154  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
at  the  Kearsarge  conglomerate.  In  this  mine  is  the  eastern  edge  of 
the  basin  in  which  the  Kearsarge  conglomerate  was  deposited. 
Newtett,72  in  1895,  gives  a  sketch  of  the  Marquette  iron-bearing 
district  of  Michigan,  and  publishes  a  geological  map  of  the  district, 
compiled  from  a  map  of  the  Upper  Peninsula  in  the  possession  of 
the  Michigan  Geological  Survey.  The  iron  ores  occur  in  the  Huro- 
nian  rocks,  of  which  there  are  some  thirty  members.  This  series  of 
rocks  has  been  subjected  to  enormous  lateral  pressure,  resulting  in 
foldings  in  the  strata.  In  the  folds  the  ore  is  found  generally  in  len- 
ticular masses.  The  Huronian  rocks  are  cut  by  eruptive  rocks,  which 
have  played  an  important  part  in  assisting  in  the  concentration  of 
the  ores. 
Clements,73  in  1895,  describes  the  volcanic  rocks  of  the  Michi- 
gamme  district  of  Michigan.  The  succession  in  the  district  from  the 
base  up  is:  (1)  Granite  and  gneiss,  cut  by  basic  dikes;  (2)  quart- 
zose  limestone  formation,  with  an  estimated  thickness  of  1,500  to 
2,000  feet;  (3)  a  great  series  of  volcanics,  with  an  average  thickness  of 
about  3,000  feet;  (1)  a  set  of  sedimentaries  consisting  of  quartzites, 
slates,  and  iron  formation  material.  The  volcanics  include  apobasalts, 
apoandesites,  and  aporhyolites,  each  occurring  both  as  lavas  and  as 
tuffs.     The  lavas  are   frequently  amygdaloidal. 
Van  Hise,74  in  1896,  describes  base-levels  in  the  crystalline  rocks  of 
central  Wisconsin  and  Keweenaw  Point.  In  the  Wisconsin  district 
the  Archean  and  Huronian  rocks  occupying  the  area  are  truncated  to 
an  even  base-level  with  an  apparent  southerly  slope.  The  altitude  is 
about  1,450  feet. 
On  Keweenaw  Point  the  peaks  of  the  main  trap  range  rise  to  so 
nearly  the  same  altitude  that  they  form  an  apparent  plain,  which  is 
considered  an  ancient  base-level.  The  altitude  of  this  plain  is  about 
1,350  feet.  Certain  peaks,  consisting  of  hard  quartz  porphyry  and 
felsite,  have  resisted  weathering,  and  stand  above  this  plain. 
The  central  Wisconsin  plain  has  not  been  so  deeply  dissected  as 
the  Keweenaw  Point  area,  but  this  is  explained  by  the  fact  that  it  is 
not  so  near  either  Lake  Superior  or  Lake  Michigan,  and  therefore 
erosion  has  not  been  so  effective  over  it. 
From  the  proximity  of  the  central  Wisconsin  and  Keweenaw  Point 
base-levels,  and  from  the  fact  that  they  have  nearly  the  same  altitude, 
it  is  concluded  that  the  base-levels  of  the  two  districts  are  probably 
but  parts  of  a  far  more  extensive  base-leveled  region  resulting  princi- 
pally from  the  subaerial  erosion  of  Cretaceous  time,  and  perhaps  also 
in  part  from  the  marine  denudation  of  the  Cretaceous. 
Gresley,75  in  1896,  describes  peculiar  markings  in  iron  ore  from  the 
Chapin  mine  of  Iron  Mountain,  Michigan,  which  are  thought  by  II.  S. 
Williams,  by  Schuchert,  and  by  Walcott  to  be  trails  of  organic  origin. 
