418  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
are  gneisses  of  the  typical  Laurentian  varieties,  finely  stratified  and 
regularly  arranged  in  anticlinal  and  synclinal  folds,  the  angles  of  dip 
usually  not  being  far  from  45°,  but  lesser  and  greater  dips  being 
found.  Red  and  gray  varieties  are  in  about  equal  proportion,  and 
they  alternate  with  each  other  in  thick  and  thin  sheets.  Mica  gneisses 
are  predominant.  No  beds  of  crystalline  limestone  are  found  west 
of  Iron  Island  in  Lake  Nipissing.  Limestones  are  associated  with 
the  gneisses  on  some  of  the  islands  of  the  eastern  part  of  this  lake  and 
at  Lake  Talon  on  the  Mattawa.  In  the  Parry  Sound  district  are  five 
distinct  bands  of  Laurentian  limestone.  These  rocks  are  classified 
with  the  Upper  Laurentian  rocks  of  the  counties  of  Ottawa  and 
Argenteuil. 
The  Laurentian  rocks' northwest  of  the  Huronian  belt  are  heavy 
contorted  gneisses  of  the  Lower  Laurentian.  Associated  with  the 
gneisses  are  red  granites  which  are  classed  with  the  Laurentian,  but 
which  may  be  really  Huronian.  These  may  have  formed  by  soften- 
ing of  the  gneiss  by  heat,  combined  with  recrystallization,  or  they 
may  be  due  to  the  alteration  of  the  Huronian  arkoses  or  graywackes, 
or  they  may  be  mainly  eruptive.  These  granites  are  along  the  contact 
line  between  the  Laurentian  and  the  Huronian.  Along  the  line  of 
contact  between  the  granites  and  the  Huronian  quartzites  and  schists 
the  rocks  are  much  broken.  It  is  not  improbable  that  a  fault  exists 
at  the  line  of  junction  between  the  Laurentian  and  Huronian  rocks. 
The  great  Huronian  belt  consists  of  a  large  variety  of  rocks,  such 
as  crystalline  schists,  quartzites,  conglomerates,  agglomerates,  clay 
slates,  greenstones,  dolomites,  etc.,  the  majority  of  which  are  pyro- 
clastic.  The  rocks  are  usually  tilted  at  high  angles.  There  are  nu- 
merous instances  where  there  is  a  gradual  transition  from  the  Hu- 
ronian to  the  lower  series.  A  few  instances  of  local  want  of  con- 
formity between  the  two  are  no  evidence  that  the  two  systems  are  not 
conformable  on  a  grand  scale.  The  few  known  instances  where  there 
appears  to  be  a  want  of  parallelism  are  more  probably  due  to  fault- 
ing. The  pyroclastic  rocks  show  the  agency  of  water  in  their  forma- 
tion, and  were  largely  derived  from  igneous  matter,  which  had  been 
more  or  less  recently  erupted.  The  newest  rock  of  the  Sudbury  dis- 
trict is  a  volcanic  breccia,  which  forms  a  continuous  range  of  hills  for 
a  distance  of  36  miles,  with  a  breadth  in  the  center  of  8  miles.  Within 
the  Huronian  rocks  are  intrusive  red  granites. 
Barlow,34  in  1892,  states  that  the  Huronian  system  is  the  oldest 
sedimentary  strata  of  the  north  shore  of  Lake  Huron,  and  that  the 
Laurentian  gneiss  or  Basement  Complex  is  the  original  crust  of  the 
earth  or  floor  on  which  the  first  sediments  were  laid  down.  This 
floor,  as  shown  by  the  pebbles  of  the  Huronian,  was  granite  which 
had  in  many  places  a  foliated  or  gneissic  structure.  In  many  places 
the  subsequent   folding  and   fracturing  of  the  comparatively  thin 
