LAKE  SUPEKIOR  REGION.  183 
'  fossils  and  numerous  fragments  from  the  older  rocks,  lies  upon  and 
wedged  in  between  the  tilted  ledges  of  the  Huronian.  Exactly  similar 
unconformity  is  to  be  seen  at  the  Dalles  of  the  St.  Croix  between  the 
Potsdam  and  the  Copper  series. 
Wight,108  in  1877,  describes  the  horizontal  Potsdam  sandstone  as 
resting  on  the  uneven  and  tilted  surface  of  the  underlying  igneous  or 
crystalline  rocks  at  St.  Croix  Falls.  The  sandstone  contains  numer- 
ous well-preserved  fossils,  even  where  almost  in  contact  with  the  trap. 
At  Pine  Island,  in  Kettle  River,  the  Superior  red  sandstone  con- 
tains abundant  fragments  of  the  adjacent  trap,  forming  a  brecciated 
conglomerate  kindred  to  the  conglomerate  which  extends  from  Ke- 
weenaw Point  along  the  northern  base  of  the  Porcupine  and  Penokee 
mountains.  Everywhere  this  conglomerate  is  composed  of  frag- 
ments of  the  more  elevated  Huronian  or  trap  ridges.  The  Superior 
red  sandstone,  wherever  it  borders  the  trap  ridges,  shows  that  it  has 
been  tilted,  broken  up,  or  crushed.  It  appears  that  the  trap,  whether 
erupted  or  upheaved  convulsively  or  slowly,  encountered  this  forma- 
tion in  its  ascent.  On  the  contrary,  the  Potsdam  sandstone  every- 
where rests  in  a  horizontal,  undisturbed  position  on  the  bedding  of 
the  trap.  Either  the  Superior  red  sandstone  is  older  than  the  Pots- 
dam or  the  trap  rocks  in  conjunction  with  the  Superior  red  sandstone 
are  younger  than  those  in  conjunction  with  the  Potsdam. 
Irving,109  in  1877,  describes  the  Archean  rocks  which  cover  all  of 
Marathon,  most  of  Wood,  and  much  of  Clark,  Jackson,  and  Portage 
counties,  in  Wisconsin.  The  Laurentian  is  a  great  mass  of  crystalline 
rocks — granite,  gneiss,  chloritic,  micaceous,  and  hornblendic  schists — 
which  are  folded  and  eroded  so  as  to  offer  the  greatest  obstacles  to 
their  detailed  study.  On  the  south  side  of  the  Laurentian  core,  on 
Black  River,  and  in  isolated  masses,  are  ferruginous  schists,  quartz- 
ites,  and  quartz  porphyries,  which  are  probably  Huronian.  The 
presence  of  these  rocks  on  the  south,  of  the  quartzites  of  Chippewa 
and  Barron  counties  on  the  west,  and  of  the  Huronian  rocks  of  the 
Penokee  range  on  the  north  leads  to  the  suggestion  that  the  Huronian 
O  Bo 
rocks  entirely  surround  the  Laurentian  core  of  northern  Wisconsin. 
The  line  of  junction  between  the  Archean  area  and  the  Potsdam 
formation  to  the  south  is  exceedingly  irregular.  The  latter  always 
rests  in  a  horizontal  position  upon  the  crystalline  formations  with 
the  most  marked  unconformity,  the  exact  contacts  being  found  at 
several  places.  The  most  abundant  of  the  crystalline  rocks  is  gneiss, 
and  the  original  bedded  condition  of  the  whole  series  is  evident,  not 
only  from  the  prevailing  gneissoid  and  schistose  character,  but  also 
from  the  existence  of  distinct  bedding  planes,  which  can  generally, 
even  in  the  granitoid  kinds  of  rocks,  be  readily  made  out.  The  pro- 
cesses of  metamorphism  and  disturbance  have  been  carried  to  the  last 
