96  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
Penokee-Gogebic  Huronian,  as  shown  by  the  fact  that  the  former  series 
is  iu  contact  in  different  places  with  various  members  of  the  Huronian. 
There  is  also  an  unconformity  between  the  Huronian  and  Laurentian. 
As  supporting  the  view  that  the  pre- Silurian  systems  are  distinct 
periods,  attention  is  called  to  the  lithological  differences  between  the 
three  series,  as  well  as  to  the  intensity  of  folding  to  Avhich  they  have 
been  subjected.  The  detrital  members  of  the  copper  series  consist  of 
friable  sandstone  showing  no  greater  metamorphism  than  the  Silurian, 
and  it  is  folded  only  in  regular  magnificent  sweeps,  the  same  strike  and 
dip  continuing  in  some  cases  for  about  150  miles.  The  Huronian  series 
consists  of  greenstones,  various  schists,  clay-slates,  quartzites,  marbles, 
with  gneisses  and  granites  containing  no  copper,  and  having  conforma- 
ble beds  of  the  various  oxides  of  iron,  and  is  everywhere  sharply  folded 
into  narrow  troughs  and  irregular  basins  trending  in  every  direction. 
The  Laurentian  is  still  more  plicated  and  metamorphosed,  the  stratifi- 
cation often  being  entirely  obliterated.  Whether  the  Laurentian  rocks 
can  be  separated  into  two  or  more,  nonconformable  systems,  as  in 
Canada,  no  opinion  is  ventured.  Since  Keweenaw  peninsula  is  a  strik- 
ing geographical  feature  in  lake  Superior,  and  is  the  locality  where  the 
copper  series  is  best  exposed,  the  name  Keweenawian  is  suggested  for 
this  period. 
Brooks107,  in  1876,  gives  a  list  of  the  rocks  of  the  Huronian  series 
in  order  of  their  abundance  and  as  they  occur  in  strati  graphical  suc- 
cession in  the  Marquette,  Menominee,  Penokee,  and  Gogebic  series. 
Lithologically  the  rocks  are  divided  into  (1)  Fragmental  rocks,  exclu- 
sive of  limestone ;  (2)  Metamorphic  rocks,  not  calcareous  ;  (3)  Calcareous 
rocks ;  (4)  Igneous  rocks. 
The  Fragmental  rocks  include  quartz-conglomerates,  which  occur  in 
the  middle  horizon,  both  in  the  Marquette  and  Menominee  and  in  the 
latter  at  the  base  of  the  series  where  it  holds  pebbles  of  granite,  gneiss, 
and  quartz.  In  the  Metamorphic  rocks  not  calcareous  are  included 
many  varieties ;  the  mica-bearing  series  includes  granite,  syenite, 
gneiss,  mica- schists,  hornblende-schists,  mica-slates,  clay-slates,  diorites, 
diabases,  quartzites,  siliceous  schists,  chert  and  jasper  rocks,  iron  ores, 
as  well  as  many  others.  Among  the  eruptive  rocks  is  a  feldspathic 
series,  including  granite  dikes ;  hornblendic  and  pyroxenic  series,  in- 
cluding diabase  and  similar  rocks,  and  hydrous  magnesian  schistose 
rocks  which  are  found  in  dike-like  forms  crossing  the  quartzites,  iron 
ores,  and  greenstones. 
The  succession  in  the  Marquette  region  from  the  base  upward  is  (a) 
syenite,  diorite,  diabase,  hornblende- schists,  slates,  conglomeratic 
quartzites,  and  various  quartzose  iron  ores  5  (b)  quartzite  graduating 
into  protogine,  and  containing  interstr  a  tiffed  beds  of  dolomi tic  marble; 
(c)  ferruginous  quartzose  schist,-  (d)  hornblendic  rocks  with  green- 
stones; (e)  ferruginous  quartzose  flags,  clay-slates,  and  quartzites;  (/) 
hornblendic  rock,  related  to  diorite  and  diabase;  (g)  siliceous  hemati tic 
