LAKE    SUPERIOR   REGION.  197 
than  1,500  feet.  At  Redwood  Falls  are  found  gneiss  and  granitic 
rocks  of  Laurentian  age.  The  quartzites  are  regarded  as  of  the  age 
of  the  Huronian  of  Canada  and  equivalent  to  the  quartzites  of  Wis- 
consin. 
White,130  in  1870,  describes  the  quartzites  of  Iowa,  Minnesota,  and 
Dakota.  They  are  completely  metamorphosed,  intensely  hard  rocks, 
although  the  lines  of  stratification  are  distinct  and  there  are  fre- 
quently seen  distinct  ripple  marks  upon  the  bedded  surfaces.  Not  in- 
frequently the  quartzite  is  conglomeratic.  In  them  no  fossils  have 
been  found.  They  are,  however,  regarded  as  belonging  to  the  Azoic 
age  because  of  their  complete  metamorphic  character,  because  of  their 
disturbed  condition,  and  because  the  Lower  Magnesian  limestone  at 
New  Ulm  rests  upon  the  quartzites  unconformably,  and  in  this  part 
of  North  America  no  disturbances  are  known  to  have  occurred  be- 
tween the  commencement  and  close  of  Paleozoic  time. 
Winchell  (N.  H.),137  in  1873, states  that  the  granitic  and  metamor- 
phic rocks  occupy  a  great  portion  of  Minnesota.  These  are  regarded 
as  Laurentian  and  Huronian.  Their  lithological  and  miner alogical 
characters  are  complex  and  variable.  The  original  nucleus  was  gran- 
ite and  syenite,  and  around  these  are  arranged  the  metamorphosed 
slates  and  gneisses  in  upturned  or  even  vertical  beds,  while  interca- 
lated with  them  are  numerous  injected  beds  or  dikes  of  trap.  The 
Sioux  quartzite  and  those  of  New  Ulm  are  placed  as  a  part  of  the 
Potsdam  sandstone.  The  Potsdam  was  laid  down  before  the  close  of 
the  volcanic  disturbance,  for  the  St.  Croix  beds  of  later  age  rest 
unconformably  upon  the  Laurentian  as  well  as  upon  the  upturned 
beds  of  the  Potsdam.  In  lithological  character  the  Potsdam  beds  dif- 
fer from  those  of  the  St.  Croix,  being  hard  and  vitreous  and  usualty 
of  a  red  color.    The  Potsdam  has  a  thickness  of  at  least  400  feet. 
Winchell  (N.  H.),138  in  1874,  gives  details  as  to  the  geology  of 
the  Minnesota  Valley.  The  quartzites  in  the  vicinity  of  New  Ulm 
and  Redstone,  referred  to  the  Potsdam,  are  conglomeratic  in  places. 
In  the  valley  there  are  rather  numerous  outcrops  of  granite,  which  are 
sometimes  cut  by  trap  dikes.  At  Granite  Falls  there  are  sudden 
changes  from  real  granite  to  hornblende  schist. 
Wight,  in  1877,  discusses  the  relations  of  Lake  Superior  sand- 
stone to  Keweenawan  traps  along  Kettle  River.  See  summary  in 
section  2,  northern  Wisconsin,  page  183. 
Streng  and  Kloos,139  in  1877,  describe  in  the  upper  Mississippi, 
a  set  of  granitic,  syenitic,  dioritic,  and  gabbro-like  rocks  whch  are 
referred  to  the  Laurentian,  while  north  of  these  is  a  zone  of  meta- 
morphic schist — mica  slate,  talc  slate,  and  clay  slate — with  gneiss- 
like rocks,  which  may  be  Huronian.  South  of  Vermilion  Lake  is  a 
region  of  granite,  gneiss,  and  crystalline  slate  which  belong  to  the 
Laurentian   formation,  while  occupying  a  wide  extent  of  country 
