vanhise.]  LAKE    SUPERIOR    REGION.  197 
(a  closely  folded  seniicrystalline  series);  unconformity;  Upper  Huro- 
nian  (a  gently  folded  and  plainly  clastic  series,  although  indurated 
by  cementation  and  metasomatic  changes);  unconformity;  completely 
unaltered  Keweenawan;  unconformity;  lake  Superior  sandstone.  In 
addition  to  the  above  are  great  masses  of  eruptive  rocks  in  all  the  series, 
both  basic  and  acidic,  including  granite-gneiss,  gabbro,  porphyry,  dia- 
base, etc. 
I  can  not  close  without  comparing  this  succession  of  lake  Superior 
formations  with  that  given  by  Logan  in  his  remarkable  paper  published 
in  1847.  It  is  as  follows:  u  (1)  Granite  and  syenite;  (2)  gneiss;  (3) 
chloritic  and  partially  talcose  and  conglomeratic  slates;  (4)  bluish 
slates  or  shales  interstratified  with  trap;  (5)  sandstones,  limestones, 
indurated  marls,  and  conglomerates,  interstratified  with  trap."  Be- 
tween 1,  2,  3,  and  4,  5  there  was  said  to  be  an  unconformity.  The 
granite,  syenite,  and  gneiss  are  Archean ;  the  chloritic  and  partly  tal- 
cose and  conglomeratic  slates,  Lower  Huronian ;  the  bluish  slates  or 
shales  interstratified  with  trap,  Upper  Huronian ;  and  the  sandstones, 
limestones,  indurated  marls,  and  conglomerates  interstratified  with 
trap,  Keweenawan.  Of  course,  Logan  at  that  time  did  not  appreciate 
all  the  structural  relations  which  obtain  between  these  various  series, 
although  the  greatest  of  the  unconformities  was  discovered,  nor  did  he 
suppose  that  they  are  all  pre-Cambrian,  and  in  his  mapping  in  1863,  1 
and  2  are  placed  together  as  Lower  Lauren tian,  and  4  and  5  together  as 
a  part  of  the  Quebec  group  above  the  Potsdam;  yet  that  he  appreciated 
that  in  this  region  there  are  five  fundamentally  different  kinds  of  rock, 
that  he  gave  an  accurate  characterization  of  the  Keweenaw  series,  com- 
prehending that  it  is  one  of  great  thickness,  not  less  than  10,000  or 
12,000  feet,  and  that  this  series  rests  unconformably  upon  the  granite 
and  gneiss,  can  not  be  too  highly  spoken  of.  Not  only  was  this  paper 
the  first  announcement  of  all  of  the  above  great  conclusions,  but  it  gave 
the  first  mention,  as  has  been  seen,  of  the  synclinal  structure  of  lake 
Superior. 
•Looking  toward  the  future  as  to  the  possible  modifications  of  this 
arrangement  by  further  work,  the  point  of  greatest  doubt  lies  as  to 
whether  the  unconformities  here  recognized  as  universal  in  the  lake- 
Superior  3gion  are  really  so.  Is  it  not  possible  that  the  unconformity 
at  the  baj  ,  of  the  Animikie  is  at  a  different  position  from  that  between 
the  Pen<j.!  .ee  proper  and  the  Oherty  Limestone,  and  may  not  these  be 
different  horizons  from  that  between  the  Upper  and  Lower  Marquette? 
May  not  the  break  above  the  Lower  Marquette  be  at  a  different  posi- 
tion from  that  above  the  Lower  Vermilion.  These  questions  can  not 
be  positively  answered  in  the  negative,  although  all  the  evidence  at 
hand  bears  strongly  in  this  direction.  It  may  be  found  in  the  lake 
Superior  basin,  so  extensive  in  area,  that  while  the  folding  and  erosion 
producing  an  unconformity  in  one  part  was  occurring,  at  some 
other  distant  part  deposition  was  going  on.     In  all  probability  this  is 
