184  PRE-CAMBRIAtt    ROCKS    OF    NORTH    AMERICA.  [nmx.SB. 
which  separates,  even  in  a  more  pronounced  manner,  the  Auimikie  and 
Kaministiquia  series  and  the  upper  and  lower  Vermilion  lake  series 
on  the  other  side  of  the  lake  Superior  basin.  The  break,  being  thus  so 
strongly  marked  at  points  so  far  separated,  would  argue  that  it  extends 
over  a  very  considerable  area  of  the  lake  Superior  region,  not  improb- 
ably from  the  most  distant  rock  series  before  mentioned,  the  Dakota 
quartzites  and  the  Original  Huronian  of  the  north  shore  of  lake 
Huron.  It  would  not  be  expected  that  a  like  succession  is  now  recog- 
nizable in  each  of  the  areas  parallelized,  even  if  they  all  belong  to  the 
same  geological  series.  In  the  first  place,  the  rocks  in  some  districts 
are  not  sufficiently  tilted  to  make  it  certain  that  all  of  the  layers  are 
exposed.  Further,  nine-tenths  or  more  of  the  surface  of  the  country 
over  large  areas  is  heavily  covered  by  the  drift,  so  that  it  is  all  but 
certain  that  some  of  the  formations  which  exist  at  the  rock  surface  have 
not  been  discovered.  Still  further,  no  satisfactory  explanation  has  yet 
been  made  of  the  subordinate  succession  of  formations  in  the  Mar- 
quette, Felch  mountain,  Menominee  and  Vermilion  lake  districts ;  so 
it  is  not  yet  known  how  far  the  order  found  in  one  of  the  districts  is 
equivalent  with  that  of  another.  From  recent  work  it  is  probable  that 
future  investigations  will  show  that  this  likeness  is  greater  in  the 
series  below  correlated  than  has  been  suspected.  But  even  supposing 
the  disagreements  are  as  great  as  the  present  known  facts  might  lead 
one  to  suppose,  it  would  not  be  any  very  strong  evidence  against  the 
correlations;  for  it  is  not  to  be  expected  that  the  same  conditions  of 
sedimentation  have  prevailed  at  all  times  in  a  geological  basin  800 
miles  in  diameter.  While  in  one  part  of  the  basin  fragmental  sedi- 
ments were  accumulating,  it  would  not  be  very  strange  if  chemical 
sediments  or  organic  sediments  were  accumulating  elsewhere.  Below, 
it  is  seen  that  the  Penokee  and  Auimikie  series  are  the  equivalents  of 
each  other  in  the  broadest  sense  of  the  term.  It  is  not  necessarily  true 
that  sedimentation  began  or  ended  simultaneously  in  both  districts,  but 
only  that  in  the  main  they  stand  as  time  equivalents.  How  far  a  corre- 
spondence can  be  made  out  among  the  subordinate  members  of  the  vari- 
ous districts  can  be  determined  only  by  a  detailed  investigation  of  each 
of  the  areas. 
EQUIVALENTS   OF   THE    ORIGINAL  HURONIAN   SERIES. 
Passing  now  to  the  Original  Huronian,  shall  this  series  be  corre- 
lated with  the  Tipper  or  Lower  Marquette,  or  is  it  the  equivalent  of 
both? 
Alex.  Winchell  lately  announced  that  the  Lower  Slate  conglomer- 
ate and  the  underlying  formations  of  the  Original  Huronian  are  sepa- 
rated from  the  Upper  Slate  conglomerate  and  the  overlying  formations 
by  an  unconformity.  No  contacts  are  described,  the  conclusion  being 
based  upon  general  relations.  No  characteristic  debris  of  the  Lower 
Hurouian  i s  said  to  occur  in  the  Upper  Huronian.     The  locality  at  which 
