120  PRE-CAMBRIAN   ROCKS    OF   NORTH   AMERICA.  [bull.  8* 
beds  of  the  Potsdam.  In  lithological  character  the  Potsdam  beds  dif- 
fer from  those  of  the  St.  Croix,  being  hard  and  vitreous  and  usually  of  a 
red  color.    The  Potsdam  has  a  thickness  of  at  least  400  feet. 
Winchell  (N.  H.),154  in  1874,  gives  details  as  to  the  geology  of  the 
Minnesota  valley.  The  quartzites  in  the  vicinity  of  New  Ulm  and  Eed 
stone,  referred  to  the  Potsdam,  are  conglomeratic  in  places.  In  the 
valley  there  are  quite  numerous  outcrops  of  granite  which  are  some- 
times cut  by  trap  dikes.  At  Granite  falls  there  are  sudden  changes 
from  real  granite  to  hornblende  schist. 
Streng  and  Kloos,155  in  1877,  describe  in  the  Upper  Mississippi  a 
set  of  granitic,  syenitic,  dioritic,  and  gabbro-like  rocks  which  are  re- 
ferred to  the  Laurentian,  while  north  of  these  is  a  zone  of  metamorphic 
schist — mica-slate,  talc-slate,  and  clay-slate — with  gneiss-like  rocks, 
which  may  be  Huronian.  South  of  Vermilion  lake  is  a  region  of  gran- 
ite, gneiss,  and  crystalline  slate  which  belong  to  the  Laurentian  forma- 
tion, while  occupying  a  wide  extent  of  country  about  the  St.  Louis 
river  are  roofing  slates  and  quartzites  which  are  probably  the  repre- 
sentative of  the  Huronian.  The  igneous  rocks  at  the  west  end  of  lake 
Superior  are  without  doubt  of  Potsdam  age.  On  the  St.  Croix  river  is 
a  melaphyre  which  lies  unconformably  below  a  sandstone  and  conglom- 
erate bearing  fossils  of  Lower  Silurian  age,  which  relation  points  to 
the  Huronian  age  of  the  melaphyre. 
Winchell  (N.  H.),156  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  the  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.),157  in  1879,  gives  the  geological  results  of  an 
examination  of  the  northeastern  part  of  the  state  of  Minnesota.  The 
formations  that  conrpose  the  coast  line  of  lake  Superior  include,  in  de- 
scending order,  (1)  metamorphic  shales,  sandstones,  and  quartzites  cut 
by  dikes  and  interteedded  with  igneous  rocks,  perhaps  Sir  William 
Logan's  Quebec  group.  (2)  Sandstones,  metamorphosed  into  basalti- 
form  red  rock,  inter  stratified  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)  Syenites,  granites  and 
other  rocks  which  have  been  classed  as  Laurentian.  (8)  The  igneous 
rocks  known  as  the  Cupriferous  series.  The  Cupriferous  series  seems 
to  overlie  several  formations  unconformably,  and  isinterstratified  with 
some  of  the  later,  and  especially  with  Nos.  1  and  2. 
Winchell  (N.  H),158  in  1880,  describes  the  Cupriferous  series  of 
Duluth.     At  Duluth  the  most  important  rock  is  the  gabbro.     This  is 
