vaxhise.]  LAKE    SUPERIOR    REGION.  87 
schists  existed  prior  to  the  intrusion  of  the  ore.  At  the  School-house, 
New  York,  and  Jackson  mines  the  overlying  rock  contains  debris  of 
the  underlying-  ore  and  jasper.  The  diorices,  felsites,  and  diabases  are 
intrusive  rocks.  The  soft  hematites  diifer  only  from  the  hard  ore  and 
jasper  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  crystal- 
line rocks  of  Presque  isle  are  peridotite  and  serpentine  which  has  re- 
sulted 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  was  found  in  Avhich 
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  were 
laid  down  in  the  usual  way ;  were  then  disturbed  by  the  eruption  of  the 
jasper  and  ore.  Much  of  the  original  rock  still  remained  horizontal, 
and  new  sedimentary  deposits  continued  to  be  formed  out  of  the  jasper 
and  other  rocks.  Next  came  the  eruption  of  the  diorite,  which  com- 
pleted 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  and  were  successively  laid  down  one  upon  the 
other,  are  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  has  been  maintained  to  rest 
against  the  trap  and  sandstone  series  unconformably,  is  found  inter- 
laminated  with  the  melaphyres,  and  therefore  settles  the  long-disputed 
question  of  the  relative  age  of  the  traps  and  Eastern  sandstone  of  lake 
Superior.  The  last  melaphy  re  sheet  wl  1  ich  underlies  th  e  sand  stone  has 
a dip  to  the north  west  of  20°.  A3  the  Douglass  Houghton  ravine  is 
followed  downward  the  dip  gradually  declines  in  steepness,  although 
still  to  the  northwest,  the  last  dip  measured  being  5°.  The  junction 
between  the  Eastern  sandstone  and  the  trap,  described  by  Agassiz  and 
Fumpelly,  is  not  the  junction  at  all,  it  being  some  distance  below  in- 
stead of  at  the  falls.  In  the  Torch  lake  sandstone  quarry  t  he  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  between  them  are  of  the  same 
geological  age. 
