112  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
and  Lower  Silurian  as  does  the  typical  Laurentian  of  Canada,  and  be- 
cause they  have  the  same  general  lithological  peculiarities.  There  can 
be  no  reasonable  doubt  that  they  are  directly  continuous  with  the 
Canada  Laurentian.  The  prevailing  rocks  are  granite  and  gneiss. 
These  rocks  are  greatly  folded  and  have  certainly  an  enormous  thick- 
ness.    It  is  evident  that  these  rocks  are  of  true  sedimentary  origin. 
The  granites  are  generally  without  distinct  bedding,  but  no  eruptive 
granite  recognizable  as  such  has  been  observed. 
Lying  immediately  against  the  Laurentian,  and  sharply  defined  from 
it,  extending  from  Montreal  river  to  lake  Numak agon,  is  a  belt  of  schis- 
tose rocks  which  are  beyond  question  the  westward  extension  of  the 
iron-bearing  series  of  the  upper  peninsula  of  Michigan.  This  belt  has 
an  aggregate  thickness  of  strata  of  13,000  feet.  The  subdivisions,  be- 
ginning below,  are  (1)  crystalline  limestone  ;  (2)  quartz-schist  and  argil- 
litic  mica-schist;  (3)  tremolitic  magnetite-schists,  magnetic  and  specular 
quartzites,  lean  magnetic  and  specular  ores;  (4)  alternations  of  black 
mica- slate  with  diorite  and  schistose  quartzites;  (5)  mica-schists  with 
coarse  intrusive  granite.  These  major  divisions  are  again  subdivided 
at  Penokee  gap  and  vicinity.  The  system  always  dips  north,  usually 
at  a  high  angle,  the  strikes  are  oblique  to  the  underlying  Laurentian 
gneiss,  proving  the  un  conform  ability  of  the  two  systems,  the  actual 
contact  of  which  can  be  seen  in  several  places.  These  rocks  are  re- 
garded as  the  equivalent  of  the  Huronian  of  Canada,  because  they  are 
the  direct  continuation  of  the  iron-bearing  system  of  Marquette,  because 
the  grand  divisions  of  the  Bad  river  and  Marquette  system  are  simi- 
lar, because  they  show  the  same  relations  to  the  Laurentian  and  Ke- 
weenawan systems  as  found  in  the  Huronian  of  Canada,  i.  e.,  newer 
than  the  former  and  older  than  the  latter,  and  because  the  Marquette 
and  Menominee  sediments  are  in  unconformable  contact  with  the  Lower 
Silurian  sandstone. 
The  Keweenawan  system  is  a  distinctly  stratified  one,  in  large  meas 
ure  made  up  of  eruptive  rocks  in  the  form  of  flows.    These  constitute 
the  lower  10,000  feet  of  the  system,  and  above  these  are  found  the  de 
trital  rocks,  increasing  in  frequency,  until  they  wholly  exclude  the  igne- 
ous rocks  in  the  upper  15,000  feet.    The  eruptives  of  the  system  are 
chiefly  diabase,  melaphyre,  and  gabbro.    The  succession  on  the  Mon- 
treal river  is  (1)  chiefly  diabase  and  diabase  amygdaloid,  with  littl 
satisfactory  appearance  of  bedding  and  having  a  width  of  about  33,00 
feet;  (2)  alternations  of  (1)  with  red  sandstone  and  shale,  1,200  feet;  (3 
bowlder  conglomerate,  1,200  feet;  (4)  alternations  of  shale  and  quartz 
less  sandstone,  350  feet;  (5)  red  sandstone  and  shale,  12,000  feet.    If 
the  series  is  regarded  as  a  continuous  one  it  is  at  least  50,000  feet  thick/ 
There  are  two  prominent  belts  of  the  Keweenaw  rocks  in  northern  Wis- 
consin lying  parallel  to  each  other  and  having  between  them  a  synclinal ; 
depression  which  is  occupied  by  Chequamegon  bay.     The  Keweenawan 
system  is  evidently  newer  than  the  Penokee  system.    That  the  two  sys- ; 
