194  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
have  the  position  of  the  arch  of  an  anticline.  This  basal  group  is 
intruded  by  later  formations  of  rhyolite,  diorite,  and  granite.  Sedi- 
mentary rocks  have  not  been  found  in  contact  with  this  group. 
The  next  group  of  rocks  is  of  sedimentary  origin  and  consists  of 
quartzite,  slate,  and  graywacke.  These  formations  include  the 
quartzite  of  Rib  Hill  and  vicinity,  the  quartzite  of  Powers  Bluff  and 
in  the  vicinity  of  Junction  and  Rudolph,  a  wide  belt  of  slate  in 
northwestern  Marathon  County,  and  graywackes  in  the  vicinity  of 
Wausau.  These  formations  are  almost  entirely  of  fragmental  origin 
and  only  rarely  contain  phases  of  carbonaceous,  calcareous,  and  fer- 
ruginous deposits.  The  floor  upon  which  this  group  of  sediments  is 
deposited  can  not  be  definitely  determined,  for  in  all  observed  cases 
the  contacts  are  either  with  later  intrusive  igneous  rocks  or  with 
later  overhang  conglomerate.  The  quartzites  are  throughout  ex- 
tremely metamorphosed  and  to  all  appearances  completely  recrystal- 
lized.  The  slates  and  graywackes  do  not  reveal  as  much  meta- 
morphism  as  the  quartzite,  although  in  places  rocks  presumably  be- 
longing with  the  slate  formation  have  been  changed  to  schists  bearing 
staurolite,  cordierite,  and  garnet.  These  sedimentary  formations 
appear  to  bear  the  relation  of  great  fragmentary  masses  intersected 
and  surrounded  by  later  igneous  intrusives.  They  constitute  the 
lower  or  older  sedimentaries  of  this  area. 
The  next  younger  group  of  rocks  is  of  igneous  origin.  This  group 
forms  about  75  per  cent  of  the  rocks  of  the  area,  and  in  the  order  of 
their  intrusion  are  (1)  rhyolite,  (2)  a  basic  series  of  diorite,  gabbro, 
and  peridotite,  and  (3)  a  series  consisting  of  granite,  quartz  syenite, 
nepheline  syenite,  and  related  rocks.  Of  these  the  last-named  series 
is  the  most  abundant,  the  granite  alone  forming  about  50  per  cent 
of  the  surface  rocks  of  the  area.  The  various  members  of  this  group 
are  found  intrusive  in  the  basal  group  of  the  area  and  also  in  the 
lower  sedimentary  formations.  They  are  in  turn  overlain  by  later 
pre-Cambrian  sediments.  The  period  involved  in  the  outflow  of  the 
various  igneous  formations  of  this  group  must  have  been  a  very  long 
one  and  evidently  constituted  an  important  portion  of  the  pre-Cam- 
brian era,  for  the  granite-syenite  series  itself  represents  a  complex 
magma  of  variable  though  related  rocks  intruded  at  different  dates. 
In  the  stratigraphy  of  this  area,  therefore,  this  group  of  igneous 
intrusives  is  one  of  the  most  important  and  occupies  a  well-defined 
position  between  the  lower  and  upper  pre-Cambrian  sedimentaries. 
The  latest  pre-Cambrian  of  the  area  consists  mainly  of  conglom- 
erate and  quartzite  overlying  all  the  other  formations  above  referred 
to.  This  group  is  represented  by  conglomerate  and  quartzite  north 
of  Wausau,  conglomerate  and  quartzite  at  Arpin  and  North  Mound, 
and  conglomerate  at  Marathon  and  Mosinee. 
