vanhise.]  LAKE    SUPERIOR    KEGION.  69 
argillaceous  matrix  with  rounded  pebbles  of  syenite  and  granite  of  vari- 
ous kinds,  and  some  of  the  other  Huronian  rooks,  but  very  seldom  of 
gneiss;  the  other  with  a  dioritic  matrix,  and  often  with  rounded  pebbles 
also.  But,  in  perhaps  th  e  majority  of  cases,  what  were  formerly  considered 
as  pebbles  are  really  concretions  of  a  lenticular  form,  and  differing  but 
slightly  from  the  matrix  in  color  and  composition.  They  are  best  seen 
on  wetted  surfaces  of  cross  sections  of  the  rock,  where  they  appear  as 
parallel  elongated  patches  tapering  to  a  point  at  each  end.  Both  hema- 
tite and  magnetic  iron  ores  are  common  in  these  rocks.  Gneiss  is  not 
common  in  the  Huronian,  and  it  differs  from  the  ordinary  Laurentian 
gneiss  in  being  imperfect  and  slightly  calcareous. 
In  the  upper  division  of  the  Huronian  probably  the  most  abundant 
rock  in  Ontario  is  what  may  be  called  a  graywacke,  but  which  in  the 
older  reports  was  often  styled  a  "  slate  conglomerate;"  bnt  it  also  in- 
cludes clay-slates,  argillites,  felsites,  quartzites,  ordinary  conglomer- 
ates, jasper  conglomerates,  breccias,  dolomites,  serpentine,  etc.  In 
some  localities  the  nearly  vertical  bands  of  quartzite,  having  withstood 
denudation  better  than  the  other  rocks,  remain  as  conspicuous  hills  or 
ridges,  and  this  circumstance  has  caused  their  relative  volume  in  the 
series  to  be  overrated  by  superficial  observers.  The  materials  forming 
the  gray wackes  and  the  stratified  quartzose  diorites  have  been  derived 
from  volcanic  sources.  The  igneous  character  of  the  Huronian  is  farther 
shown  by  the  large  masses  and  areas  ot  greenstone  (diorites  or  dia- 
bases), granites,  syenites,  and  other  eruptive  rocks  which  are  so  largely 
mingled  with  both  the  lower  and  upper  portions  of  the  Huronian  sys- 
tem in  all  parts  of  their  distribution,  forming  indeed  one  of  its  charac- 
teristic features.  The  crystalline  greenstones  occur  either  as  compact 
areas,  wide  elongated  masses,  dikes,  or  thick  inter  stratifying  beds  in 
nearly  all  the  Huronian  areas.  In  many  cases  the  dioritic  schists  may 
have  been  originally  massive,  but  assumed  the  cleaved  structure  by 
pressure  when  incorporated  among  stratified  masses.  The  commonest 
position  of  the  granite  and  syenite  areas  is  within  but  toward  the  bor- 
ders of  the  Huronian  tracts;  but  they  sometimes  occur  in  the  Lauren- 
tian country,  in  their  immediate  vicinity,  or  at  a  distance  from  them  in 
the  direction  of  the  longer  axis  of  the  Huronian  areas. 
Unconformably  above  the  Huronian  is  the  Cambrian  system,  which 
comprises,  in  the  ascending  order,  the  Animikie,  Mpigon,  and  Potsdam 
formations. 
The  Animikie  formation,  in  ascending  order,  consists  of  arenaceous 
conglomerate,  with  pebbles  of  quartz,  jasper,  and  slate,  seen  on  the 
north  shore  of  Thunder  bay;  of  thinly  bedded  cherts,  mostly  of  dark 
colors,  with  argillaceous  and  dolomitic  beds;  of  black  and  dark  argil- 
lites, and  flaggy  black  shales,  with  sandstones  and  ferruginous  dolo- 
mitic  bands  and  arenaceous  beds,  often  rich  in  magnetic  iron,  together 
with  layers  and  intrusive  masses  of  trap  (diabase).  The  Animikie  for- 
mation occupies  a  great  triangular  area  north  and  west  of  lake  Supe- 
