442  pre-oambkjajn   logics  of  north  America.         (zvik.M. 
the  time  when  life  appeared,  and  represents  a  period  vastly  longer  than 
the  Silurian,  although  it  may  occupy  a  less  superficial  area. 
Logan  and  Hunt,4  in  1855,  gave  a  geological  sketch  of  Canada.  The 
ancient  rocks  are  divided  into  the  Laurentian  system  and  the  Cam- 
brian and  Huronian  system. 
The  rocks  of  the  Laurentian  system  are  almost  without  exception 
old  sedimentary  beds  that  have  become  highly  crystalline;  they  have 
been  greatly  disturbed,  and  form  mountain  ranges  running  about  north- 
east and  southwest,  and  sometimes  rising  to  heights  of  800  or  1,000 
meters,  and  even  beyond.  The  rocks  of  this  formation  are  the  oldest 
known  on  the  American  continent,  and  probably  correspond  to  the 
oldest  gneisses  of  Finland  and  Scandinavia,  and  to  similar  rocks  in 
the  north  of  Scotland.  The  rocks  of  the  Laurentain  formation  are  in 
large  part  crystalline  schists,  mostly  gneissoid  or  hornblendic.  Asso- 
ciated with  these  schists  are  seen  heavy  stratified  masses  of  a  crystal- 
line rock,  which  is  almost  entirely  composed  of  feldspar  with  a  base  of 
lime  and  soda.  With  these  schists  and  these  feldspars  are  found  strata 
of  quartzite,  associated  with  crystalline  limestones  which  have  a  rather 
important  place  in  this  formation.  The  limestones  form  beds  from  1 
meter  to  more  than  100  meters  thick,  and  often  present  a  succession  of 
thin  beds,  intercalated  in  beds  of  gneiss  or  quartzite.  The  quartzites 
sometimes  present  themselves  under  the  form  of  conglomerates,  and  in 
certain  cases  have  a  paste  of  dolomite.  Beds  of  dolomite  or  of  more 
or  less  magnesian  limestone  are  often  intercalated  with  pure  limestones. 
These  schists,  feldspars,  quartzites,  and  limestones,  such  as  we  have 
described  them,  constitute  the  stratified  part  of  the  Laurentian  system ; 
but  there  are,  furthermore,  intrusive  granites,  syenites,  and  diorites 
which  form  quite  important  masses;  the  granites  are  sometimes  albitic 
and  often  contain  tourmaline,  mica  in  large  flakes,  sphene,  and  sulphate 
of  molybdenum.  Associated  with  the  limestones  are  important  beds  of 
hematite  and  limonite.  Graphite  is  very  frequently  disseminated  in 
little  flakes  in  the  crystalline  limestones,  and  forms  also  veins  having 
sometimes  a  considerable  thickness.  Two  of  these  are  found  near  Gren- 
ville,  on  the  Ottawa.  The  graphite  exists  in  three  detached  bands, 
each  having  a  thickness  of  about  12  centimeters. 
In  the  Cambrian  and  Huronian  system  are  found  the  rocks  on  the 
north  shores  of  lakes  Huron  and  Superior  where  are  present  a  series  of 
schists,  sandstones,  limestones,  and  conglomerates,  interspersed  with 
heavy  layers  of  diorite,  and  resting  un  conform  ably  on  the  Laurentian 
system.  As  these  rocks  are  lower  than  the  Silurian  terrane,  and  as 
they  have  thus  far  not  yielded  a  single  fossil,  they  may  well  be  referred 
to  the  Cambrian  system  (the  Lower  Cambrian  of  Sedgwick).  The 
schists  of  this  system,  on  lake  Superior,  are  bluish,  and  inclose  layers 
of  horn  flint,  having  calcareous  bands,  and  whose  cracks  are  often  filled 
with  anthracite.  These  rocks  are  covered  with  a  considerable  thickness 
of  trap,  on  which  are  superposed  heavy  beds  of  white  and  red  sand- 
