vanhise.]  LAKE    SUPERIOR    REGION.  107 
Irving,124  in  1872,  maintains  that  the quartzites  of  Sank  county  are 
un conformably  below  the  Potsdam,  because  they  are  uptilted  at  a 
high  angle,  while  the  Potsdam  is  horizontal,  and  the  horizontal  sand- 
stone abuts  against  the  quartzite  and  holds  fragments  derived  from 
it.     These  quartzites  are  either  Laurentian  or  Huronian. 
Murrish,125  in  1873,  describes  the  quartzite  ranges  of  Baraboo  as  a 
metamorphic  sandstone  of  the  Potsdam  age.  On  the  Black  and  Yel- 
low rivers  are  found  granitic  and  hornblendic  Azoic  and  pi utonic  rocks. 
At  Black  river  falls  are  knobs  of  magnetic  ore  in  a  series  of  elongated 
knobs  or  mounds,  associated  with  quartz  and  micaceous  slate.  At 
Grand  rapids,  on  the  Wisconsin  river,  Azoic  rocks  similar  to  those  on 
Black  river  are  found.  A  quartzite  mound  at  Necedah  occupies  a 
geological  position  similar  to  the  iron  ores  at  Black  river  falls. 
Eaton,126  in  1873,  maps  the  quartzites  of  the  Baraboo  river.  At; 
Ableman's  the  highly  tilted  quartzites  are  flanked  on  both  sides  by 
horizontal  sandstone  and  conglomerate,  the  latter  having  angular  frag- 
ments of  the  quartzite  of  varying  magnitudes.  The  overlying  sand- 
stone is  exactly  like  that  described  by  Irving  at  Devil's  lake  as  con- 
taining Potsdam  fossils.  The  sandstone  is  above  horizontal  limestone 
containing  Pleurotomaria.  The  quartzite  is  then  an  old  Azoic  reef  of 
tilted  rocks  which  has  suffered  enormous  erosion  before  washed  by  the 
waves  of  the  Potsdam  sea. 
Irving,127  in  1873,  maintains  the  pre-Potsdam  age  of  the  Portland 
quartzite  on  the  same  ground  as  the  pre-Potsdam  age  of  the  Baraboo 
ranges.  There  is  a  close  similarity  between  the  Baraboo  and  Portland 
quartzites  and  the  rocks  in  northern  Wisconsin  and  Michigan  which 
are  now  regarded  as  Huronian. 
Irving,128  in  1874,  describes  as  occurring  in  northern  Wisconsin  four 
distinct  groups  of  rocks,  the  Laurentian,  Huronian,  Copper -bearing, 
and  Lower  Silurian.  The  Laurentian  consists  of  granites,  gneisses, 
and  syenites  for  the  most  part,  although  there  may  be  various  schist 
beds  present.  The  Huronian  rocks  consist  of  siliceous  schists,  quartz 
rock,  and  black  slates,  magnetic  and  specular  schists  and  slates, 
metamorphic  diorite  and  diorite-schists.  Its  lowest  portion  is  of 
simple  siliceous  schist  with  some  granular  white  quartz,  gray  quartz- 
ite, and  black  slate.  The  central  portion  consists  of  magnetic  and 
specular  slates  and  schists  in  which  all  the  ores  are  found,  and  the 
highest  and  northernmost  portion  consists  of  diorites,  diorite- slates, 
diorite-schists,  and  quartz-slates.  .The  Copper-bearing  series  is  next 
north  of  and  immediately  overlies  the  Huronian,  and  is  of  enormous 
thickness,  never  less  than  4  miles.  The  lower  portions  of  the  group 
are  probably  in  part  of  igneous  origin,  but  the  upper  portions  are  be- 
yond all  doubt  exclusively  the  results  of  sedimentation.  The  group 
consists  of  shales,  sandstones,  conglomerates,  amygdaloids,  and  traps. 
The  sedimentary  series  do  not  altogether  overlie  the  trappean  beds, 
but  are  near  their  junction  directly  and  unmistakably  interstratiiied 
