VANHisR.]  LAKE    SUPERIOR    REGION.  151 
the  schists  also  have  alike  origin,  because  of  the  gradations  between 
them. 
The  Penokee  series  proper  is  made  up  of  three  members,  Quari  z  Slate, 
Iron-Bearing,  and  Upper  Slate,  and  these  rest  unconformable  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  shows  no  evidence  as  a  whole  of  mechanical  origin, 
although  occasionally  a  small  amount  of  detrital  material  is  found  in  it. 
It  is  regarded  either  as  a  chemical  or  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  thick- 
ness. 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  uppermost  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  other  times  to  be  a  con- 
glomerate. 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  maguetitic  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  Animi- 
kie  shows  that  it  consists  of  the  same  kind  of  rocks,  which  have  been 
derived  from  an  iron  carbonate  in  the  same  manner  as  those  of  the  iron 
formation  of  the  Penokee  series.  The  iron  ores  are  found  to  rest  for 
the  most  part  upon  the  underlying  quartzite  and  upon  a  seriesof  dikes 
which  have  cut  the  stratified  layers.  The  ores  in  this  position  are  sec- 
ondary Concentrations  regarded  as  produced  at  the  same  time  as  the 
