182  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
Sweet,100  in  1876,  describes  the  junction  of  the  Laurentian  and 
Huronian  rocks  on  Bad  Kiver.  Here,  at  the  base  of  the  Huronian 
series,  is  a  siliceous  marble  dipping  6G°  N.,  while  100  feet  to  the  south 
is  a  ledge  of  gneissoid  granite  showing  a  well-defined  dip  of  77°  S. 
There  can  be  no  doubt  of  the  unconformity  of  these  formations. 
The  Penokee  series  is  found  to  be  about  5,000  feet  thick,  to  be  every- 
where conformable,  and  to  dip  about  66°  N. 
On  the  Chippewa  is  found  a  quartzite  which  has  a  layer  the  lowest 
stratum  of  which  is  a  reddish  metamorphic  conglomerate  having  a 
thickness  of  300  feet.  The  pebbles  of  this  conglomerate  are  either 
jasper  or  amorphous  quartz.  The  conglomerate  and  quartzite  are  dis- 
tinctly and  heavily  bedded.  South  of  this  quartzite  are  syenitic  gran- 
ites which  are  assumed  to  be  of  Laurentian  age,  and  the  quartzites 
and  conglomerates  are  assumed  to  overlie  them  unconformably.  A 
short  distance  north  of  the  mouth  of  Snake  River  cupriferous  mela- 
phyres and  amygdaloids  are  overlain  by  horizontal  beds  of  light- 
colored  Potsdam  sandstone,  while  a  few  miles  to  the  north  conglom- 
erates and  shales  conformably  overlie  the  cupriferous  strata.  The  con- 
glomerate is  heavily  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  St.  Croix 
River,  Potsdam  sandstone  containing  fossils  are  found  in  a  horizontal 
position  within  a  few  feet  of  the  cupriferous  rocks.  Depressions  and 
pockets  in  the  surface  of  the  cupriferous  rocks  are  filled  with  hori- 
zontal layers  of  the  sandstone,  and  detritus  from  the  crystalline  rocks 
are  found  in  its  layers.  The  Lake  Superior  syncline  is  traced  west- 
ward across  Wisconsin  and  enters  Minnesota.  It  is,  then,  more  than 
300  miles  in  length  and  from  30  to  50  miles  in  width. 
Irving,107  in  1877,  summarizes  the  facts  proved  as  to  the  older  rock 
series  of  Wisconsin.  There  are  here  four  series:  The  oldest  are  (1) 
gneisses  and  granites  with  other  rocks;  these  are  overlain  uncon- 
formably 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  uncon- 
formably covered  by  (1)  a  series  of  unaltered  horizontal  sandstone 
including  numerous  fossils,  many  of  which  are  closely  allied  to  those 
of  the  Potsdam  sandstone  of  New  York,  and  all  of  which  have  a 
marked  Primordial  aspect.  To  the  Laurentian  and  Huronian  systems 
of  Canada  are  referred  (1)  and  (2)  because  they  bear  the  same  re- 
lations 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 
