Future  consequences  of  the  continued  uptilt  within  the  lake
Mgion.  —  One  of  the  most  distinguished  of  American  geologists,
Dr.G.  K.  Gilbert,  in  order  to  determine  whether  the  uptilt  revealed
by  canted  beach  lines  is  still  in  progress,  carried  out  an  elaborate
rtudy  upon  the  gauge  records  preser\'ed  at  the  various  gauging
itions  about  the  Great  Lakes.  Upon  the  basis  of  these  stiidies,
concluded  that  the  -uplift  continues,  that  the  axes  of  equal
l^ilift  (isobases)  take  their  course  alxiut  fifteen  degrees  north  of
*wt,  80  that  the  lines  of  greatest  uptilt  should  be  perpendicular  to
lisdirection,  or  fifteen  degrees  e^st  of  north.  He  further  believed
lat  the  basin  was  undergoing  an  up-cant  in  the  simple  manner  of
trap  door,  the  hinge  of  which  lay  to  the  southward  of  Chicago,
lad  the  study  of  the  gauge  records  led  him  to  believe  that  "  the
Bteof  change  is  such  that  the  twoendsota  line  one  hundred  miles
3ng  and  l>Tng  in  a  south-aouthwest  direction  are  relatively  dis-
iWed  four  tenths  of  a  foot  in  one  hundred  years."

Gilbert's  prophecy  of  a  future  outlet  of  the  Great  Lakes  to
4e  Mississippi.  —  The  natural  rock  sill,  over  which  the  waters
*1  Lake  Chicago  once  flowed  to  the  Mississippi,  is  to-day  but
ight  feet  above  the  common  mean  level  of  Lakes  Michigan  and
luron,  and  if  the  tilting  of  the  lake  region  were  to  continue  upon
rilbert's  assumption  of  a  canting  plane  with  the  hinge  of  the
wvenient  to  the  south  of  Chicago,  a  time  must  come  when  the
Chicago  outlet"  will  again  come  into  use  and  the  lakes  once


I


348  EARTH  FEATURES  AND  THEIR  MEANING

more  drain  to  the  Mississippi  and  the  Gulf.  Upon  the  basis  d
his  measurements,  Gilbert  ventured  the  prophecy  that  the  first
high-water  discharge  into  the  Mississippi  should  occur  in  ttwo
five  hundred  to  six  hundred  years,  and  for  continuous  discbajge
in  fifteen  hundred  years.  In  twenty-five  hundred  years  Niagara
Falls  should  at  low  water  stages  be  dry  from  this  cause,  and  in
tbirty-five  hundred  years  it  should  have  become  extinct.
