346  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
Keweenawan  had  been  built  up,  the  rising  lava  from  the  deep-seated 
reservoirs  penetrated  the  earlier  Keweenawan  precisely  as  it  did  the 
series  earlier  than  the  Keweenawan.  These  intrusions  continued 
throughout  the  major  portion  of  middle  Keweenawan  time,  and  thus 
earlier  intrusives  are  cut  by  later  intrusives. 
UPPER    KEWEENAWAN. 
The  upper  Keweenawan  is  confined  to  northern  Wisconsin  and 
Michigan,  where  it  constitutes  a  great  sedimentary  group  consisting 
of  conglomerates,  sandstones,  and  shales.  It  has  been  divided  into 
three  formations,  from  the  base  up:  (1)  The  "Outer  conglomer- 
ate," consisting  mainly  of  conglomerate  with  red  sandstone;  (2)  the 
Nonesuch  formation,  consisting  of  black  shale  and  gray  sandstone; 
and  (3)  the  Freda  sandstone,  the  main  body  of  red  sandstone.  The 
"  Outer  conglomerate  "  has  been  traced  for  a  distance  of  between  175 
and  200  miles.  Its  thickness  has  been  estimated  at  different  places 
as  varying  from  350  feet  to  nearly  5,000  feet.  The  Nonesuch  forma- 
tion has  been  traced  125  miles.  Its  thickness  varies  from  125  to  500 
feet.  This  formation,  unlike  the  sandstones  and  conglomerates  at 
lower  horizons,  contains  a  large  and  sometimes  dominant  amount  of 
basic  detritus.  The  Freda  sandstone  constitutes  much  the  larger 
portion  of  the  upper  division  of  the  Keweenawan.  At  Montreal 
River  Irving  estimates  the  thickness  of  this  sandstone  at  12,000  feet. 
This  sandstone,  like  the  Nonesuch  formation,  is  very  largely  com- 
posed of  basic  detritus.  In  places  it  is  nearly  quartzless.  Thus  it  is 
clear  that  its  source  is  dominantly  the  basic  igneous  rocks  and  only 
subordinately  the  acidic  igneous  rocks  of  the  Keweenawan.  Also  in 
places  the  pre-Cambrian  rocks  have  contributed  subordinate  amounts 
of  material. 
STRUCTURE. 
As  was  shown  by  Irving,  the  Keweenawan  as  a  whole  is  a  great 
synclinorium,  the  margin  of  which  for  the  eastern  half  of  Lake  Su- 
perior closely  corresponds  with  the  shore,  but  which  for  the  western 
half  of  the  lake  extends  for  a  considerable  distance  inland.  Subordi- 
nate to  the  greater  syncline  are  other  lesser  synclines,  such  as  the 
extensions  of  Chequamegon  Bay,  Fond  du  Lac  Bay,  and  Nipigon 
Lake.  The  dips  of  the  rocks  are  very  variable,  ranging  from  almost 
flat  in  the  neighborhood  of  Thunder,  Black,  and  Nipigon  bays  to 
angles  of  60°,  70°,  or  even  80°  north  of  the  Gogebic  district  in  Michi- 
gan and  Wisconsin.  In  general  the  dips  to  the  south  on  the  north 
shore  of  Lake  Superior  are  less  steep  than  the  dips  to  the  north  on 
the  south  shore.  Also  in  any  section  the  dips  are  steeper  at  the  lower 
horizons  than  at  the  higher  horizons.  Correlative  with  the  folding 
there  is  extensive  faulting.     The  best  known  case  of  faulting  occurs 
