66  PRE-CAMBRTAN    GEOLOGY    OF    NORTH    AMERICA. 
conic  system  form  a  belt  on  both  sides  of  the  Blue  Ridge.  On  the 
west  it  is  continuous  from  Canada  to  Georgia.  On  the  east  it  is 
wider  in  certain  places  than  on  the  west  side,  but  its  continuity  is 
broken.  The  Taconic  system  rests,  then,  on  the  following  points: 
The  formations  of  the  series  are  physically  unlike  the  Lower  Silu- 
rian ;  it  supports  the  Lower  Silurian  unconformably  at  numerous 
places;  it  is  a  system  in  which  life  existed,  and  the  remains  of  organ- 
isms are  unlike  those  of  the  Lower  Silurian;  it  carries  back  many 
stages  farther  the  time  when  life  appeared,  and  represents  a  period 
vastly  longer  than  the  Silurian,  although  it  may  occupy  a  less  super- 
ficial area. 
Logan  and  Hunt,4  in  1855,  gave  a  geological  sketch  of  Canada. 
The  ancient  rocks  are  divided  into  the  Laurentian  system  and  the 
Cambrian  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 
northeast-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  Laurentian  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  con- 
glomerates, 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  lime- 
stones, such  as  we  have  described  them,  constitute  the  stratified  part 
of  the  Laurentian  system;  but  there  are,  furthermore,  intrusive  gran- 
ites, 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.  Graph- 
ite 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  Grenville,  on  the  Ottawa. 
The  graphite  exists  in  three  detached  bands,  each  having  a  thickness 
of  about  12  centimeters. 
