SUMMAKY   OF    GENERAL   LITERATURE.  93 
the  iron  ores  occur  in  greater  masses  than  in  deposits  of  organic 
origin,  and  their  mode  of  occurrence  is  opposed  to  any  theory  of  this 
kind.  The  Upper  Laurentian  rocks  seem  to  be  much  more  limited  in 
geographical  extent  than  the  Lower  Laurentian. 
The  Huronian  system  in  Canada  has  a  great  thickness  and  variety 
of  strata,  for  the  most  part  crystalline,  but  in  less  degree  than  the 
Laurentian,  together  with  many  unstratified  igneous  masses.  Like 
the  Laurentian  it  is  azoic,  or  devoid  of  any  trace  of  organic  life,  so 
that  the  distinction  between  the  two  systems  is  based  entirely  on  lith- 
ological  grounds.  The  difference  in  this  respect  is  great,  and  is 
easily  recognized  by  those  who  have  paid  any  attention  to  geology. 
The  prevailing  dark-green  and  dark-gray  colors  of  the  Huronian 
offer  a  marked  contrast  to  the  lighter  grays  and  reddish  grays  of  the 
Laurentian.  The  latter  are  massive  and  coarsely  crystalline,  while 
the  former  are  usually  fine  grained  and  schistose  or  fissile,  this  cleav- 
age structure  constituting  a  striking  difference  from  the  solid  Lauren- 
tian. There  are  some  exceptions  to  this  rule,  such  as  the  light- 
colored  quartzites  and  the  granites  and  syenites  of  the  Huronian,  to 
be  noticed  further  on.  The  change  in  passing  from  one  to  the  other 
is  often  sudden  and  complete,  but  sometimes  beds  of  passage  are 
met  with.  The  Huronian  is  the  great  metalliferous  system  of  Can- 
ada. Although  the  Huronian  strata  have  generally  been  thrown  into 
sharp  folds,  or  stand  at  high  angles,  they  are,  as  a  rule,  less  bent 
about  or  contorted  than  the  Laurentian.  The  total  volume  of  the 
system  is  very  great,  probably  not  far  from  40,000  or  50,000  feet,  or 
perhaps  even  more. 
In  Canada,  as  far  as  our  investigations  have  gone,  the  two  systems 
appear  to  be  everywhere  conformable  to  each  other,  but  in  rocks 
of  so  ancient  date  which  have  undergone  so  profound  structural 
changes,  owing  to  pressure,  etc.,  affecting  alike  the  stratified  aud 
unstratified  portions,  this  appearance  may  not  everywhere  indicate  a 
truly  conformable  sequence.  Both  sets  of  rocks  having  been  thrown 
by  lateral  pressure  into  sharp  folds,  standing  at  high  angles  to  the 
horizon,  the  Huronian  often  appears  to  dip  under  the  older  Lauren- 
tian, but  this  is  merely  the  effect  of  overturning,  and  does  not  show 
that  a  part  of  the  Laurentian  is  newer  than  the  locally  underlying  Hu- 
ronian. Notwithstanding  the  geographical  relations  of  the  two  sets 
of  rocks,  their  general  difference  in  character  and  composition  would 
indicate  that  some  great  change  in  terrestrial  conditions  had  occurred 
when  the  formation  of  the  one  system  ended  and  that  of  the  other 
began.  In  the  Laurentian  an  "  acid  "  or  siliceous  composition  pre- 
vails, whereas  the  Huronian  rocks  as  a  whole  are  more  basic,  chem- 
ically speaking.  The  latter  can  be  shown  to  be  very  largely  of  vol- 
canic origin,  although  this  may  not  always  be  obvious  at  first  sight. 
