  this  terrain.  The  beds  of  the  Upper  Laurentian 
also  are  upraised  at  high  angles,  but  the  structure  of  this  terrain,  which 
has  evidently  undergone  part  of  the  movements  that  affected  the  lower, 
has  not  yet  been  studied.  The  lower  terrain  is  traversed  in  several 
localities  by  igneous  rocks,  and  there  have  been  ascertained  at  least 
four  epochs  of  effusion,  three  of  which  are  anterior  to  the  Silurian 
period.  These  eruptive  rocks  are  syenites,  quartziferous  porphyries, 
and  dolerites. 
Under  the  name  of  Huronian  terrain  is  designated  a  series  of  rocks, 
more  or  less  altered,  resting  unconformably  on  the  Lower  Lauren- 
tian terrain,  and  probably  also  on  the  Labradorian  terrain.  This 
series  is  composed  of  quartzites,  of  more  or  less  chloritic  or  epidotic 
schists,  sometimes  with  impure  serpentines,  and  with  diorites,  which 
constitute  very  important  masses  in  the  series.    The  quartzites,  as  well 
