410  PBE-CAMBKTAN    GEOLOGY   OF    NORTH   AMERICA. 
acter.  The  latter  appear  to  constitute  the  green  chlorite  schist  of 
Logan,  which  is  either  an  ancient  or  much  altered  eruptive  or  highly 
altered  sedimentary  material.  The  quartzites  contain  angular  frag- 
ments of  such  a  character  that  they  seem  to  be  derived  from  this  dia- 
base schist ;  and  this  circumstance  countenances  the  theory  that  the  lat- 
ter are  older  and  probably  sedimentary  in  origin.  The  Huronian  of 
Canada  is  as  follows  in  descending  order :  Otter  Tail  white  quartzite, 
4,000  feet;  Thessalon  red  and  gray  quartzite,  5,000  feet;  Otter  Tail 
cherty  limestone,  100  feet;  Upper  Plummer  conglomeratic  and  sili- 
ceous argillite,  500  feet ;  red  f elsite,  granulite,  and  quartzite,  100  feet ; 
lower  conglomeratic  and  siliceous  argillite,  7,400  feet;  Bruce  lime- 
stone, 100  feet;  Mississagui  vitreous  quartzite,  3,750  feet.  This  suc- 
cession includes  neither  the  lower  nor  the  upper  limit  of  the  Huron- 
ian. At  St.  Josephs  Island  the  Huronian  is  immediately  overlain 
by  a  fossiliferous  limestone,  apparently  the  Chazy.  It  thus  appears 
that  the  Huronian  is  a  system  immediately  below  the  Lower  Silurian, 
and  that  if  no  intervening  terranes  are  wanting  it  occupies  the  posi- 
tion of  the  Taconic  of  Emmons  and  the  Lower  Cambrian  of  Sedgwick. 
The  lower  limit  of  the  Huronian  must  be  succeeded  by  a  formation  of 
vitreous  quartz,  red  jasper,  and  graywacke,  besides  greenstones,  red 
granulite,  red  gneiss,  and  mica-bearing  granite,  since  fragments  of 
all  these  are  found  in  the  Huronian.  It  may  be  that  the  quartzite 
pebbles  are  derived  from  the  Mississagui  quartzite,  but  the  red  jasper 
and  greywacke  must  have  been  derived  from  a  terrane  older  than  the 
Huronian  and  newer  than  the  crystalline  masses  of  the  Laurentian. 
Bonney,23  in  1888,  discusses  the  development  of  the  crystalline 
schists  in  the  neighborhood  of  Sudbury.  The  semicrystallines  can 
be  easily  separated  from  the  thoroughly  crystalline  rocks  of  the  Lau- 
rentian. In  the  Huronian  rocks  two  groups  may  be  distinguished, 
one  of  which  is  slightly  altered  and  the  other  very  much  more  exten- 
sively modified.  The  semicrystallines  are  compared  with  those  of 
like  character  in  Great  Britain. 
Barlow,24  in  1890,  describes  the  relations  between  the  Huronian 
and  the  Laurentian  north  of  Lake  Huron.  At  many  localities  the 
contact  is  found  to  be  an  irruptive  one,  the  granite  and  gneiss  intrud- 
ing the  Huronian  elastics.  Very  often  the  Huronian  strata  dip  into 
or  under  the  gneiss,  although  often  the  Huronian  beds  are  superim- 
posed upon  the  gneiss  in  perfect  conformity,  and  occasionally  gneiss 
is  seen  dipping  away  from  the  vertical  Huronian  strata.  Huronian 
rocks  are  also  seen  resting  uncon  form  ably  upon  the  upturned  edges 
of  Laurentian  gneiss.  The  Huronian  strata  are  often  metamor- 
phosed 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. 
