198  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
about  St.  Louis  River  are  roofing  slates  and  quartzites  which  are 
probably  the  representative  of  the  Huronian.  The  igneous  rocks  at 
the  west  end  of  Lake  Superior  are  without  doubt  of  Potsdam  age. 
On  St.  Croix  River  is  a  melaphyre  which  lies  unconformably  below 
a  sandstone  and  conglomerate  bearing  fossils  of  Lower  Silurian  age, 
which  relation  points  to  the  Huronian  age  of  the  melaphyre. 
Winchell  (N.  H.),140  in  1878,  describes  the  crystalline  rocks  along 
the  Northern  Pacific  Railroad.  Syenites  and  granites  occur  at  Little 
Falls  on  the  Mississippi,  and  at  Thompson  on  St.  Louis  River  are 
slates,  the  former  varying  into  a  mica  schist.  In  Pipestone  and' 
Rock  counties  are  large  exposures  of  quartzite  which  are  lithologically 
like  those  of  New  Ulm,  and  like  them  are  placed  in  the  Potsdam. 
Winchell  (N.  H.),141  in  1879,  gives  the  geological  results  of  an 
examination  of  the  northeastern  part  of  Minnesota.  The  formations 
that  compose  the  coast  line  of  Lake  Superior  include,  in  descending 
order,  (1)  metamorphic  shales,  sandstones,  and  quartzites,  cut  by 
dikes  and  interbedded  with  igneous  rocks,  perhaps  Sir  William 
Logan's  Quebec  group;  (2)  sandstones,  metamorphosed  into  basalti- 
form  red  rock,  interstratified  with  igneous  rock  along  the  Palisades 
and  at  Black  Point;  (3)  a  quartzose  conglomerate  at  the  Great 
Palisades  and  at  Portage  Bay  Island;  (4)  the  quartzites  and  slates 
of  Grand  Portage  Bay;  (5)  the  jasper,  flint,  and  iron-bearing  belt 
of  Gunflint  Lake,  Vermilion  Lake,  and  Mesabi ;  (6)  the  slates  and 
schists  which  the  Canadian  geologists  designate  Huronian;  (7)  Syen- 
ites, granites,  and  other  rocks  which  have  been  classed  as  Laurentian ; 
(8)  the  igneous  rocks  known  as  the  Cupriferous  series.  The  Cuprif- 
erous series  seem  to  overlie  several  formations  unconformably,  and 
is  interstratified  with  some  of  the  later,  especially  with  Nos.  1  and  2. 
Winchell  (N.  H.),142  in  1880,  describes  the  Cupriferous  series  of 
Duluth.  At  Duluth  the  most  important  rock  is  the  gabbro.  This  is 
intimately  associated  with  a  syenitic  granite  which  is  a  metamorphic 
rock,  all  stages  being  seen,  from  the  perfectly  crystalline  granite  to 
the  unchanged  sedimentary  layers.  The  Cupriferous  series  is  re- 
garded as  Potsdam. 
Upham,143  in  1880,  describes  granites  and  gneisses  at  numerous 
localities  in  the  Minnesota  Valley.  In  the  conglomerate  opposite  New 
Ulm  and  in  the  quartzite  at  Redstone  are  found  numerous  pebbles  of 
quartz  and  jasper,  but  no  granite  pebbles  are  seen,  although  granite 
outcrops  close  to  the  west. 
Hall  (C.  W.),144  in  1880,  describes  the  rocks  between  the  mouths 
of  Poplar  and  Devils  Track  rivers  on  Lake  Superior  to  be  dark- 
colored  basic  rocks  of  igneous  origin  belonging  to  the  Cupriferous 
series,  with  the  exception  of  a  few  beds  of  sandstone  and  conglomerate 
interbedded  with  the  igneous  rocks.     The  Sawteeth  Mountains  are 
