364  PRE- CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
of  the  same  age.  Bell  at  first  thought  the  Keweenawan  Permian  or 
Triassic,  while  cognizant  of  the  fact  that  the  Lake  Superior  sandstone 
is  older  than  the  Triassic,  but  recently  this  writer  places  the  sandstone 
as  probably  unconformably  above  the  Keweenawan.  Foster,  Wads- 
worth,  Whitney,  Winchell  (N.  H.),  Lane,  and  Seaman,  after  com- 
parisons and  studies  of  the  problem,  have  maintained  that  the 
Lake  Superior  and  Keweenawan  sandstones  belong  to  the  same  series. 
Agassiz,  Brooks,  Chamberlin,  Dawson,  Houghton,  Irving,  Logan, 
Macfarlane,  Bell,  Owen,  Pumpelly,  Rominger,  Selwyn,  Strong, 
Sweet,  and  Wooster  have  held  as  their  latest  view  that  the  Kewee- 
nawan series  is  older  than  the  Lake  Superior  sandstone.  Agassiz  at 
first  regarded  all  the  sandstones  of  the  same  age,  but  afterward  came 
to  the  conclusion  that  the  sandstone  was  deposited  against  the  upturned 
Keweenawan  series.  Agassiz,  Brooks,  Chamberlin,  Dawson,  Irving, 
Owen,  Pumpelly,  Rominger,  Strong,  Sweet,  Wooster,  and  Hall  main- 
tain a  great  unconformity  between  the  two.  Macfarlane  held  that 
there  was  a  doubtful  unconformity  between  the  Keweenawan  series 
and  the  Lake  Superior  sandstone,  the  former  occupying  an  inferior 
position.  Houghton's,  Logan's,  and  Selwyn's  position  is  that  the 
Keweenawan  series  is  a  downward  extension  of  the  Lake  Superior 
red  sandstone.  The  latter  is  regarded  by  Logan  as  probably  Chazy, 
and  the  Keweenawan  therefore  "  Calciferous  "  or  Potsdam.  Selwyn 
and  Bell  now  place  the  Keweenawan  as  Cambrian. 
The  relations  of  the  horizontal  sandstone  in  northern  Wisconsin  to 
the  melaphyres  and  traps  regarded  as  Keweenawan  have  been  de- 
scribed by  all  observers  to  be  those  of  unconformity,  the  horizontal 
sandstone  resting  upon  the  upturned  edges  of  the  Keweenawan  series. 
The  only  point  of  difference  has  been  whether  this  sandstone  is  Pots- 
dam or  not.  It  is  so  regarded  by  the  Wisconsin  geologists  and  by 
Owen,  but  is  by  N.  H.  Winchell  called  "  St.  Croix  "  and  placed  above 
the  Potsdam.  No  one  doubts  that  it  belongs  near  the  base  of  the 
northwestern  Paleozoics.  The  extensive  area  of  horizontal  sandstone 
about  Agogebic  Lake,  between  the  two  highly  tilted  trap  ranges,  was 
long  ago  cited  by  Brooks  and  Pumpelly  as  evidence  that  the  Lake 
Superior  sandstone  is  far  later  in  age,  it  being  found  not  distant  from 
the  highly  tilted  Keweenawan  eruptives. 
The  controversy  has  been  most  extended  as  to  the  relations  of  the 
two  series  on  Keweenaw  Point.  A  part  of  what  has  been  regarded  as 
the  Lake  Superior  sandstone,  adjacent  to  the  trap  range,  has  been 
shown  by  Wadsworth  to  belong  with  the  Keweenawan  series,  and 
just  where  at  certain  places  the  Lake  Superior  sandstone  begins  and 
the  Keweenawan  series  ends  is  even  yet  a  debatable  question,  because 
by  all  it  is  now  agreed  that  near  this  contact  is  an  ancient  fault  of 
great  magnitude,  along  which  post-Potsdam  slipping  has  taken  place. 
