LAKE    SUPERIOR   REGION. 
stone,  trap,  and  amygdaloid  similar  in  their  general  aspect  to  the 
contemporaneous  ranges  of  igneous  rocks  which  occur  in  the  mining 
district  of  Michigan.  Besides  this  trap  formation  there  is  an  entirely 
distinct  trap  system,  in  immediate  juxtaposition  with  which  strata 
have  been  discovered  which  are  as  old  as  if  not  older  than  the 
Lingula  beds  of  the  Potsdam  sandstone  of  New  York. 
Whittlesey,100  in  1852,  gives  geological  descriptions  of  part  of 
Wisconsin  south  of  Lake  Superior.  Passing  from  the  lake  southerly 
four  great  classes  of  rocks  are  seen  in  each  section:  (1)  Sedimentary, 
including  red  sandstone,  black  slate,  and  conglomerate.  (2)  Trappose 
rocks,  or  those  of  volcanic  origin,  including  amygdaloid,  greenstone, 
augitic,  hornblendic,  and  feldspathic  rocks,  embracing  syenites  and 
granites  of  the  same  age.  (3)  Metamorphosed  rocks,  including  horn- 
blende slates,  iron  slates,  black  slates,  talcose  slates,  and  slaty  quartz. 
(4)  Granitic,  including  syenite  and  granite.  The  granites  and  sye- 
nites of  the  interior  are  the  most  ancient  rocks  of  the  district.  Since 
the  protrusion  of  these  granitic  masses  many  changes  have  occurred. 
The  sandstone  of  Lake  Superior  must  have  been  deposited  subsequent 
to  the  granites  of  the  Wisconsin,  Chippewa,  and  Montreal  rivers,  for 
that  period  has  been  one  of  long  and  intense  igneous  action  in  which 
the  trap,  hornblendic,  and  greenstone  masses  have  been  ejected,  and 
also  with  them  protrusions  of  recent  granites  and  syenite.  The  meta- 
morphic  slates  have  been  elevated  during  these  convulsions,  and  the 
sedimentary  rocks  thrust  far  northward  and  tilted  up  at  high  angles. 
The  old  granites  and  syenites  have  been  rent  with  fluid  matter,  such 
as  quartz  and  hornblende.  The  northern  part  of  the  Penokee  range 
shows  evidence  of  four  formations  of  trappose  rocks,  which  fill  a 
geological  epoch  of  no  great  duration  between  the  area  of  the  red 
sandstone  deposits  and  the  metamorphic  uplifts.  There  are  cases 
where  the  trap,  instead  of  being  forced  across  the  strata,  has  spread 
out  between  the  beds,  forming  alternate  strata  of  trap  and  sandstone 
without  any  visible  conglomerate. 
Perctval,  in  1856,  discusses  the  geology  of  this  district.  See  sum- 
mary in  Chapter  XII,  section  1,  Baraboo,  page  717. 
Lapham,101  in  1860,  describes  the  Penokee  iron  range.  Here  is 
found  a  mountain  mass  of  iron  ore  in  an  ancient  chloride  slate  which 
rests  upon  a  light-colored  quartz  rock.  Above  and  north  of  the  ore 
the  slate  is  hardened,  probably  by  some  volcanic  agency.  The  whole 
series  dips  to. the  north. 
Half,  (James),102  in  1861,  describes  the  quartzite  ranges  of 
northern  Wisconsin,  and  particularly  those  of  Spirit  Lake,  as  having 
been  original  stratified  sandstones  which  have  undergone  subsequent 
metamorphism.  These  rocks  are  folded  with  their  axes  lying  in  an 
east-west  direction  and  had  become  uplifted  and  metamorphosed  be- 
fore the  commencement  of  Potsdam  time.     In  the  quartzites  in  two 
