LAKE    SUPERIOR   REGION.  129 
and  diabases  are  intrusive  rocks.  The  soft  hematites  differ  from  the 
hard  ore  and  jasper  only  in  that  they  have  been  leached  by  thermal 
waters  and  changed  to  their  soft  condition.  The  granite  is  found  at 
numerous  points  to  cut  the  schists  and  gneisses.  At  several  places, 
also,  it  cuts  a  quartzite,  one  of  which  resembles  the  ordinary  Huronian 
quartzite.  The  crystalline  rocks  of  Presque  Isle  are  peridotite  and 
serpentine  which  has  resulted  from  the  alteration  of  the  peridotite. 
The  only  evidence  that  the  Huronian  unconformably  overlies  the 
Laurentian  is  the  fact  that  the  foliation  of  the  latter  does  not  conform 
in  its  dip  to  that  of  the  former.  However,  no  point  wTas  found  in 
which  it  was  possible  to  trace  the  rock  continuously  from  well-marked 
and  mapped  Laurentian  into  the  Huronian.  The  general  structure 
of  the  iron  region  seems  to  be  as  follows :  The  'schists  and  sandstones 
Avere  laid  down  in  the  usual  way,  and  were  then  disturbed  by  the 
eruption  of  the  jasper  and  ore.  Much  of  the  original  rock  still  re- 
mained horizontal,  and  new  sedimentary  deposits  continued  to  be 
formed  out  of  the  jasper  and  other  rocks.  Next  came  the  eruption 
of  the  diorite,  which  completed  most  of  the  local  folding  and  tilting 
of  the  strata.  Finally  the  granite  eruption  took  place  on  both  sides 
of  the  Huronian,  uplifting  and  contorting  the  strata  near  it,  and 
perhaps  laterally  compressing  the  inclosed  iron-bearing  rocks. 
The  conclusion  reached  by  Foster,  Whitney,  and  Marvine  that  the 
traps  and  lava  flows  were  successively  laid  down  one  upon  the  other, 
and  covered  by  sandstones  and  conglomerates,  is  agreed  with.  The 
sandstones  and  conglomerates  when  overlain  by  traps  are  usually 
baked  and  indurated.  At  the  Douglass  Houghton  ravine  and  Hun- 
garian River  the  Eastern  sandstone,  which  it  has  been  maintained 
rests  against  the  trap  and  sandstone  series  unconformably,  is  found 
interlaminated  with  the  melaphyres,  and  this  settles  the  long-disputed 
question  of  the  relative  age  of  the  traps  and  Eastern  sandstone  of 
Lake  Superior.  The  last  melaphyre  sheet  which  underlies  the  sand- 
stone has  a  dip  to  the  northwest  of  20°.  As  the  Douglass  Houghton 
ravine  is  followed  downward  the  dip,  although  still  to  the  northwest, 
gradually  declines  in  steepness,  the  last  dip  measured  being  5°.  The 
junction  between  the  Eastern  sandstone  and  the  trap,  described  by 
Agassiz  and  Pumpelly,  is  not  the  junction  at  all,  it  being  some  dis- 
tance below  instead  of  at  the  falls.  In  the  Torch  Lake  sandstone 
quarry  the  sandstone  layers,  instead  of  being  horizontal,  as  they  have 
been  regarded,  have  a  dip  of  15°,  the  former  supposed  bedding  being 
due  to  joints.  As  the  Eastern  sandstone  conformably  underlies  the 
traps,  the  Eastern  and  Western  sandstones  and  the  traps  lying  be- 
tween them  are  of  the  same  geological  age. 
Rominger,43  in  1881,  gives  a  general  account  of  the  Marquette  and 
Menominee  iron  region,  with  very  voluminous  details  as  to  particular 
localities. 
55721— Bull.  3G0— 09 9 
