vanhisb.]  LAKE    SUPERIOR    REGION.  91 
quently  before  the  elevation  of  the  great  Azoic  area.  After  tlte  el<\  a- 
tion  of  these  rocks,  and  after  they  had  assumed  their  essential  litholog- 
ical  characters,  came  the  deposition  of  the  lake  Superior  sandstone  and 
its  accompanying  shales  as  a  product  of  the  erosion  of  the  older  rocks, 
and  containing  fossils  which  show  them  to  belong  to  the  Lower  Silurian. 
At  several  places  have  been  detected  a  lack  of  conformity  between  the 
Laureiitian  and  Huronian  in  the  upper  peninsula  of  Michigan,  but  when 
the  Huronian  and  Cupriferous  are  seen  in  contact  there  seems  to  be  a 
well  marked  concordance  between  them. 
Brooks,101  in  1873,  divides  the  rocks  of  the  upper  peninsula  in  de- 
scending order  into  Lower  Silurian, Copper-bearing  rocks,  Iron-bearing 
rocks,  and  Granitic  rocks,  and  gives  a  systematic  account  of  the  last 
two,  and  especially  of  the  economic  geology  of  the  iron-bearing  series. 
The  copper- bearing  rocks  correspond  with  the  Upper  Copper-bearing 
rocks  of  the  Canadian  geologists  and  occupy  a  narrow  belt  on  the  north- 
western edge  of  the  upper  peninsula.  This  series  includes  sandstones, 
which  are  nearly  or  quite  identical  with  the  Silurian  in  appearance,  but 
their  great  mass  is  made  up  of  different  varieties  of  trap,  often  amyg- 
daloidal,  interstratined  with  beds  of  peculiar  conglomerates.  The  lay- 
ers of  these  rocks  are  inclined,  dipping  northwest  and  north  toward 
lake  Superior,  from  vertical  to  as  low  as  23°  on  Keweenaw  point.  The 
iron  bearing  rocks  are  assumed  to  correspond  with  the  Huronian  system 
of  Canada.  They  may  have  ;i  thickness  of  5,000  feet  and  consist  of  a 
series  of  extensively  folded  beds  of  diorite,  quartzite,  chlorite- schists, 
clay-slate,  mica-slate,  and  graphitic  shales,  among  which  are  interca- 
lated extensive  beds  of  several  varieties  of  iron  ore.  The  most  abun- 
dant rock  is  greenstone  or  diorite,  in  which  the  bedding  is  usually  ob- 
scure, but  the  intercalated  schists  and  slates  usually  bear  strong  marks 
of  stratification.  The  dips  are  usually  at  a  high  angle  and  are  more 
apt  to  be  north  or  south  than  any  other  direction.  The  granitic  rocks 
are  believed  to  be  the  equivalents  of  the  Laurentian  of  Canada.  In 
these  the  bedding  indications  are  still  more  obscure  and  often  entirely 
wanting.  Also,  if  possible,  there  is  more  irregularity  in  strike  and  dip 
than  in  the  Huronian. 
A  full  lithological  description  of  tin'  different  phases  of  rocks  found 
in  the  Huronian  system,  and  the  various  sections  at  the  many  mines  in 
the  upper  peninsula  are  given  in  detail. 
The  formations  of  the  Huronian  system  in  the  Marquette  region  com- 
prise nineteen  members,  numbered  from  the  base  upward.  I,  11,  11 1,  and 
IV  are  composed  of  beds  of  siliceous  ferruginous  schist^  alternating^  ith 
cfeloritic  schists  and  diorites,  the  relations  of  which  have  not  been  fully 
made  out;  V  is  a  quartzite,  sometimes  containing  marble  and  beds  of 
argillite  and  novaculite;  VI,  VIII,  and  X  are  siliceous  ferruginous 
schists;  VII,  IX,  and  XI  are  dioritic  rocks,  varying  much  in  character; 
XIII  is  the  bed  which  contains  all  the  rich  specular  and  magnetic  ore, 
associated  with  mixed  ore  and  ma gnesian  schist;  XIV   is  a  quart/.ite, 
