LAKE    SUPERIOR   REGION.  193 
rocks.  The  Upper  Keweenawan  appears  in  a  broad  belt  in  the  south- 
eastern part  of  the  county  between  the  two  belts  of  Lower  Keweena- 
wan rocks.  It  is  a  series  of  conglomerates,  sandstones,  and  shales. 
In  a  belt  north  of  the  northern  belt  of  Lower  Keweenawan  rocks, 
extending  from  these  rocks  to  the  shore  of  Lake  Superior,  is  the  Lake 
Superior  sandstone  (Cambrian).  This  is  either  flat  lying  or  dips 
slightly  toward  Lake  Superior.  The  junction  of  the  sandstone  with 
the  Lower  Keweenawan  is  marked  by  a  fault,  along  which  the  Lake 
Superior  sandstone  has  been  depressed,  in  some  places  probably  as 
much  as  several  hundred  feet. 
The  Upper  and  Lower  Keweenawan  belts  form  a  syncline,  the  axis 
of  which  runs  northeast-southwest  through  the  center  of  the  tract 
underlain  by  Upper  Keweenawan  rocks. 
While  the  Keweenawan  rocks  of  this  area  are  the  same  in  kind 
and  in  age  as  are  the  productive  copper-bearing  rocks  of  Keweenaw 
Point,  the  probable  unproductive  character  of  the  Douglas  County 
Cocks  is  intimated. 
Weidman,126  in  1903,  describes  a  pre-Potsdam  peneplain  of  the  pre- 
Cambrian  of  north-central  Wisconsin  and  shows  that  it  slopes  grad- 
ually to  the  south,  where  it  is  covered  by  Paleozoic  sediments.  Several 
monadnocks  stand  above  the  pre-Potsdam  peneplain.  Extensive  clay 
deposits  near  the  contact  of  the  Paleozoic  and  the  pre-Cambrian  are 
believed  to  have  developed  during  the  pre-Potsdam  base-leveling. 
Burling,127  in  1905,  maps  and  describes  the  quartzite  of  Barron 
County.  There  is  a  prevailing  westward  dip,  indicating  a  thickness 
of  quartzite  of  5,000  feet,  if  folding  and  faulting  do  not  duplicate 
the  series. 
Weidman,128  in  1905,  maps  and  describes  a  pre-Cambrian  area  of 
north-central  Wisconsin  containing  about  7,200  square  miles  and  in- 
cluding the  counties  of  Marathon,  Portage,  Wood,  Clark,  Taylor, 
Lincoln,  and  adjacent  parts  of  Gates,  Price,  and  Langlade.  From  90 
to  95  per  cent  of  the  pre-Cambrian  rocks  of  this  area  are  of  igneous 
origin. 
The  rocks  believed  to  be  the  oldest  consist  of  a  complex  mixture  of 
rocks  such  as  contorted  and  crumpled  granite  gneiss,  diorite  gneiss, 
granite  schist,  syenite  schist,  and  diorite  schist.  The  gneisses  and 
schists  form  a  belt  or  group  which  can  be  fairly  well  outlined,  ex- 
tending from  the  vicinity  of  Stevens  Point  and  Grand  Rapids  in  a 
northwesterly  direction  through  Neillsville.  This  group  of  rocks  is 
referred  to  as  the  basal  group.  Its  various  members  are  closely  inter- 
woven and  intermingled  with  one  another,  and  have  been  subjected 
to  extensive  folding  and  metamorphism.  The  zone  or  belt  in  which 
this  group  is  largely  comprised  lies  between  areas  of  later  igneous  and 
sedimentary  rock  to  the  north  and  to  the  south,  and  hence  appears  to 
55721— Bull.  300—01) 13 
