118  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
instances  poured  out,  in  others  sublimed,  from  the  interior  of  the 
earth.  Where  the  ores  are  in  a  state  of  purity,  in  the  form  of  irregu- 
lar masses  in  preexisting  depressions,  or  where  the  incumbent  strata 
are  metamorphosed  or  traversed  by  the  dikes  of  ferruginous  matter, 
they  are  without  doubt  eruptive.  Where  impregnating  metamorphic 
products,  such  as  jasper,  hornstone,  or  chert,  quartz,  chlorite,  and  talc 
slate  are  found,  not  only  between  the  laminae  but  intimately  incorpo- 
rated with  the  mass,  giving  it  a  banded  structure,  they  are  regarded  as 
the  results  of  sublimation.  The  supposition  that  the  ore  may  be  a  sec- 
ondary product  resulting  from  the  decomposition  of  a  pyrite  or  the 
metamorphism  of  bog  iron  is  inadequate  to  account  for  the  accumula- 
tion of  such  mountain  masses  and  to  explain  the  relations  to  the  asso- 
ciated rocks. 
The  bed  of  Lake  Superior,  embracing  an  area  of  about  32,000 
square  miles,  is  occupied  almost  exclusively  by  the  Potsdam  sandstone. 
The  sandstone  in  the  vicinity  of  the  trappean  rocks  attains  the  enor- 
mous thickness  of  5,000  feet,  and  often  consists  of  conglomerates 
composed  of  trappean  pebbles,,  while  away  from  these  lines  of  dis- 
turbance, 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  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 
Jackson,  nor  any  other  form  characteristic  of  the  Niagara  formation; 
but,  on  the  other  hand,  they  are  pronounced  as  belonging  to  Lower 
Silurian  types. 
Foster  and  Whitney,22  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  denomi- 
nated the  Iron  Age  of  geology,  while  the  Silurian  epoch  with  equal 
propriety  might  be  designated  the  Copper  Age. 
Foster  and  Whitney,23  in  1851,  further  speak  of  the  age  of  the 
Lake  Superior  sandstone.  The  sandstone  of  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 
volcanic  action  along  the  line  of  fissure.  The  mural  faces  of  the  trap- 
pean ranges  are  almost  without  exception  turned  toward  the  south, 
and  the  sandstone  on  that  side  is  elevated  at  a  high  angle,  sometimes 
