362  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
(3)  To  the  south  of  the  Marquette  belt  of  the  Huronian,  in  the 
Crystal  Falls,  Felch,  Menominee,  and  Iron  Eiver  districts,  the  upper 
Huronian  is  underlain  by  formations  lithologically  similar  to  the 
lower  Huronian  of  the  Marquette  district,  but  with  slight  evidence  of 
unconformity. 
The  question  therefore  arises  as  to  the  explanation  of  this  differ- 
ence. It  seems  probable  that  the  Marquette  district  was  near  a  shore 
line  during  Huronian  time.  During  this  period  there  were  two 
episodes  of  orogenic  movement  with  uplift  and  subsidence,  so  that  the 
Huronian  series  is  divided  into  three  unconformable  groups.  Farther 
to  the  south,  in  the  Crystal  Falls,  Felch,  Menominee,  and  Iron  River 
areas,  if  such  movements  occurred,  they  were  less  strongly  marked, 
and  thus  the  different  formations  appear  to  be  in  conformity. 
Whether  or  not  there  were  interruptions  in  the  sedimentation  is  diffi- 
cult to  determine,  but  it  should  be  remembered  that  the  absence  of 
positive  evidence  of  an  unconformity  does  not  prove  that  the  uncon- 
formity does  not  exist.  While  it  is  uncertain  how  far  south  the  un- 
conformities found  in  the  Marquette  and  Gogebic  districts  extended, 
we  have  little  difficulty  in  correlating  the  formations  on  the  basis 
of  succession  of  sediments  of  like  kind  and  the  continuity  of  the  upper 
Huronian. 
The  criteria  for  discriminating  subaqueous  and  subaerial  deposi- 
tion will  not  be  discussed  here  in  detail,  but  the  essential  evidence  may 
be  noted.  Favoring  the  subaerial  deposition  of  many  of  the  Kewee- 
nawan  sediments  are  the  red  color  of  the  sands,  the  local  banding  of 
purple,  red,  and  yellow  colors,  lack  of  assortment,  mud  cracks,  ripple 
marks  characteristic  of  streams  rather  than  shores,  and  interlamina- 
tion  with  surface  igneous  rocks  having  typical  subaerial  textures  in 
contrast  with  textures  developed  in  igneous  rocks  of  the  Huronian 
below.  The  subaqueous  character  of  most  of  the  Huronian  sediments 
of  the  south  shore  is  shown  by  their  clean  assortment,  their  green 
rather  than  red  colors  (indicating  presence  of  ferrous  iron),  their 
abundant  ripple  marks  of  the  kind  characteristic  of  shore  action,  and 
their  association  with  basalts  showing  pillow  or  ellipsoidal  structure, 
which  in  many  parts  of  the  world  has  been  ascribed  to  submarine 
extrusion. 
SECTION  7.    HISTORY  OF  DEVELOPMENT  OF  KNOWLEDGE 
OF  LAKE  SUPERIOR  REGION. 
LAKE  SUPERIOR  SANDSTONE. 
The  horizontal  red  sandstone  of  Lake  Superior  was  recognized  as 
the  most  extensive  formation  of  the  lake  by  the  earliest  geological 
voyagers,  and  in  what  follows  this  formation  will  be  called  the  Lake 
Superior  sandstone.  The  early  travelers,  Schoolcraft,  Bigsby,  and 
Bayfield,  regarded  it  as  the  "  Old  Ked  sandstone,"  although  Bayfield 
