118  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
subsiding  during  the  time  of  the  formation  of  these  beds.  While  tilted, 
they  are  not  contorted  or  metamorphosed.  There  is  in  this  period  no 
direct  evidence  of  the  existence  of  life.  Over  the  great  conglomerate 
of  the  Penokee  and  Porcupine  mountain  regions  is  a  black  shale  that 
simulates  those  of  later  ages  formed  in  association  with  life.  After 
the  close  of  the  Keweenawan  period,  before  the  Potsdam  sedimentation 
began,  there  was  a  period  of  erosion.  How  great  this  interval  was  has 
not  yet  been  determined;  quite  possibly  the  lower  Cambrian  forma- 
tions of  Great  Britain  and  Bohemia  bridge  the  entire  interval. 
Irving,146  in  1883,  gives  a  systematic  account  of  the  lithology  of 
Wisconsin.  Among  the  eruptive  rocks  are  placed  diabase,  melaphyre, 
gabbro,  norite,  diorite,  peridotite,  syenite,  porphyry,  and  granite. 
Among  the  schistose  rocks  are  gneiss,  mica-schist,  hydromica-schist, 
actinolite- schist,  tremolite- schist,  hornblende- schist,  augite- schist,  chlor- 
ite-schist,  talc-schist,  magnetite- schist,  hematite-schist,  quartz-schist, 
quartzite  in  part,  chert- schist,  and  jasper- schist.  Among  the  half  f rag- 
mental  rocks  are  quartzite,  clay-slate,  and  novaculite.  The  explana- 
tion of  the  origin  of  gneiss  or  lamellar  granite  by  metamorphism,  the 
structure  being  regarded  as  residual  sedimentation,  is  regarded  as  un- 
satisfactory. Many  rocks  which  have  been  called  metamorphic  are 
placed  as  eruptive,  and  it  seems  not  improbable  that  the  same  origin 
is  to  be  attributed  to  some  rocks  with  a  strongly  developed  schistose 
structure.  The  hornblende- schists  are  regarded  as  altered  forms  of 
augite- schists. 
Wooster,147  in  1884,  describes,  upon  the  St.  Croix  river  near  Osceola 
mills,  the  Potsdam  sandstone  carrying  fossils,  which  grades  down  into 
a  conglomerate  containing  pebbles  from  the  Keweenawan,  Laurentian, 
and  Huronian  series.  The  sandstone  and  conglomerate  rest  unconform- 
ably  upon  the  underlying  Keweenawan  rocks. 
Fulton,148  in  1888,  describes  the  Huronian  rocks  of  the  eastern  Me- 
nominee region  as  consisting  of  three  formations.  The  basal  formation 
is  a  crystalline  siliceous  limestone  at  least  1,200  feet  thick,  which  out- 
crops at  many  localities  along  the  range,  especially  north  of  the  Norway, 
Quinnesec,  and  Chapin  mines.  The  next  group,  estimated  at  a  thou- 
sand feet  in  thickness,  is  the  Quinnesec  ore  formation.  It  consists  of 
siliceous  or  jasper  slates,  largely  impregnated  with  iron  oxides.  These 
are  succeeded  by  argillaceous  hydromica  slates  and  flesh-colored  slates. 
This  formation  embraces  the  deposits  of  iron  ore.  The  third  formation 
is  a  series  of  dark  gray,  slaty,  or  schistose  rocks,  with  occasional 
quartzose  bands,  having  a  thickness  of  2,000  feet,  and  is  called  the  lake 
Hanbury  slate  group.  Detailed  sections  at  the  East  Yulcan,  Curry, 
Norway,  Cyclops,  and  Quinnesec  mines  are  described.  In  some  cases 
the  ore  is  associated  with  the  Potsdam  sandstone,  which  rests  unconfor- 
mably  in  a  horizontal  position  upon  the  flexed  and  denuded  Huronian 
rocks.  The  iron  ore  beds  in  the  Huronian  are  generally  associated 
with  aluminous  slates  or  soap  stones. 
