406  PRE-CAMBRTAN    GEOLOGY   OF    NORTH    AMERICA. 
spreads  over  the  country  to  the  south  and  east,  while  the  slates,  con- 
glomerates, limestones,  quartzites,  and  greenstones  of  the  Huronian 
occupy  the  northern  and  western  parts.  The  difference  in  lithologi- 
cal  character  between  the  two  formations  is  always  sufficiently  appa- 
rent, but  though  both  were  found  a  short  distance  apart  the  imme- 
diate point  of  contact  was  always  obscure,  and  a  mass  of  greenstone 
of  rather  coarse  grain  was  usually  the  first  intimation  of  the  prox- 
imity of  the  higher  rocks.  Whether  this  greenstone  is  a  contempo- 
raneous flow  or  a  subsequent  intrusion  has  not  been  ascertained.  The 
lower  slates  stand  nearly  vertical  on  Sturgeon  River  near  the  gneiss. 
The  following  is  the  general  succession  within  the  Huronian,  in  as- 
cending order :  Fine-grained  siliceous  slates ;  slate  conglomerate,  con- 
taining profuse  syenite  and  occasional  jasper  pebbles;  limestone; 
slate  conglomerate,  like  the  first;  green  siliceous  chloritic  slate;  close- 
grained  quartzite  of  various  colors,  running  into  a  conglomerate,  the 
pebbles  of  which  include  white  quartz  and  red  and  green  jasper. 
The  thickness  of  the  Huronian  is  calculated  at  10,000  feet  and  cor- 
responds with  the  determination  of  the  thickness  of  the  quartz-rock 
series  on  Spanish  River. 
Logan,11  in  1858,  applies  the  term  Huronian  to  the  copper-bearing 
rocks  of  Lake  Huron.  A  limestone  near  the  middle  of  the  series  is 
used  to  trace  out  the  structure. 
Murray,12  in  1858,  gives  a  continuation  of  his  study  of  the  rocks 
north  of  Lake  Huron.  He  places  the  rocks  of  French  River,  de- 
scribed in  the  report  of  1857,  as  Laurentian.  A  belt  of  limestone 
200  feet  thick  is  used  in  working  out  the  structure  of  the  Huronian. 
The  Huronian  is  also  called  the  copper-bearing  rocks. 
Logan,13  in  1858,  gives  a  general  description  of  the  pre-Silurian 
Azoic  rocks  of  Canada,  which  occupy  nearly  a  quarter  of  a  million 
square  miles.  These  are  a  series  of  very  ancient  sedimentary  deposits 
in  an  altered  position.  They  are  of  great  thickness  and  are  capable 
of  division  into  stratigraphical  groups.  In  the  formation  about 
Lake  Temiscaming  are  sandstones,  quartzose,  conglomerates,  and 
slate  conglomerates,  the  slate  conglomerates  holding  pebbles  and 
bowlders  derived  from  the  subjacent  gneiss.  The  bowlders  display 
red  'feldspar,  translucent  quartz,  green  hornblende,  and  black  mica, 
arranged  in  parallel  layers,  which  present  directions  accordant  with 
the  attitude  in  which  the  bowklers  were  accidentally  inclosed.  From 
this  it  is  evident  that  the  slate  conglomerate  was  not  deposited  until 
the  subjacent  formation  had  been  converted  into  gneiss,  and  very 
probably  greatly  disturbed;  for  while  the  dip  of  the  gneiss,  up  to 
the  immediate  vicinity  of  the  slate  conglomerates,  was  usually  at 
high  angles,  that  of  the  latter  did  not  exceed  9°.  A  similar  set  of 
clastic  rocks  is  found  on  the  north  shore  of  Lake  Huron,  except  that 
the  series  is  here  intersected  and  interstratined  with  greenstone  trap, 
