vanhise.]  LAKE    SUPERIOR    REGION.  HI 
Very  numerous  details  are  given  as  to  particular  localities,  showing 
the  manner  of  occurrence  and  relations  of  the  different  varieties  of 
rocks  and  the  unconformities  which  exist  between  them  and  the  Silu- 
rian. On  Mosinee  and  Rib  hills  are  found  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  magriesian  schist 
or  slate,  having  together  an  approximate  thickness  of  at  least  5,000 
feet.     Gneiss  and  granite  are  also  here  found. 
Isolated  from  the  main  Archean  area  are  quite  numerous  exposures 
of  crystalline  rocks  which  protrude  in  mound-like  forms  from  beneath 
the  horizontal  strata.  The  largest  of  these  are  the  quartzite  ranges  of 
Baraboo.  About  many  of  these  areas  are  found  horizontal  sandstone 
lying  immediately  against  the  tilted  crystalline  rocks  and  carrying  peb- 
bles and  bowlders  derived  from  them,  proving  that  they  are  all  of 
greater  antiquity  than  the  surrounding  sandstone  layers.  These  rela. 
tions  are  particularly  well  shown  in  the  Baraboo  quartzite  ranges.  Aside 
from  these  ranges,  the  more  important  areas  are  the  Marcellon,  Observ- 
atory hill,  Moundville,  Seneca  (Pine  bluff),  Marquette  and  Berlin 
quartz-porphyries,  the  Montello  and  Marion  granites,  and  the  Necedah 
quartzite. 
Irving,134  in  1878,  describes  at  Potato  river  the  siliceous  slate,  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  layers  dip  to  the  south  and  strike  in  a  direction 
oblique  to  that  of  the  slate  layers. 
Chamberlin,135  in  1878,  describes  ou  the  Gogogashugun,  in  the 
Penokee  district,  the  exact  junction  between  the  Laurentian  and  Hu- 
ronian series.  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 
fche  surfaces  of  the  gneiss  that  the  two  formations  are  interlocked,  and 
a  hand  specimen  was  obtained,  one  portion  of  which  is  Laurentian 
gneiss  and  the  other  Huronian  schist,  the  two  being  unconformable, 
rhe  Huronian  siliceous  schists  are  overlain  by  beds  of  white  and  red 
piartzite,  and  these  graduate  into  alternating  layers  of  quartzite  and 
ron  ore.  The  iron  ore  horizon  is  here  hematitic  and  soft,  but  is  the 
equivalent  of  the  hard  magnetic  horizon  to  the  west.  In  this  part  of 
3he  belt  is  presented  the  greatest  probabilities  of  the  existence  of  work- 
ible  ore. 
Irving,136  in  1880,  gives  a  comprehensive  account  of  the  general 
structure  of  northern  Wisconsin.  Here  are  found  four  great  systems? 
;he  Laurentian,  Huronian,  Keweenawan,  and  Lower  Silurian,  all  of 
Lhich  are  unconformable  with  each  other.  The  rocks  of  the  crystal- 
line nucleus  are  correlated  with  the  Laurentian  of  Canada  because  they 
sustain  the  same  structural  relations  to  the  Huronian,  Keweenawan, 
