LAKE    SUPERIOR   REGION.  115 
Silver  Mountain  rises  as  an  isolated  and  dome-shaped  mass  to  the 
height  of  1,000  feet.  The  summit  of  the  rock  consists  of  labradorite 
and  hornblende,  while  the  surrounding  plain  is  covered  with  clay, 
resting  on  sandstone  in  a  nearly  horizontal  position.  The  sandstones 
and  interbedded  traps  of  Keweenaw  Point  and  Isle  Royal  dip  in 
opposite  directions  and  form  a  synclinal  trough.  Near  L'Anse  is  a 
limited  patch  of  limestone  which  has  a  dip  to  the  east  of  25°  to  30°, 
the  limestone  being  distinctly  stratified.  The  sandstone  about  a 
quarter  of  a  mile  to  the  north  is  horizontal,  and  it  seems  evident  that 
the  limestone  overlies  the  sandstone,  although  the  position  of  the  in- 
clined beds  of  the  more  southerly  portion  of  the  limestone  is  diffi- 
cult to  explain,  since  the  surrounding  country  is  low  and  level  and 
underlain  b}^  sandstone  and  horizontal  beds.  It  seems  evident  that 
at  this  point  the  country  has  been  disturbed  and  upheaved  by  igneous 
action  beneath,  which  has  raised  the  strata  without  any  appearance 
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  Calciferous  sandstones,  the  Chazy, 
Bird's-eye,  and  Black  River  limestones,  perhaps  of  the  Trenton  or 
even  Hudson  River  groups.  In  T.  47  N.,  R.  25  W.,  Michigan,  is  a 
conglomerate^  the  pebbles  of  which  comprise  granite,  hornblende, 
slate,  greenstone,  and  iron  ore. 
Foster  and  Whitney,21  in  1851,  report  on  the  iron  region  of  the 
Lake  Superior  land  district,  and  give  an  account  of  its  general 
geology.    The  older  rocks  are  classified  as  follows: 
Igneous  (of  various  ages)., 
Plutonic 
rocks. 
Trappean 
or 
volcanic 
rocks. 
["Granite. 
Syenite. 
[Feldspar  and  quartz  rock. 
Greenstone,  or  dolerite,  porphyry,  ba- 
salt, amygdaloid,  hornblende  and 
serpentine  rocks,  masses  of  specular 
and  magnetic  oxide  of  iron. 
,,  ,  ,  .       /  A     •  fGneiss,  mica  and  hornblende  slate. 
Metamorphic    (Azoic    sys-l    ,  ,     .'  ■  ...  .  . 
^Chlorite,  talcose  and  argillaceous  slate. 
[Beds  of  quartz  and  saccharoidal  marble. 
tern). 
Aqueous  (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  sys- 
tems.   These  are  traversed  by  two  systems  of  greenstone  dikes  which 
