42  PRE-CAMBRIAN   ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
tian.  In  the  Huronian  rocks  two  groups  may  be  distinguished,  one  of 
which  is  slightly  altered  and  the  other  very  much  more  extensively 
modified.  The  semi3rystallines  are  compared  With  those  of  like  char- 
acter in  Great  Britain. 
Barlow,50  in  1890,  describes  the  relations  between  the  Huronian 
and  Laurentian  north  of  lake  Huron.  At  many  localities  the  contact 
is  found  to  be  an  irruptive  one,  the  granite  and  gneiss  intruding  the 
Huronian  elastics.  Very  often  the  Huronian  strata  dip  into  or  under 
the  gneiss,  although  often  the  Huronian  beds  are  superimposed  upon 
the  gneiss  in  perfect  conformity,  and  occasionally  gneiss  is  seen  dipping 
away  from  the  vertical  Huronian  strata.  Huronian  rocks  are  also  seen 
resting  unconformably  upon  the  upturned  edges  of  Laurentian  gneiss. 
The  Huronian  strata  are  often  metamorphosed  where  in  contact  with 
the  gneiss.  These  different  phenomena  are  all  explained  by  the  later 
irruptive  character  of  the  gneiss.  It  is  concluded  that  the  Huronian 
system  is  the  oldest  series  of  sedimentary  strata  known  in  this  region. 
Bell51,  in  1890,  states  that  stretching  from  lake  Huron  to  lake  Temis- 
camang  is  the  greatest  area  of  Huronian  rocks  in  Canada.  The  most 
prevalent  rock  in  this  region  is  graywacke,  often  conglomeratic.  An- 
other rock  of  great  abundance  is  a  quartz-diorite.  These  two  are  the 
parent  rocks  of  the  Huronian.  The  quartzites  and  clay- slates  are  but 
phases  of  the  graywacke.  The  rocks  of  this  region  show  three  ways  by 
which  gneiss  may  be  formed,  namely,  by  the  direct  conversion  of  the 
thin  bedded  or  slaty  varieties  of  graywacke,  by  the  alteration  of  the 
mixed  quartz  and  feldspar  rock  derived  from  other  varieties  of  it,  and 
by  the  alteration  of  the  modified  quartz-diorites.  The  dolomites  are  of 
a  concretionary  or  segregated  nature,  derived  from  the  hornblende  and 
augite  of'the  rocks  with  which  they  are  associated.  During  the  process 
of  conversion  from  graywacke  into  syenite,  strings  and  veins  of  mag- 
netite have  formed. 
Winchell  (Alex.)52,  in  1890,  gives  further  observations  on  the  orig- 
inal Huronian  region.  Northwest  of  Echo  lake  is  found  a  series  of 
argillites,  slates,  quartzites,  and  schists,  which  are  frequently  conglom- 
eratic and  in  one  place  contain  outcrops  of  hematite.  These  strata  are 
close  to  a  vertical  attitude,  strike  nearly  east  and  west,  and  resemble  the 
Knife  lake  series  and  Ogishki  conglomerates  of  Minnesota.  These 
rocks  can  not  belong  to  the  same  system  as  the  quartzites,  upper 
slate  conglomerates,  and  limestone  of  the  Huronian,  which  dip  at  an 
angle  of  20°.  There  is  here  a  genuine  discordance  of  stratification,  and 
two  series,  not  one,  as  mapped  by  Logan.  The  lower  system  is  the 
formation  which  occurs  at  Gros  cap,  Goulais  bay,  and  Dore  river,  which 
was  identified  by  Logan  with  the  Huronian  of  lake  Huron.  The  author 
is  convinced  also  of  their  identity  with  the  vertical  strata  in  Minnesota 
and  Canada  known  as  the  Keewatin  system.  It  is  also  clear  that  these 
gnarled,  green  pebble  slates  are  the  prolongation  of  the  lower  slate  con- 
glomerate of  the  Thessalon  valley. 
