178  PKE-CAMBRIAN    GEOLOGY    OF    NORTH    4MERICA. 
exposed  to  a  minimum  thickness  of  60,000  feet,  and  the  real  thickness 
is  a  large  fraction  of  this,  even  after  making  allowance  for  faulting, 
initial  dip,  and  uncomformity.  It  seems  probable  that  the  full  thick- 
ness exists  in  the  center  of  the  Lake  Superior  basin,  but  that  around 
the  margin  of  the  basin,  owing  to  erosion,  denudation,  and  uplift, 
the  bottom  beds  were  never  buried  by  any  such  thickness.  The  dip 
of  the  Keweenawan  at  the  south  end  of  the  section  is  78°  and  at  the 
north  end  20°,  in  part  as  a  result  of  sinking  of  the  Lake  Superior 
basin  contemporaneous  with  deposition,  but  possibly  in  part  due  to 
unconformity.  A  conglomerate  below  the  base  of  the  Nonesuch 
sandstone  contains  more  than  the  usual  number  of  jasper  fragments 
from  the  Animikie  formation  and  may  possibly  represent  such  a 
break,  but  if  this  break  were  present  bowlders  from  the  basic  rocks 
making  up  the  base  of  the  series  should  be  more  numerous  in  the 
conglomerate.  Many  dip  faults  are  found.  Strike  faults  are  not 
observed,  but  are  believed  to  be  present.  From  change  in  grain  of 
the  gabbro  from  the  surface  inward  Lane  argues  that  its  initial  tem- 
perature of  crystallization  wTas  1,850°.  In  a  note  on  the  pre-Ke- 
weenawan  rocks  Lane  calls  attention  to  the  existence  of  basic  tuf- 
faceous  slate,  not  certainly  in  place,  in  the  area  of  the  iron-bearing 
formation  near  Bessemer,  possibly  representing  contemporaneous 
volcanic  activity. 
For  fuller  summaries  of  articles  referring  to  the  Michigan  ore- 
bearing  districts  see  Monographs  V,  XIX,  XXVIII,  XXXVI,  and 
XLVI  of  the  United  States  Geological  Survey. 
SECTION  2.     NORTHERN  WISCONSIN. 
SUMMARY    OF   LITERATURE. 
Owen,98  in  1847,  gives  many  details  of  the  formations  of  the  in- 
terior of  the  Chippewa  land  district  and  of  the  formation  of  Lake 
Superior.  In  the  first  district  are  seen  many  varieties  of  granite, 
syenite,  greenstones,  hornblende  rock,  gneiss,  and  mica  slate.  Mag- 
nesian  and  magnetic  slates  are  capped  unconformably  by  pebbly 
sandstones  for  nearly  a  mile  along  Black  River.  The  red  sandstone 
of  Lake  Superior  on  Raymonds  Creek  is  estimated  by  Randall  to  be 
10,000  feet  in  thickness. 
Owen,09  in  1847,  finds  the  horizontal  sandstone  to  overlap  the  crys- 
talline and  metamorphic  formations  at  the  southern  portion  of  the 
Chippewa  land  district  near  the  falls  of  the  streams  flowing  into  the 
Mississippi.  The  region  to  the  north  is  based  upon  crystalline,  gran- 
itic, and  other  intrusive  rocks.  North  of  the  summit  levels  of  the 
Chippewa  land  district  the  peculiar  formations  of  the  Lake  Superior 
country  commence.  These  are  red  sandstones,  marls,  and  conglomer- 
ates, occasionally  penetrated  by  intrusive  ranges  of  hornblende,  green- 
