272  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
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  hematite  and  mag- 
netic iron  ore  are  common  in  these  rocks.  Gneiss  is  not  common  in 
the  Huronian,  and  it  differs  from  the  ordinary  Laurentian  gneiss  in 
being  imperfectly  and  slightly  calcareous. 
In  the  upper  division  of  the  Huronian  is  probably  the  most  abund- 
ant rock  in  Ontario,  which  may  be  called  a  graywacke,  but  which  in 
the  older  reports  was  often  styled  a  "  slate  conglomerate ;  "  but  it 
also  includes  clay  slates,  argillites,  felsites,  quartzites,  ordinary  con- 
glomerates, jasper  conglomerates,  breccias,  dolomites,  serpentine,  etc. 
In  some  localities  the  nearly  vertical  bands  of  quartzite,  having  with- 
stood 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  graywackes  and  the  stratified  quartzose  diorites  have 
been  derived  from  volcanic  sources.  The  igneous  character  of  the 
Huronian  is  further  shown  by  the  large  masses  and  areas  of  green- 
stone (diorites  or  diabases),  granites,  syenites,  and  other  eruptive 
rocks  which  are  so  largely  mingled  with  both  the  lower  and  the  upper 
portions  of  the  Huronian  system  in  all  parts  of  their  distribution, 
forming  indeed  one  of  its  characteristic  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  borders  of  the  Huronian  tracts ; 
but  they  sometimes  occur  in  the  Laurentian  country,  in  the  immediate 
vicinity  of  the  Huronian  tracts,  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  ascending  order,  the  Animikie,  Nipigon,  and  Potsdam 
formations. 
The  Animikie  formation  consists,  in  ascending  order,  of  arenaceous 
conglomerate,  with  pebbles  of  quartz,  jasper,  and  slate  seen  on  the 
north  shore  of  Thunder  Bay;  thinly  bedded  cherts,  mostly  of  dark 
color,  with  argillaceous  and  dolomitic  beds,  and  black  and  dark  ar- 
gillites 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 
formation  occupies  a  great  triangular  area  north  and  west  of  Lake 
Superior,  the  base  of  which  is  60  miles  in  length  and  the  arms  40  and 
80  miles,  respectively. 
