184  FEE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
extreme,  as  shown  by  the  highly  crystalline  character  of  the  rocks  and 
the  fact  that  the  gneiss  grades  into  granite,  as  well  as  by  the  greatly 
contorted  condition  of  the  gneiss  laminae  and  the  close  folding  of  the 
Whole  series.  While  the  series  as  a  whole  is  bedded,  distinctly  intru- 
sive granite  occurs,  as  shown  by  the  way  in  which  it  joins  and  pene- 
trates the  bedded  rocks.  The  main  area  of  the  crystalline  rocks  cer- 
tainly belongs  to  the  Laurentian ;  only  a  small  area  on  the  south  of 
the  district  is  doubtfully  Huronian. 
Very  numerous  details  are  given  as  to  particular  localities,  showing 
the  manner  of  occurrence  and  the  relations  of  the  different  varieties 
of  rocks  and  the  unconformities  which  exist  between  them  and  the 
Silurian.  On  Mosinee  and  Rib  hills  are  large  exposures  of  quartzite. 
At  Black  River  Falls  the  regularly  bedded  succession  of  highly  tilted 
strata  of  many  members  consists  in  large  part  of  regularly  laminated 
schistose  rocks,  such  as  ferruginous  quartz  schist  and  magnesian 
schist  or  slate,  having  together  an  approximate  thickness  of  at  least 
5,000  feet.     Gneiss  and  granite  are  also  found  here. 
Isolated  from  the  main  Archean  area  are  numerous  exposures  of 
crystalline  rocks  which  protrude  in  moundlike  forms  from  beneath 
the  horizontal  strata.  The  largest  of  these  are  the  quartzite  ranges 
of  Baraboo.  About  many  of  these  areas  is  found  horizontal  sand- 
stone lying  immediately  against  the  tilted  crystalline  rocks  and 
carrying  pebbles  and  bowlders  derived  from  them,  proving  that  they 
are  all  of  greater  antiquity  than  the  surrounding  sandstone  layers. 
These  relations  are  particularly  well  shown  in  the  Baraboo  quartzite 
ranges.  Aside  from  these  ranges,  the  more  important  areas  are  the 
Marcellon,  Observatory  Hill,  Moundville,  Seneca  (Pine  Bluff),  Mar- 
quette, and  Berlin  quartz  porphyries,  the  Montello  and  Marion  gran- 
ites, and  the  Necedah  quartzite. 
Irving,110  in  1878,  describes  the  siliceous  slate  at  Potato  River, 
which  is  one  of  the  lower  members  of  the  Huronian,  as  in  contact 
with  the  chloritic  gneiss  of  the  Laurentian.  The  slate  inclines  at  a 
high  angle  to  the  north,  while  the  gneiss  la}7ers  dip  to  the  south  and 
strike  in  a  direction  oblique  to  that  of  the  slate  layers. 
Chamberlin,111  in  1878,  describes  the  exact  junction  between  the 
Laurentian  and  Huronian  series  on  the  Gogogashugun,  in  the  Peno- 
kee  district.  The  Laurentian  member  consists  of  a  peculiar  gneissoid 
rock,  like  that  which  occupies  a  similar  relation  at  Penokee  Gap. 
The  Huronian  lies  in  absolute  contact  with  this.  Its  siliceous 
material  at  the  time  of  its  deposition  so  insinuated  itself  into  the 
irregularities  of  the  surfaces  of  the  gneiss  that  the  two  formations 
are  interlocked,  and  of  a  hand  specimen  that  was  obtained  one  por- 
tion is  Laurentian  gneiss  and  the  other  Huronian  schist,  the  two 
being  unconformable.     The  Huronian  siliceous  schists  are  overlain 
