90  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  r bull.  86 
this  system  in  Michigan  consist  of  talcose,  chloritic,  and  siliceous  slates, 
quartz,  and  beds  of  marble.  In  it  are  found  the  specular  and  magnetic 
ores  of  lake  Superior.  The  Lake  Superior  sandstone  is  placed  in  the 
lower  Silurian  system.  The  solid  quartzose  character  of  the  rock  of 
St.  Joseph's  and  Sulphur  islands  suggests  the  idea  of  its  being  Azoic; 
but  the  gradual  transition  from  the  unaltered  sandstone  of  the  Sault 
to  the  altered  sandstone  of  Neebish  rapids  and  jasper  conglomerates  of 
the  western  shore  of  Oampement  d'Ours,  favors  the  idea  of  the  equiva- 
lency of  the  sandstone  and  quartzite,  as  it  is  also  the  fact  that  the  i'os- 
siliferous  Chazy  limestone  is  found  directly  upon  the  quartzite  at  Sul- 
phur island. 
Credner,"  in  1869,  describes  in  the  upper  peninsula  of  Michigan  the 
Laurentian  and  Huronian  systems.  The  Laurentian  system  is  the 
gneiss-granite  formation,  which  includes  many  varieties  of  massive 
rocks,  as  well  as  hornblende,  chlorite,  and  other  schists,  and  also  thin 
layers  of  dolomitic  limestones. 
The  Hnronian  system  is  the  iron-bearing  formation.  The  general 
succession,  beginning  at  the  base,  is  quartzite,  in  its  upper  parts  often 
iron  stained,  2,500  feet;  crystalline  dolomitic  limestone,  containing 
argillite,  chlorite-sehist,  and  layers  of  quartz,  seldom  conglomeratic, 
2,500  to  3,500  feet;  more  or  less  siliceous  hematite,  000  to  1,000  feet; 
ferruginous  chlorite-sehist,  1,200  feet;  dark  clay-slate,  with  beds  of  hard 
quartzite,  8,500  feet;  chlorites-chist,  with  beds  of  diorite,  1,300  feet; 
talc  schist,  ivith  various  impurities,  100  feet;  aphanitic  to  granular  di- 
orite, 2,300  feet;  talc-schist,  with  various  impurities,  1,500  feet. 
In  the  iron  group  is  a  granite  dike  on  the  Sturgeon  river  12  feet  wide, 
which  breaks  through  the  iron  ore  and  jasper  at  a  right  angle  to  the 
schist.  Over  the  iron  formation,  at  the  Miehigamme  mine,  is  found  a 
conglomerate  of  jasper  and  fragments  of  quartz  in  an  iron  and  quartz 
base,  There  is  a  discordance  between  the  Laurentian  and  Huronian. 
The  Potsdam  sandstone  rests  upon  the  Huronian  and  Laurentian  un- 
coil form  ably. 
Brooks  and  Pumpelly,100  in  1872,  maintain  that  the  copper-bearing 
rocks  of  lake  Superior  are  unconformably  below  the  Lower  Silurian 
sandstone,  This  is  shown  by  the  fact  that  the  horizontal  strata  abut 
against  the  steep  faces  of  the  cupriferous  series  on  Keweenaw  point, 
the  latter  dipping  away  from  the  sandstone  at  an  angle  of  from  G0°  to 
40°.  Also  for  a  long  distance  between  the  Montreal  river  and  lake 
Gogebic  the  cupriferous  series  conform  in  strike  and  dip  to  the  linro- 
nian  schists,  dipping  steeply  to  the  north  at  an  angle  of  from  50°  to  70°, 
while  the  Silurian  sandstone  to  the  north,  in  a  flat- lying  condition,  cov- 
ers an  extensive  country.  In  Sec.  13,  T.  40  1ST.,  E.  41  W.,  the  Silurian 
sandstone  is  found  in  a  nearly  horizontal  position,  while  4  miles  distant 
the  cupriferous  series  dip  to  the  north  at  an  angle  of  50°. 
It  is  concluded  that  the  cupriferous  series  was  formed  before  the  tilt- 
ing of  the  Huronian  beds  upon  which  it  rests  conformably,  and  conse- 
