152  PRE-CAMBRIAN    EOGKS    OF    NORTH    AMERICA.  [bull.  86. 
modifications  of  the  IromBearing  member  and  due  to  downward  perco- 
lating water,  which  has  removed  silica  and  has  substituted  iron  oxide. 
The  Upper  Slate  member  follows  above  the  Iron-Bearing  member.  It 
is  of  great  and  variable  thickness,  the  maximum  being  over  12,000  feet, 
and  it  varies  from  this  to  entire  disappearance,  the  overlying  series 
coming  in  contact  with  the  Iron-Bearing  or  lower  members.  The  rocks 
here  comprised  are  mica-schists  and  mica-slates,  graywackes  and  gray- 
wacke-slates,  clay-slates  or  phyllites,  and  quartzites  and  conglomerates, 
all  of  which  are  of  original  mechanical  detrital  origin.  The  mica- 
schists  and  mica-slates  are  traced  by  imperceptible  stages  back  to  their 
original  little  altered  or  unaltered  condition. 
These  three  members  constitute  the  Penokee  series  proper.  The 
Eastern  area  of  the  series  is  found  to  differ  in  many  respects  from  the 
main  area  already  described.  This  was  the  center  of  great  contempo- 
raneous volcanic  activity,  and  consequently  the  succession  includes 
large  thicknesses  of  lava  flows  and  volcanic  tuffs,  which  are  not  paral- 
leled by  the  rocks  found  in  the  western  area,  and  as  a  result  of  thi 
disturbing  force  the  detrital  succession  is  not  so  simple  and  regular. 
With  the  Penokee  series  are  found  eruptives  of  two  classes,  dikes 
cutting  the  formation  and  interbedded  sheets,  which  are  probably  in 
trusions  of  the  same  age  as  the  dikes.  These  eruptives  are  usually 
diabases,  which  are  like  the  dikes  found  in  the  complex  below  the 
Penokee  series,  and  which  chemically  are  like  the  overlying  Keweenaw 
series. 
The  Penokee  series  has  approximately  an  east-  and  west  strike,  is 
unfolded,  and  dips  to  the  north  at  an  angle  varying  usually  from  60° 
to  80°.  There  are  sharp  flexures  at  a  few  points  and  small  faults  at 
only  two  localities. 
While  the  strikes  and  dips  of  the  Penokee  series  are  persistent,  those  of 
the  underlying  schists  are  variable  and  often  are  at  marked  discordance 
with  the  Penokee  succession.  The  granites  which  cut  the  fine  grained 
schists  of  the  underlying  complex  are  never  seen  to  intersect  the  lime 
stone  or  quartz- slate.  At  quite  a  number  of  places  the  limestone  or 
quartz-slate  is  found  immediately  adjacent  to  or  in  actual  contact  with 
the  underlying  complex,  when  it  is  always  found  to  bear  numerous 
water-worn  fragments  from  the  Southern  Complex,  the  condition  of  which 
is  that  of  the  rock  from  which  it  is  derived.  When  the  contact  is  with 
the  green  schists  the  schistose  structure  of  the  underlying  rocks  abuts 
against  the  strike  of  the  quartz-slate,  while  the  fragments,  with  their 
greater  length  parallel  to  the  schistose  structure,  are  found  with  their 
longer  diameters  in  the  direction  of  the  bedding  of  the  slate,  showing 
that  their  schist osity  was  produced  before  they  were  broken  from  their 
original  position.  It  is  then  concluded  that  the  Southern  Complex  is 
separated  from  the  Penokee  series  by  a  great  unconformity,  and  that  as 
the  quartz-slate  is  persistent  for  a  distance  of  many  miles,  that  the 
underlying  complex  had  nearly  reached  a  base  level  before  the  overly- 
ing series  was  deposited. 
