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vanhisb.]  LAKE    SUPERIOR    REGION.  70 
pearance  of  trappean  rocks  at  the  surface.  This  is  indicated  by  the 
fact  that  at  no  great  distance  to  the  south  an  elevation  occurs  at  which 
the  strata  of  sandstone  dip  on  all  sides,  although  there  is  no  igneous 
rock  visible.  From  the  fossils  entombed  in  this  limestone  Hall  con- 
cludes that  the  rocks  may  be  regarded  as.the  equivalent  of  any  of  the 
following:  The  Potsdam  and  Oalciferous  sandstones,  the  Chazy,  Bird's- 
eye,  and  Black  river  limestones,  perhaps  of  the  Trenton  or  even  Hudson 
river  groups.  In  T.  47  K,,  R.  25  W.,  Michigan,  is  a  conglomerate,  the 
pebbles  of  which  comprise  granite,  hornblende,  slate,  greenstone,  and 
iron  ore.  m 
Foster  and  Whitney73,  in  1851,  report  on  the  iron  region  of  lake 
Superior  land  district,  and  give  and  an  account  of  its  general  geology. 
The  older  rocks  are  classified  as  follows: 
Plutonic  |  Granite. 
rocks.         ^T"lte-         . 
{  ieldspar  and  quartz  rock. 
Trappean  f  Greenstone,  or  dolerite,  porphyry,   basalt, 
or  I      amygdaloid,    hornblende  and   serpentine 
volcanic  j      rocks,  masses  of  specular    and  magnetic 
rocks.  [     oxide  of  iron. 
f  Gneiss,  mica  and  hornblende  slate.  • 
.  <  Chlorite,  talcose  and  argillaceous  slate. 
^  Beds  of  quartz  and  saccharoidal  marble. 
o 
2  1  Silurian  system. 
The  igneous  rocks  are  found  in  all  these  sedimentary  systems.  The 
oldest  igneous  rocks,  consisting  of  hornblende,  feldspar,  and  serpen- 
tine rocks  are  contemporary  with  the  Azoic  system.  The  granites  and 
syenites  are  intermediate  in  age  between  the  Azoic  and  Silurian  systems. 
These  are  traversed  by  two  systems  of  greenstone  dikes  which  are 
anterior  to  the  purely  sedimentary  deposits.  Contemporaneous  with 
the  lower  portion  of  the  Silurian  system  are  the  bedded  traps  and 
amygdaloids  of  Keweenaw  point,  isle  Eoyale  and  the  Ontonagon  region, 
which  are  conrposed  of  nearly  the  same  constituents  as  many  of  the 
older  igneous  rocks,  although  there  is  no  difficulty  from  the  diversity 
in  external  characters  in  drawing  the  line  of  demarcation  between 
them. 
Below  all  the  fossiliferous  groups  of  the  region  is  a  class  of  rocks  con- 
sisting of  crystalline  schists,  beds  of  quartz,  and  saccharoidal  marble, 
which  is  denominated  the  Azoic  system,  a  term  first  applied  by  Mur- 
chison  and  de  Yerneuil  to  designate  the  crystalline  masses  which  pre- 
ceded the  Paleozoic  strata.  This  term  as  here  used  is  limited  to  rocks 
which  are  detrital  in  their  origin  and  which  have  been  formed  before 
