LAKE    SUPERIOR   REGION.  119 
dipping  almost  vertical  at  the  junction  of  the  two  formations,  but  in 
proceeding  southward  becoming  almost  immediately  horizontal.  The 
appearance  is  as  if  the  strata  had  been  broken  and  elevated,  just  like 
the  southern  edge  of  the  igneous  mass.  Where  the  sandstones  and 
traps  are  interlaminated  it  is  difficult  to  determine  the  junction  when 
the  sandstone  lies  upon  the  trap,  but  when  below  it  the  line  of  separa- 
tion is  sharp.  As  further  showing  that  the  sandstone  is  Lower  Silu- 
rian, a  small  deposit  of  Lower  Silurian  limestone  rests  upon  the  sand- 
stone ;  this  effectually  completes  the  chain  of  evidence. 
Jackson,24  in  1853,  maintains  that  the  red  and  gray  sandstones  of 
Lake  Superior  are  above  the  rocks  of  Devonian  age.  They  rest  hor- 
izontally around  Silurian  limestone,  which  has  an  inclination  on 
Sturgeon  River  near  Keweenaw  Point  of  30°.  In  point  of  fact  the 
sandstones  of  Lake  Superior  are  the  exact  equivalents  of  those  of 
Nova  Scotia,  where  trap  rocks  of  the  same  age  as  those  of  Lake 
Superior  pass  through  them.  The  amygdaloidal  trap  of  Keweenaw 
Point  and  Isle  Royal  is  a  vesicular  rock  formed  by  the  interfusion 
of  sandstone  and  trap  rock. 
Jackson,25  in  1800,  again  asserts  that  the  red  sandstones  of  Kewee- 
naw Point  are  certainly  coeval  with  the  sandstone  of  Nova  Scotia, 
Connecticut  River,  and  New  Jersey,  as  proved  by  identity  of  composi- 
tion, mode  of  disruption,  character  of  associated  minerals,  and,  above 
all,  by  the  fact  that  they  rest  upon  Devonian  limestones.  Orthoceratite 
at  Copper  Falls  mine  and  Pentamerus  in  the  underlying  limestone  of 
Sturgeon  River  show  that  the  sandstones  are  not  Potsdam.  This  is 
also  shown  by  the  occurrence  of  pitchstone  porphyry  on  Isle  Royal 
such  as  is  found  in  the  Isle  of  Arran  of  Triassic  or  Devonian  age. 
The  author  is  not  disposed  to  place  the  sandstones  of  the  Pictured 
Rocks  in  the  same  formation  with  Keweenaw  Point  and  Isle  Royal. 
Rogers,20  in  1860,  maintains  that  the  argillaceous  shales  and  con- 
glomerates of  a  part  of  the  southern  shore  of  Lake  Superior  are  the 
equivalent  of  the  Primal  series.  The  Cupriferous  series  is  in  direct 
association  with  the  Potsdam,  and  therefore  the  argument  for  Triassic 
age  on  account  of  texture  and  color  is  entirely  valueless. 
Winciiell  (Alexander),27  in  1861,  gives  a  general  sketch  of  the 
geology  of  Michigan.  Among  the  stratified  rocks  are  placed  the 
Azoic,  while  the  unstratified  rocks  are  divided  into  Volcanic,  includ- 
ing lava,  trap,  etc.,  and  Plutonic,  including  granite,  syenite,  etc.  The 
Azoic  rocks  are  of  immense  thickness  and  are  interposed  between  the 
crystalline,  plutonic,  and  volcanic  rocks  and  the  Lake  Superior  sand- 
stone. The  rocks  in  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.  Josephs  and  Sulphur  islands 
