196  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
LAKE   SUPERIOR  BASIN. 
The  synclinal  structure  of  the  formations  about  lake  Superior  was 
noted  as  early  as  1847  by  Logan  in  his  remarkably  accurate  general 
account  of  the  lake  Superior  region.  His  five  general  formations  were 
found  to  recur  in  reverse  order  on  both  sides  of  the  lake,  dipping  to  the 
ceuter.  Rogers  and  Agassiz,  in  1848  and  1850,  maintained  that  the 
shores  of  the  lake  are  due  to  dikes.  Owen,  in  1851,  in  his  studies  north 
and  south  of  the  west  part  of  the  lake,  saw  that  the  formations  occur 
in  reverse  order,  and  reached  independently  the  same  conclusion  as  did 
Logan.  Bigsby  and  Whitney  followed  Logan  and  Owen  in  describing 
the  lake  Superior  basin  as  synclinal.  The  next  exact  contributions  to 
the  structure  of  the  lake  Superior  synclinal  .were  by  Swee£  and  Irving, 
who  found  it  to  continue  to  the  southwestward  in  Wisconsin  and  Min- 
nesota, the  rocks  adjacent  to  the  shore  on  the  south  side  of  the  west 
end  of  the  lake  being  on  the  north  side  of  the  synclinal  basin.  Sweet, 
in  1876,  spoke  of  the  lake  Superior  synclinal  as  over  300  miles  in  length 
and  30  to  50  miles  in  width.  That  the  lake  Superior  formations  are 
not  only  a  synclinal  in  an  east  and  west  direction,  the  rocks  dipping 
respectively  from  the  north  shore  south  and  the  soutn  shore  north,  but 
that  it  is  a  basin  in  the  exact  sense  of  the  term,  the  rocks  on  the  east 
shore  dipping  to  the  west,  while  the  western  termination  of  the  synclinal 
in  Minnesota  plunges  to  the  east  or  northeast,  was  shown  by  Irving. 
This  author  went  further  and  also  showed  that  the  major  bays  of  the 
lake  are  due  either  to  faults  or  subordinate  flexures  within  the  Kewee- 
nawan.  The  basin  is  clearly  a  product  of  Keweenawan  time.  Chamber- 
lin  suggests  that  it  began  early  in  the  Keweenawan.  The  Upper 
Huronian  series  partakes  in  several  districts  in  large  measure  of  the 
basin  structure.  This  is  apparent  from  the  fact  that  the  Huronian  of 
the  Penokee  and  Animikie  series  was  so  long  regarded  as  conformable 
with  the  Keweenawan.  It  has  been  seen  that  there  is  between  these 
series  a  great  unconformity,  although  not  one  so  vast  as  the  other 
physical  breaks  about  the  lake.  As  a  consequence  of  this  the  Upper 
Huronian  series  corresponds  only  locally  with  the  synclinal  structure, 
and  chiefly  about  the  west  half  of  the  lake.  The  structures  of  the  Lower 
Huronian  and  Archean  have  no  reference  to  that  of  the  lake  Superior 
basin. 
CONCLUSION. 
It  appears  that  in  the  lake  Superior  region  is  a  general  succession 
which  may  be  recognized,  and  that  there  is  really  a  much  greater 
degree  of  harmony  than  has  been  thought  in  the  conclusions  which  the 
various  writers  have  held  most  steadfastly  as  to  the  lake  Superior 
stratigraphy.  From  the  base  upward  it  is  as  follows :  Archean,  includ- 
ing Laurentian  granite  and  gneiss,  the  origin  of  which  is  largely  un- 
known, but  which  were  certainly  in  their  present  condition  earlier  than 
the  formation  of  the  Lower  Huronian ;  unconformity;  Lower  Huronian 
