82  PRE- CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
of  trappean  pebbles,  and  away  from  these  lines  of  disturbance,  where 
it  abuts  against  the  Azoic  rocks,  it  is  a  purely  siliceous  sand.  At 
Granite  point  masses  of  granite  are  overlain  by  horizontal  sandstones. 
The  granites  are  cut  by  dikes  of  greenstone,  which  in  no  case  penetrate 
the  overlying  rock.  The  same  phenomena  are  seen  at  Presque  isle,  at 
Middle  island  below  Presque  isle,  and  at  Carp  river.  On  the  Menominee 
river,  near  the  foot  of  Chippewa  island,  layers  of  sandstone  are  found 
on  the  upturned  edges  of  the  Azoic  slates. 
Whitney  does  not  find  the  Pentamerus  oblongus  referred  to  by  Jack- 
son, nor  any  other  form  characteristic  of  the  Magara  formation;  but,  on 
the  other  hand,  they  are  pronounced  as  belonging  to  Lower  Silurian 
types. 
Foster  and  Whitney,74  in  1851,  repeat  the  general  conclusions 
contained  in  their  report  on  the  iron  region,  and  remark  that  the  Azoic 
system  is  characterized  by  such  immense  deposits  of  specular  and 
magnetic  oxide  of  iron  that  it  might  with  propriety  be  denominated 
the  Iron  Age  of  geology,  while  the  Silurian  epoch  with  equal  propriety 
might  be  designated  the  Copper  Age. 
Poster  and  Whitney,75  in  1851,  farther  speak  of  the  age  of  the; 
lake  Superior  sandstone.  The  sandstone  of  the  St.  Marys  river  is 
traced  to  the  south  shore  of  Keweenaw  point  and  is  found  to  increase 
in  thickness  gradually,  until  in  the  vicinity  of  the  trappean  rocks  it 
becomes  of  great  thickness,  accompanied  by  wide  belts  of  conglomer- 
ate. The  conglomerates  of  Keweenaw  point  are  the  result  of  igneous 
rather  than  aqueous  forces,  being  caused  by  friction  and  mechanical 
voleai lie  action  along  the  line  of  fissure.  The  mural  faces  of  the  trag 
pean  ranges  are  almost  without  exception  turned  toward  the  south, 
and  the  sandstone  on  that  side  is  elevated  at  a  high  angle,  sometimes 
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  as  the 
southern  edge  of  the  igneous  mass.  Where  the  sandstones  and  trapsi 
are  iiiterlaminated  it  is  difficult  to  determine  the  junction  when  thei 
sandstone  lies  upon  the  trap,  but  when  below  it  the  line  of  separation 
is  sharp.  As  further  showing  that  the  sandstone  is  Lower  Silurian,  a 
small  deposit  of  Lower  Silurian  limestone  resting  upon  the  sandstone! 
effectually  completes  the  chain  of  evidence. 
Owen,76  in  1851,  mentions  various  metamorphie  slates,  quaitzites,  anw 
other  crystalline  and  trappean  rocks  as  occurring  on  the  south  shore  of 
lake  Superior.  On  the  north  shore,  in  Minnesota,  between  Pond  duS 
Lac  and  the  British  possessions  there  is  a  repetition  in  inverse  order  of 
the  same  formations,  forming  a  synclinal  trough  with  the  red  sandstona 
nearest  the  lake,  while  the  slates,  oonglomerates,  and  associated  trap! 
are  crossed  in  succession  in  proceeding  into  the  interior,  and  these  ai 
followed  by  the  nietamorphic  slates  and  granitic  rocks. 
