LAKE   SUPERIOR   REGION.  365 
The  latest  work  is  by  Lane  and  Seaman,  who  conclude  that  the  neo- 
Cambrian  or  Potsdam  or  Lake  Superior  sandstone  includes  (1)  the 
sandstones  west  of  the  Copper  range,  to  which  the  new  term  Freda 
sandstone  is  applied;  (2)  the  sandstones  east  of  the  Copper  range,  to 
which  the  term  Jacobsville  is  applied,  and  (3)  the  sandstones  in  the 
bluffs  back  of  Munising,  to  which  the  term  Munising  sandstone  is 
applied.  The  relations  of  the  Freda  sandstone  to  the  Lake  Superior 
sandstone  are  uncertain,  but  they  are  regarded  as  probably  one  forma- 
tion. The  Keweenawan  series  is  regarded  as  representing  the  lower 
and  middle  Cambrian  and  not  the  upper  part  of  the  pre-Cambrian 
section.  They  are  described  as  Cambrian  beds  disturbed  by  coeval 
volcanic  activity  and  faulting,  and  seem  comparable  with  inter- 
Keweenawan  phenomena,  while  the  Lake  Superior  sandstone  and  the 
upper  Keweenawan  appear  closely  associated,  not  merely  litholog- 
ically  but  stratigraphically. 
CHARACTER   OF  THE   KEWEENAWAN   SERIES. 
Of  the  forms  of  the  word  proposed  for  this  series,  Keweenawian, 
Keweenian,  and  Keweenawan,  the  last  is  apparently  preferable  as 
being  most  directly  derived  from  the  geographical  term  Keweenaw. 
Bayfield,  in  1829,  recognized  the  detrital  character  and  source  of 
the  debris  of  the  Keweenawan  conglomerates,  and  concluded  that 
they  must  be  later  than  the  traps. 
The  existence  of  a  succession  of  interbedded  elastics  and  volcanics 
12,000  feet  thick  about  Lake  Superior  was  recognized  by  Logan  as 
early  as  1847,  while  in  1852  the  same  author  accurately  characterized 
the  series  as  an  alternation  of  sandstones  and  conglomerates,  amygcla- 
loids,  and  traps.  The  work  of  Foster  and  Whitney  on  the  south  shore 
established  about  the  same  time  the  existence  of  a  similar  great  suc- 
cession on  Keweenaw  Point,  although  the  conglomerates  were  by 
these  authors  regarded  as  friction  conglomerates  caused  mainly  by 
volcanic  action  upon  the  earlier  sandstones.  Jackson  recognized  that 
the  conglomerates 'are  of  true  detrital  origin.  The  first  clear  appre- 
ciation of  the  contemporaneous  interstratified  relations  between  the 
volcanics  and  the  detrital  rocks  on  the  south  shore  was  reached  by 
Pumpelly  and  Marvine.  Their  work  on  this  series  of  rocks  was  much 
fuller  than  any  that  had  gone  before,  and  as  a  consequence  of  this  the 
rocks  were  recognized  as  a  distinct  division,  to  which  the  term  "  Kewee- 
naw group  "  was  applied.  Logan,  on  the  north  shore,  included  in  his 
"  Upper  Copper-bearing  "  rocks  what  is  here  called  Keweenawan  and 
the  unconformably  underlying  Animikie.  He  recognized,  however, 
that  the  two  have  a  very  different  lithological  character.  The  felsites, 
quartz  porphyries,  and  other  acidic  rocks — in  the  earlier  reports  fre- 
