But  suppose  the  Keweenawan  traps  average  a  density  of  about  2.85,
that  is,  weigh  about  2,850  ounces  to  the  cubic  foot.  Then  they  will
weigh  about  200  ounces  to  the  cubic  foot  more  than  granites,  which  run
about  2.65  in  specific  gravity.  Now  the  granitic  areas  around  Lake
Superior  seem  to  be  remnants  of  a  dissected  peneplain  about  1,800  feet
above  sea  level.  Now  as  Lake  Superior  is  1,000  feet  deep  in  its  deepest
part  and  over  a  large  area  is  over  600  feet  deep,  and  its  elevation  is
about  600  A.  T.  we  have  some  2,200  feet  of  rock  in  the  Huron  Mountains
(5,820,000  ounces  per  square  foot)  over  against  1,000  feet  of  water
(1,000,000  ounces  per  square  foot)  as  an  extreme,  or  in  a  general  way
1,600  feet  of  rock  as  against  600  feet  of  water  (4,230,000  ounces  as
against  600,000  ounces).  ^

Now  if  this  is  due  to  extra  weight  of  Keweenawan  traps  under  Lake

^The  48,000  feet  column  of  Gordon  is  about  12  miles  in  width.    This  corresponds  to  an  average  dip
of  6Qf*.     Making  its  dip  3(f  for  the  same  breadth  would  give  us  33,000  feet.
62


486  MICHIGAN  SURVEY,  1906.
Superior,  which  makes  an  equilibrium,  there  would  have  to  be,  under*  the
suppositions  24,100  to  18,250  feet  of  them.
It  is  easy  to  see  that  this  argument  indicates  a  thickness  of  Kewee-
nawan  traps  under  Lake  Superior,  of  the  same  general  order  of  magni-
tude as  indicated  otherwise,  and  that  the  balance,  known  as  the  isostatic
balance,  may  easily  be  supposed  complete.*

For  closer  figures  one  would  have  to  have  fair  contour  maps  both  of
the  Lake  bottom  and  the  Huron  Mountains,  and  also  closer  figures  as
to  density.
