vanhise.1  LAKE    SUPERIOR    REGION.  109 
conformably  overlie  the  cupriferous  strata.  The  conglomerate  is  heav- 
ily bedded,  but  does  not  cover  the  melaphyres  and  amygdaloids  at  all 
points,  appearing  to  fill  pockets  and  depressions  in  them  rather  than 
being  interstratified.  At  St.  Croix  falls  on  the  St.  Croix  river  Pots- 
dam sandstone  containing  fossils  are  found  in  a  horizontal  position  wit  Inn 
a  few  feet  of  the  cupriferous  rocks.  Depressions  and  pockets  in  the  sur- 
face of  the  cupriferous  rocks  are  filled  with  horizontal  layers  of  the 
sandstone,  and  detritus  from  the  crystalline  rocks  are  found  in  its  lay- 
ers. The  lake  Superior  synclinal  is  traced  westward  across  the  state 
©f  Wisconsin  ami  enters  the  state  of  Minnesota.  It  is  then  over  300 
miles  in  length  and  from  30  to  50  miles  in  width. 
Irving,130  in  1877,  summarizes  the  facts  proved  as  to  the  older  rock 
series  of  Wisconsin.  There  are  here  four  series:  The  oldest  (1)  are 
gnrisses  and  granites  with  other  rocks;  these  are  overlain  uncon- 
formable by  (2)  a  series  of  quartzites,  schists,  diorites,  etc.,  with  some 
gneiss  and  granite;  these  in  turn  are  overlain — probably  also  uncon- 
formably,  but  this  is  not  certainly  proved — by  (3)  the  Copper  series, 
which  includes  greenstones  and  melaphyres  and  also  great  thicknesses 
of  interstratified  sandstone,  melaphyres,  amygdaloids,  and  shales,  the 
whole  having  a  thickness  of  several  miles;  these  finally  are  unconform- 
ably  covered  by  (1)  a  series  of  unaltered  horizontal  sandstone  including 
numerous  fossils,  many  of  which  are  closely  allied  to  those  of  the  Pots- 
dam sandstone  of  New  York,  and  all  of  which  have  a  marked  Primor- 
dial aspect.  To  the  Laurentian  and  Huronian  systems  of  Canada  are 
referred  (1)  and  (2)  because  they  bear  the  same  relations  to  one  another 
and  to  the  Copper  series  that  these  systems  do. 
The  exact  junction  between  the  Potsdam  sandstone  and  the  Huro- 
nian quartzite  is  seen  at  numerous  places.  The  Potsdam,  containing 
fossils  and  numerous  fragments  from  the  older  rocks,  lies  upon  and 
wedged  in  between  the  tilted  ledges  of  the  Huronian.  Exactly  similar 
unconformability  is  to  be  seen  at  the  Dalles  of  the  St.  Croix  between 
the  Potsdam  and  Copper  series. 
Wight,131  in  1877,  describes  the  horizontal  Potsdam  sandstone  as 
resting  on  the  uneven  and  tilted  surface  of  the  underlying  igneous  or 
crystalline  rocks  at  St.  Croix  falls.  Almost  in  contact  with  the  trap 
the  sandstone  contains  numerous  well  preserved  fossils.  At  Pine 
island  in  Kettle  river  the  Superior  red  sandstone  contains  abundant 
fragments  of  the  adjacent  trap,  forming  a  brecciated  conglomerate  kin- 
dred to  the  conglomerate  which  extends  from  Keweenaw  point  along 
the  northern  base  of  the  Porcupine  and  Penokee  mountains.  Every- 
where this  conglomerate  is  composed  of  fragments  of  the  more  elevated 
Huronian  or  trap  ridges.  The  Superior  red  sandstone,  wherever  it 
borders  the  trap  ridges,  shows  that  it  has  been  tilted,  broken  up.  or 
(crushed.  It  appears  that  the  trap,  whether  erupted  or  upheaved  con- 
vulsively or  slowly,  encountered  this  formation  in  its  ascent.     On  the 
ntrary,  the  Potsdam   sandstone  everywhere  rests  in  a  horizontal 
