ISOLATED  AREAS   IN    MISSISSIPPI  VALLEY.  723 
The  crystalline  rocks  are  unconformably  overlain  by  flat-lying 
Potsdam  and  Ordovician  sediments.  From  their  similarity  in  com- 
position to  the  Baraboo  volcanics,  which  are  considered  to  be  of  Ke- 
weenawan  age,  it  is  believed  that  they  belong  to  the  same  province, 
and  are  therefore  of  Keweenawan  age. 
For  literature  on  correlation  of  this  district  with  the  rocks  of  Lake 
Superior,  see  summaries  of  articles  by  Irving,  Van  Hise,  and  others 
in  Chapter  III,  Lake  Superior  region,  pages  108-252. 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
In  Fox  River  valley  are  several  small  isolated  outcrops  of  pre- 
Cambrian  crystalline  rocks  projecting  through  the  Paleozoic  sedi- 
ments. These  are  at  Berlin,  Utley,  Waushara,  Marquette,  Montello, 
Observatory  Hill,  Marcellon,  and  Endeavor.  The  rocks  are  mainly 
acidic  extrusives,  metarhyolites,  showing  gradations  into  rocks  of 
more  deep-seated  origin,  rhyolite  gneiss,  quartz  porphyry,  and  gran- 
ite, all  of  them  cut  by  basic  dikes.  -  The  characteristic  feature  in  the 
metarlryolites  is  the  presence  of  abundant  and  well-preserved  surface 
volcanic  textures,  such  as  fluxion,  perlitic,  spherulitic,  and  brecciated 
textures.  The  lithological  similarities  of  the  rocks,  the  presence  of 
the  surface  textures,  and  their  composition  as  shown  by  analysis, 
indicate  clearly  the  consanguinity  of  the  igneous  rocks  of  these  areas 
with  one  another  and  with  certain  of  the  igneous  rocks  on  the  north 
and  south  sides  of  the  Baraboo  Range.  In  the  Baraboo  district  these 
rocks  have  been  found  by  Weidman  to  be  unconformably  below  the 
sedimentary  rocks,  and  hence  the  volcanics  of  Fox  River  may  be  sup- 
posed to  be  pre-Huronian.  Up  to  1903  all  had  been  tentatively  re- 
ferred to  the  Keweenawan,  because  the  Baraboo  igneous  rocks  had 
been  supposed  to  lie  on  the  quartzite. 
NECEDAH,  NORTH  BLUFF,  AND  BLACK  RIVER. 
SUMMARY   OF  LITERATURE. 
Percival,  in  1850,  discusses  the  geology  of  this  district.  See  sum- 
mary under  Baraboo,  page  717. 
Daniels,22  in  1858,  describes  the  iron  ores  of  Black  River  Falls  as 
associated  with  the  chloritic  and  micaceous  slates  of  the  Azoic  sys- 
tem. Syenite  is  also  found  adjacent.  The  fossiliferous  horizontal 
sandstone  rests  upon  the  upturned  edges  of  the  Azoic  slates,  and  at 
the  base  of  it  is  a  brecciated  conglomerate  consisting  of  sand,  ore,  and 
slate.  In  the  lower  part  of  the  Baraboo  Valley  are  lofty  ranges  of 
hard  quartzite  which  are  the  soft,  crumbling  Potsdam  sandstone  vio- 
lently disturbed  and  changed. 
