110  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
undisturbed  position  on  the  bedding  of  the  trap.  Either  the  Superior 
red  sandstone  is  older  than  the  Potsdam  or  the  trap  rocks  in  conjunc- 
tion with  the  Superior  red  sandstone  are  younger  than  those  in  con- 
junction with  the  Potsdam. 
Chamberlin,132  in  1877,  describes  the  Archean  rocks  which  in  the 
eastern  part  of  Wisconsin  protrude  but  are  not  intrusive  in  the  Pale- 
ozoic formations.  These  are  the  Muliwa  granite,  the  Berlin  porphyry, 
the  Pine  bluff  quartz-porphry,  the  Marquette  quartz-porphyry  and  the 
quartzites  of  Portland  and  Waterloo.  The  porphyries  are  found  to* 
have  obscure  but  distinct  bedding.  The  metamorphosed  quartzites 
show  ripple  marks  and  contain  conglomeratic  layers.  The  Potsdam 
sandstone  and  Lower  Magnesian  limestone  rest  in  a  horizontal  position 
against  and  contain  fragments  from  the  crystalline  rocks.  The  quartz- 
ites are  regarded  as  originally  sandstones  and  conglomerates  which 
were  metamorphosed  before  the  deposit  of  the  neighboring  horizontal 
rocks,  and  which  have  been  tilted  and  eroded  before  the  stratified  rocks 
were  deposited.  These  quartzites  are  regarded  as  a  portion  of  the  Bar- 
aboo  quartzite  series. 
Irving,133  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  de- 
tailed  study.  On  the  south  side  of  the  Laurentian  core,. on  Black  river, 
and  in  isolated  masses,  are  ferruginous  schists,  quartzites,  and  quartz- 
porphyries,  which  are  probably  Huronian.  The  presence  of  these  rocks 
on  the  south,  the  quartzites  of  Chippewa  and  Barron  counties  on  the 
west,  and  the  Huronian  rocks  of  the  Penokee  range  on  the  north  leads \ 
to  the  suggestion  that  the  Huronian  rocks  entirely  surround  the  Lauren- 
tian 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  unconformability,  the  ex-i 
act  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  by  a  prevailing  gneissoid  and  schistose] 
character,  but  also  by  the  existence  of  distinct  bedding  planes  which ; 
can  generally,  even  in  the  granitoid  kinds  of  rocks,  be  readily  made  out.' 
The  processes  of  metamorphism  and  disturbance  have  been  carried  to« 
the  last  extreme,  as  shown  by  the  highly  crystalline  character  of  thej 
rocks  and  the  fact  that  the  gneiss  grades  into  granite,  as  well  as  byj 
the  greatly  contorted  condition  of  the  gneiss  laminae  and  the  close  fold-; 
ing  of  the  whole  series.  While  the  series  as  a  whole  is  bedded,  dis-i 
tinctly  intrusive  granite  occurs,  as  shown  by  the  way  in  which  it  joins  '■ 
and  penetrates  the  bedded  rocks.  The  main  area  of  the  crystalline' 
rocks  certainly  belongs  to  the  Laurentian,  and  a  small  area  only  on  the] 
south  of  the  district  is  doubtfully  Huronian. 
