LAKE    SUPERIOR    REGION.  135 
The  slate  beds  at  Plumbago  Brook  are  succeeded  by  schistose  beds 
richer  in  red  feldspar  and  containing  little  quartz,  which  might  by 
superficial  examination  be  mistaken  for  granite,  but  which  is  evi- 
dently a  fragmental  rock  formed  by  the  detritus  of  the  granite,  which 
near  by  forms  large  mountain  masses,  and  the  granite  of  which  is 
very  rich  in  red  feldspar  and  contains  comparatively  little  quartz. 
The  Gogebic  region  is  described  and  the  rocks  are  divided  into 
granitic,  dioritic,  iron-ore,  and  upper  slate  groups,  which  are  analo- 
gous to  the  similar  groups  in  the  Marquette  country.  Granite  seams 
were  found  here  cutting  across  the  dioritic  schists,  but  were  not  found 
to  cut  the  truly  sedimentary  strata.  Locally,  in  contiguity  with  the 
granite,  are  heavy  quartzite  strata  which  are  often  conglomeratic, 
and  are  filled  with  rounded  granite  pebbles.  The  dioritic  rocks  above 
the  granite  often  have  a  brecciated  or  conglomeratic  structure,  the 
fragments  being  various  kinds  of  diorite  cemented  by  the  same  ma- 
terial. The  diorite  is  of  eruptive  character,  as  is  shown  by  the  occur- 
rence of  belts  of  it  cutting  transversely  through  the  ore-bearing  series. 
Limestones  are  also  found,  which  occupy  the  same  position  as  the 
limestones  below  the  ore-bearing  strata  in  the  Menominee  district. 
The  succession  in  Felch  Mountain,  from  the  base  upward,  is  (1) 
granitic  or  dioritic  rocks,  (2)  quartzite  beds,  (3)  fissile  quartz  schists, 
(4)  micaceous  argillite,  (5)  crystalline  limestone  with  siliceous  seams, 
(6)  ferruginous  quartzites,  containing  the  ore  beds. 
Wadsworth,45  in  1884,  as  a  result  of  an  examination  of  a  supposed 
fossil  from  the  copper-bearing  rocks  of  Lake  Superior,  described  by 
Hall  as  being  very  like  the  Huronia  or  siphuncles  of  Orthoceratites, 
finds  it  to  be  of  inorganic  origin,  having  probably  been  formed  by  the 
flowing  of  a  pasty  lava  in  such  a  manner  as  to  raise  a  series  of  ridges, 
giving  an  appearance  closely  like  that  of  some  cephalopods.  The  in- 
terior of  the  specimens  is  in  all  respects  like  that  of  an  ordinary 
volcanic  rock. 
Irving  and  Chamberlin,46  in  1885,  give  a  systematic  account  of 
the  junction  between  the  Eastern  sandstone  and  the  Keweenaw  series 
on  Keweenaw  Point.  Detailed  descriptions  are  given  of  the  relations 
of  the  two  series  at  Bete  Grise  Bay,  the  Wall  ravine,  the  St.  Louis 
ravine,  the  Douglass  Houghton  ravine,  Torch  Lake  quarry,  the  Hun- 
garian ravine,  and  other  points.  The  conclusions,  and  the  grounds 
upon  Avhich  they  are  based,  of  Jackson,  Foster  and  AVhitney,  Agas-i/. 
Ilominger,  Credner,  and  those  who  have  followed  them,  are  discus-cl 
in  detail. 
At  Bete  Grise  Bay  the  horizontal  sandstone  is  found   upon   ap- 
proaching the  melaphyre  to  become  tilted  upward,  and  along  the 
junction  is  found  the  evidence  of  faulting,  both  in  the  fluccan  of  the 
sandstone  and  in  the  broken  character  of  the  melaphyre  at  the  con 
tact.     At  the  Wall  ravine  the  sandstone  and  conglomerate  bearing 
