LAKE    SUPERIOR   REGION.  141 
eluded  that  many  of  the  schists  also  have  a  like  origin,  because  of  the 
gradations  between  them. 
The  Penokee  series  proper  is  made  up  of  three  members,  Quartz 
slate,  Iron  group,  and  Upper  slate,  and  these  rest  unconformably 
upon  a  Cherty  limestone  member. 
The  Cherty  limestone  below  the  base  of  the  Penokee  series  proper 
varies  in  thickness  from  nothing  to  300  feet  and  is  not  continuous. 
This  member  as  a  whole  shows  no  evidence  of  mechanical  origin, 
although  occasionally  a  small  amount  of  detrital  material  is  found  in 
it.  It  is  regarded  as  either  a  chemical  or  an  organic  sediment,  and  is 
called  the  Lower  Penokee  formation. 
The  Quartz  slate  member,  resting  upon  the  Cherty  limestone  or 
upon  the  Southern  Complex,  is  a  continuous  persistent  layer  of  very 
constant  thickness  for  many  miles.  It  is  for  the  most  part  in  the 
neighborhood  of  450  feet  thick,  although  at  one  locality  it  reached 
800  feet  in  thickness.  The  rocks  of  which  it  is  composed  comprise 
feldspathic  quartz  slates,  biotitic  and  chloritic  quartz  slates,  and 
vitreous  quartzite,  the  latter  being  a  persistent  phase  at  the  upper- 
most horizon.  All  these  rocks  are  plainly  fragmental  and  for  the 
most  part  little  altered,  although  occasionally  by  metasomatic  changes 
they  have  become  semicrystalline.  The  lowest  horizon  of  the  Quartz 
slate  in  the  Penokee  series  proper  was  found  at  times  to  be  a  vitreous 
quartzite  and  at  other  times  to  be  a  conglomerate.  The  debris  of  this 
conglomerate  is  usually  derived  chiefly  from  the  Southern  Complex, 
but  at  several  localities  contains  a  large  quantity  of  chert  from  the 
Cherty  limestone  member,  and  also  includes  a  considerable  amount  of 
red  jasper  pebbles,  and  occasionally  contains  pebbles  of  white  vitreous 
quartzite. 
The  next  overlying  formation  is  the  Iron-bearing  member,  which  is 
longitudinally  coextensive  with  the  underlying  Quartz  slate.  The 
thickness  of  this  formation  is  surprisingly  uniform,  varying  for  the 
most  part  between  800  and  1,000  feet,  although  at  its  eastern  extremity 
it  apparently  becomes  thicker.  The  main  phases  of  rocks  here  in- 
cluded are  slaty  and  cherty  iron  carbonates,  ferruginous  slates  and 
cherts,  and  actinolitic  and  magnetitic  slates,  none  of  which  show  any 
evidence  of  being  of  mechanical  origin.  The  original  form  of  the 
entire  formation  is  taken  to  be  an  impure  cherty  iron  carbonate,  also 
bearing  magnesium  and  calcium  carbonate.  From  this  condition  the 
many  phases  and  varieties  of  rocks  now  found  are  traced  by  minute 
stages.  These  transformations  are  mainly  produced  by  secondary 
chemical  changes.  A  comparison  with  the  iron-bearing  formation  of 
the  Animikie  shows  that  it  consists  of  the  same  kinds  of  rocks,  which 
have  been  derived  from  an  iron  carbonate  in  the  same  manner  a-  those 
of  the  iron  formation  of  the  Penokee  series.     The  iron  ores  arc  found 
