38  PRE-CAMBRIAN   EOCKS    OF    NORTH   AMERICA.  [bull. 86. 
obscure,  and  a  mass  of  greenstone  of  rather  coarse  grain  was  usually 
the  first  intimation  of  the  proximity  of  the  higher  rocks.  Whether  this 
greenstone  is  a  contemporaneous  flow  or  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  ascending  order :  line  grained  siliceous  slates ;  slate- 
conglomerate,  containing  profuse  syenite  and  occasional  jasper  pebbles  5 
limestone  j  slate-conglomerate,  like  the  first ;  green  siliceous  chloritic 
slate  5  and  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  corresponds  with  the  determination  of  the  thickness  of  the 
quartz-rock  series  on  the  Spanish  river. 
Logan,40  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,41  in  1858,  gives  a  continuation  of  his  study  of  the  rocks 
north  of  lake  Huron.  He  places  the  rocks  of  French  river,  described 
in  the  report  of  1857,  as  Lauren tian.  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,42  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  Te- 
miscamang  are  sandstones,  quartzose  conglomerates,  and  slate  conglom- 
erates, the  slate  conglomerates  holding  pebbles  and  bowlders  derived 
from  the  subjacent  gneiss.  The  bowlders  display  red  feldspar,  trans- 
lucent quartz,  green  hornblende,  and  black  mica,  arranged  in  parallel 
layers,  which  present  directions  accordant  with  the  attitude  in  which 
the  bowlders  were  accidentally  inclosed.  From  this  it  is  evident  that 
the  slate  conglomerate  was  not  deposited  until  the  subjacent  forma- 
tion had  been  converted  into  gneiss,  and  very  probably  greatly  dis- 
turbed; 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  degrees.  A  similar  set  of  clastic  rocks  is  found  on 
the  north  shore  of  lake  Huron,  except  that  the  series  is  here  intersected 
and  interstratified  with  greenstone  trap,  and  pebbles  of  syenite  and  jas- 
per are  found.  Eastward  of  lake  Temiscamang,  in  an  area  of  200,000 
square  miles  imperfectly  examined,  no  similar  series  of  rocks  has  been 
met  with.  Because  this  clastic  series  of  rocks  occurs  in  typical  devel- 
opment on  lake  Huron  it  has  been  decided  to  designate  it  by  the  term 
Huronian. 
Murray,43  in  1859,  in  continuing  his  study  of  the  Huronian,  gives 
most  of  his  time  to  the  region  adjacent  to  the  Thes salon  and  Mississagui 
