128  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
The  Huronian  in  the  Marquette,  Menominee,  and  Gogebic  regions 
is  nonconformable  with  the  Laurentian. 
Pumpelly,40  in  1878,  describes  Lake  Superior  as  divided  into  two 
distinct  basins  by  Keweenaw  Point,  the  western  basin  being  a  geosyn- 
clinal  trough.  The  southeastern  lip  of  this  trough  consists  of  an  im- 
mense development  of  volcanic  rocks  in  the  form  of  great  beds  and 
flows  associated  with  conglomerates  and  sandstones,  both  of  which 
consist  essentially  of  porphyry  detritus.  This  Keweenaw  series  is 
more  nearly  conformable  with  the  underlying  highly  tilted  Huronian 
schists  than  with  the  Potsdam  sandstone.  The  prominent  eruptive 
rocks  of  the  Keweenaw  series  fall  under  the  two  heads,  diabase  and 
melaphyre.  The  changes  which  have  taken  place  in  the  interior  of 
the  rock  masses  since  eruption,  that  is,  the  metasomatic  development 
of  these  rocks,  is  traced  out  in  great  detail. 
Wright,41  in  1879,  describes  the  Laurentian  series  as  consisting  of 
coarsely  crystalline  massive  granites,  passing  into  gneissoid  rocks,  and 
these  graduating  into  mica  schists,  and  the  latter  even  as  impercepti- 
bly into  slates.  The  Laurentian  granite  is  regarded  as  a  metamorphic 
sedimentary  rock,  because  the  quartz  grains  contain  cavities  filled 
with  liquid,  while  igneous  granites  never  contain  such  cavities,  but 
rather  those  filled  with  glass  or  stone.  The  Lower  Huronian  strata 
have  been  chiefly  derived  from  the  ruins  of  the  Laurentian  rocks. 
The  nonconformity  between  the  Laurentian  and  Huronian  may  be 
seen  at  Penokee  Gap.  Here  the  dip  of  the  gneissoid  granite  is  about 
70°  S.,  while  the  plainly  bedded  Huronian  strata  in  direct  contact 
have  a  dip  of  65°  N.  At  the  Macomber  mine,  near  Negaunee,  is  found 
a  bed  of  manganiferous  hematitic  shale  bearing  the  impression  of 
some  fossil  which  Brush,  Verrill,  Dana,  and  Smith  pronounce  to  be- 
long to  the  lower  forms  of  life.  The  Lower  Silurian  sandstone  about 
the  city  of  Marquette  is  nearly  horizontally  bedded,  and  rests  uncon- 
formably  on  and  against  the  Huronian. 
Brooks  and  Wright,  in  1880,  discuss  the  geology  of  the  Menominee 
iron  region  in  Wisconsin.     See  section  2,  Wisconsin,  pages  188-190. 
Wads  worth,42  in  1880,  gives  notes  on  the  geology  of  the  iron  and 
copper  districts  of  Lake  Superior.  The  contacts  of  the  jasper  and  ore, 
which  are  interlaminated  and  have  a  common  origin  with  the  asso- 
ciated schists,  are  described,  and  at  numerous  points  the  contacts  are 
found  to  be  those  of  eruptive  and  sedimentary.  The  schistose  struc- 
ture is  regular,  while  the  jasper  and  ore  is  exceedingly  contorted, 
breaks  across  the  schistose  and  other  rocks,  and  contains  fragments  of 
the  schists.  Not  the  slightest  sign  of  plasticity  or  intrusion  of  the 
schists  relative  to  the  ore  and  jasper  was  seen.  ■  The  present  lamination 
of  the  schists  existed  prior  to  the  intrusion  of  the  ore.  At  the 
School-house,  New  York,  and  Jackson  mines  the  overlying  rock  con- 
tains debris  of  the  underlying  ore  and  jasper.    The  diorites,  felsites, 
