LAKE    SUPERIOR   REGION. 
geological  ages.  The  Keweenawan  is  divided  into  two  divisions,  both 
of  which  are  placed  in  the  Cambrian — the  Lower  Keweenawan, 
25,000  feet  of  interbedded  conglomerates  and  lava  flows,  with  some 
intrusives;  the  Upper  Keweenawan,  12,000  feet  of  sandstones  and 
shales,  not  separable  from  the  Potsdam  or  Eastern  sandstone. 
The  Azoic  or  Archean  system  consists  of  rocks  formed  (1)  by  me- 
chanical means,  (2)  by  eruptive  agencies,  (3)  by  chemical  action. 
The  Cascade  or  oldest  formation  of  sedimentary  and  eruptive  rocks 
consists,  commencing  with  the  oldest,  of  gneissoid  granites  or  gneiss, 
basic  eruptives  and  schists,  jaspilites  and  associated  iron  ores,  and 
granites,  although  the  above  arrangement  may  be  considered  no  more 
than  a  hypothesis,  and  it  is  probable  that  the  jaspilites  and  iron  ores 
will  be  found  to  belong  to  the  Republic  formation.  It  is  also  probable 
that  the  Cascade  formation  itself  will  prove  to  be  composed  of  two  or 
more  distinct  geological  formations,  as  shown  by  the  fact  that  the 
chief  rock  of  the  Huron  Mountains  appears  to  be  a  gneissoid  granite, 
rather  than  a  true  sedimentarv  gneiss.  True  sedimentarv  o-neisses 
are  found  in  the  Huron  Bay  and  Cascade  districts.  In  the  former 
area  they  contain  fragments  that  closely  resemble  the  gneissoid 
granites,  and  thus  they  appear  to  be  formed  from  the  debris  of  those 
rocks.  If,  however,  the  gneissoid  granites  are  metamorphosed  erup- 
tive rocks,  and  not  true  gneisses  (which  are  restricted  to  metamor- 
phosed sedimentary  rocks),  this  fact  proves  only  that  the  gneisses  are 
younger  in  order  of  time,  but  not  necessarily  of  younger  geological 
age.  Similar  statements  apply  to  the  breaks  between  the  Cascade  and 
Republic  formations,  and  the  break  between  the  Republic  and  Holy- 
oke  formations.  In  the  Huron  Bay,  Menominee,  and  other  districts 
the  Cascade  formation  holds  intrusive  granites.  Amphibole  schists  are 
also  found  intrusive  in  the  gneisses  in  the  Cascade  area.  In  the  Mar- 
quette area  the  amphibole  schists  are  cut  by  felsite  or  quartz  porphyry. 
Much  of  the  granite  and  felsite  appears  to  have  been  erupted  dur- 
ing the  time  of  the  Cascade  formation,  and  perhaps  even  later.  On 
the  Cascade  range  hornblende  gneiss  cuts  the  country  rock.  These 
(likes  are  cut  by  other  dikes  containing  crystals  of  feldspar,  while 
both  are  cut  by  gray  granite  that  is  in  turn  cut  by  red  granite. 
The  Republic  formation,  commencing  with  the  oldest  division,  is 
divided  roughly  as  follows:  Conglomerate-breccia  and  conglomerate 
schist;  quartzite;  dolomite;  jaspilite  and  iron  ore;  argillite  and 
schist  ;  granite  and  felsite:  diabase;  diorite  and  porodite ;  porphvrite. 
At  the  base  of  the  Republic  formation  is  a  series  of  conglomerates  and 
conglomerate  schists,  which  pass  into  hydrous  mica  schists.  Near 
Palmer  the  coarse  conglomerate  rests  on  the  gneiss  to  the  south,  and 
is  overlain  to  the  north  by  quartzite,  fragniental  jaspilite.  and  quartz 
schist.  The  dip  is  about  40°  N.  The  conglomerate  contains  numerous 
pebbles  of  gneiss,  as  well  as  some  of  granite,  diorite.  schist,  and  quartz 
veins.     Near  the  Volunteer  mine  quartzite  immediately  overlies  the 
