176 
PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Partial  geological  section  of  Michigan — Continued. 
Sys- 
tem. 
Series. 
Formation. 
Thickness 
(feet). 
Character  of  rocks. 
Animikean  (Neo- 
Huronian). 
Michigamme 
Bijiki 
1,000-4,000 
300-800 
0-400+ 
Slates;  black  and  graphitic,  and  gray- 
wacke  slates,  passing  into  graywacke 
arkoses,  and  quartzites;  metamor- 
phosed into  staurolite,  chiastolite, 
garnet,  and  other  mica  schists  and 
phyllites. 
Iron  formation  or  schist;  slates  with 
cherty  carbonate  and  soft  ore. 
Quartzite;  with  conglomerate  base,  and 
above  quartzite  or  red  and  green  Hags, 
also  volcanic  material. 
S 
'3 
Mio-Huronian. 
1,000+ 
600+ 
700+ 
s 
tered  to  jaspilites,  etc.,  with  efl'usives 
and  intrusives  altering  to  hornblende 
schists  and  amphibolites. 
Slates;    graywackes  and  arkoses  and 
volcanics. 
Ajibik 
Eo-IIuronian. 
300+ 
600+ 
250+ 
Slate;  black  largely. 
Dolomite;     with   siliceous  cherty   and 
slaty  (schistose)  bands. 
Quartzite;    conglomerate  and   arkose, 
becoming  a  scricite  schist  or  gneiss. 
.3 
Keewatin  (Mare- 
niscan)  =  Lau- 
rentian. 
1,000+ 
Greenstone  schists,  amphibolites,  horn- 
blende   schists,    sericite    schists    or 
crushed    felsites,     rarely    ellipsoidal 
greenstones  and  slates  and  jaspilites, 
very  largely  cut  by  granites,  the  Lau- 
rentian,  arid  numerous  other  classes 
of  injections. 
This  succession  is  essentially  the  same  as  that  of  the  later  work  of 
the  United  States  Geological  Survey  in  Michigan,  but  some  of  the 
names  differ,  the  thicknesses  and  characters  of  the  rocks  are  not  in  all 
cases  the  same,  and  there  are  differences  in  the  major  correlation. 
The  principal  points  of  difference  are  as  follows: 
The  Archean  or  Basement  Complex  of  the  United  States  Geological 
Survey  is  here  called  Laurentian.  Its  subdivision  into  Laurentian 
and  Keewatin  is  essentially  the  same.  The  Palmer  gneiss  of  the  Lau- 
rentian  of  the  United  States  Geological  Survey  is  here  regarded  as 
representing  sheared  phases  of  the  lower  part  of  the  Huronian  series, 
up  to  the  Siamo  slate. 
The  Upper,  Middle,  and  Lower  Huronian  of  the  United  States 
Geological  Survey  are  here  called  Neo-Huronian,  Mio-Huronian,  and 
Eo-Huronian.  They  include  the  same  formations.  The  Clarksburg 
volcanics,  instead  of  being  correlated  with  the  Upper  Huronian,  are 
correlated  with  both  the  Middle  and  the  Upper  Huronian.  All  of 
the  iron  formation  of  the  Upper  Huronian  is  included  in  the  Bijiki 
formation,  which  is  regarded  as  more  continuous  than  had  been  indi- 
cated in  the  work  of  the  United  States  Geological  Survey.  The 
Bijiki  schist  has  previously  included  only  one  of  the  members  of  the 
iron  formation.  The  Michigamme  slates  and  their  equivalents  are 
regarded  as  usually  not  over  1,000  to  2,000  feet  thick,  although  on 
