142  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
to  rest  for  the  most  part  upon  the  underlying  quartzite  and  upon  a 
series  of  dikes  which  have  cut  the  stratified  layers.  The  ores  in  this 
position  are  secondary  concentrations  regarded  as  produced  at  the 
same  time  as  the  modifications  of  the  Iron-bearing  member  and  due 
to  downward-percolating  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  more  than 
12,000  feet,  and  it  varies  from  this  to  entire  disappearance,  the  over- 
lying series  coming  in  contact  with  the  Iron-bearing  or  lower  mem- 
bers. The  rocks  here  comprised  are  mica  schists  and  mica  slates, 
graywackes  and  graywacke  slates,  clay  slates  or  phyllites,  and  quartz- 
ites  and  conglomerates,  all  of  which  are  of  original  mechanical 
detrital  origin.  The  mica  schists  and  mica  slates  are  traced  by  im- 
perceptible 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  this 
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-west  strike,  is  un- 
folded, 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  inter- 
sect the  limestone  or  quartz  slate.  At  a  number  of  places  the  lime- 
stone or  quartz  slate  is  found  immediately  adjacent  to  or  in  actual 
contact  with  the  underlying  complex,  when  it  is  always  found  to  bear 
numerous  waterworn  fragments  from  the  Southern  Complex,  the  con- 
dition 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  un- 
derlying rocks  abuts  against  the  strike  of  the  quartz  slate,  while  the 
