QUEBEC    NORTH    AND    WEST    OF    ST.    LAWRENCE    RIVER.  451 
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mica  and  hornblende.  The  gneisses  appear  to  attain  several  thou- 
sand feet  in  thickness,  but  are  divided  at  unequal  intervals  by  horn- 
blende and  mica  schist,  in  which  the  stratification  is  more  distinct. 
The  quartzites  are  in  considerable  volume,  two  layers  of  which,  nearly 
pure,  have  thicknesses,  one  of  400  and  the  other  of  600  feet.  The 
masses  of  limestone  are  generally  very  crystalline  and  coarse  grained, 
but  sometimes  are  saccharoidal.  The  bands  of  limestone  are  some- 
times of  great  thickness.  They  are  usually  not  pure,  but  contain 
many  other  minerals,  among  which  are  very  frequently  mica  and 
graphite.  Among  the  rarer  minerals  is  chondrodite.  The  iron  ore, 
which  is  mostly  magnetite,  is  interstratified  with  or  not  far  removed 
from  the  limestone  bands.  Associated  with  the  limestones  are  dol- 
omites, which,  however,  compose  distinct  beds. 
There  is  not  any  special  order  in  the  masses,  but  beds  of  hornblende 
rock  and  hornblende  schist  are  more  abundant  near  the  interstratified 
bands  of  limestone  than  elsewhere,  and  in  the  same  neighborhood 
there  is  usually  a  more  frequent  repetition  of  beds  of  quartzite  than 
in  other  parts.  Garnet  is  sometimes  disseminated  in  the  micaceous 
and  hornblendic  gneiss  and  quartzite,  but  is  commonly  confined  to  the 
immediate  proximity  of  the  limestones.  The  limestones  and  gneiss 
beds  as  a  whole  are  generally  conformable  to  them.  It  often  happens 
that  a  subordinate  layer  of  gneiss  will  display  contortions  of  the  most 
complicated  description.  Notwithstanding  the  highly  crystalline  con- 
dition of  the  Laurentian  rocks,  beds  of  unmistakable  conglomeratic 
character  are  occasionally  met  with.  These  generally  occur  in  the 
quartzite  or  micaceous  beds.  The  intrusives  of  the  Laurentian  con- 
sist chiefly  of  syenite  and  greenstones.  The  greenstone  dikes  are 
always  interrupted  b}^  the  syenite  when  they  have  been  found  to  come 
in  contact  with  it,  and  the  latter  is  therefore  of  posterior  date.  A 
mass  of  intrusive  syenite  occupies  an  area  of  about  36  square  miles  in 
the  townships  of  Grenville,  Chatham,  and  Wentworth.  It  is  cut  and 
penetrated  by  masses  of  a  porphyritic  character,  which  are  therefore 
of  a  still  later  date. 
The  Laurentian  series  stretches  on  the  north  side  of  the  St.  Law- 
rence from  Labrador  to  Lake  Huron,  and  occupies  by  far  the  larger 
portion  of  Canada.  Its  strata  probably  have  very  great  thickness. 
To  determine  the  superposition  of  the  various  members  of  such  an 
ancient  series  is  a  task  which  has  never  yet  been  accomplished.  The 
difficulties  attending  it  arise  from  the  absence  of  fossils  to  character- 
ize its  different  members.  Bands  of  crystalline  limestones  are  easily 
distinguished  from  the  bands  of  gneiss,  but  it  is  scarcely  possible  to 
know  from  local  inspection  whether  any  mass  of  limestone  in  one 
part  is  equivalent  to  a  certain  mass  in  another  part.  They  all  resem- 
ble one  another  lithologically.  The  dips  avail  but  little  in  tracing  out 
the  structure,  for  in  numerous  folds  in  the  series  the  dips  are  over- 
