122  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
may  have  a  thickness  of  5,000  feet  and  consist  of  a  series  of  exten- 
sively folded  beds  of  diorite,  quartzite,  chlorite  schists,  clay  slate, 
mica  slate,  and  graphitic  shales,  among  which  are  intercalated 
extensive  beds  of  several  varieties  of  iron  ore.  The  most  abundant 
rock  is  greenstone  or  diorite,  in  which  the  bedding  is  usually  obscure, 
but  the  intercalated  schists  and  slates  usually  bear  strong  marks  of 
stratification.  The  dips  are  usually  at  a  high  angle  and  are  more  apt 
to  be  north  or  south  than  any  other  direction.  The  granitic  rocks 
are  believed  to  be  the  equivalents  of  the  Laurentian  of  Canada.  In 
these  the  bedding  indications  are  still  more  obscure  and  often  entirely 
wanting.  Also,  if  possible,  there  is  more  irregularity  in  strike  and 
dip  than  in  the  Huronian. 
A  full  lithological  description  of  the  different  phases  of  rocks 
found  in  the  Huronian  system,  and  the  various  sections  at  the  many 
mines  in  the  Upper  Peninsula,  are  given  in  detail. 
The  formations  of  the  Huronian  system  in  the  Marquette  region 
comprise  nineteen  members,  numbered  from  the  base  upward.  I,  II, 
III,  and  IV  are  composed  of  beds  of  siliceous  ferruginous  schist, 
alternating  with  chloritic  schists  and  diorites,  the  relations  of  which 
have  not  been  fully  made  out ;  V  is  a  quartzite,  sometimes  containing 
marble  and  beds  of  argillite  and  novaculite;  VI,  VIII,  and  X  are 
siliceous  ferruginous  schists;  VII,  IX,  and  XI  are  dioritic  rocks, 
varying  much  in  character;  XIII  is  the  bed  which  contains  all  the 
rich  specular  and  magnetic  ore,  associated  with  mixed  ore  and  mag- 
nesian  schist ;  XIV  is  a  quartzite,  often  conglomeratic ;  XV  is  argil- 
lite  or  clay  slate ;  XVI  is  uncertain — it  contains  some  soft  hematite ; 
XVII  is    anthophyllitic    schist,    containing    iron    and    manganese; 
XVIII  is  doubtful;  XIX  is  mica  schist,  containing  staurolite,  anda- 
lusite,  and  garnets.  The  thickness  of  the  whole  Marquette  series  may 
aggregate  5,000  feet. 
The  beds  appear  to  be  metamorphosed  sedimentary  strata,  having 
many  folds  or  corrugations,  thereby  forming  in  the  Marquette  region 
an  irregular  trough  or  basin,  which,  commencing  on  the  shore  of  Lake 
Superior,  extends  west  more  than  40  miles.  The  upturned  edges 
of  these  rocks  are  quite  irregular  in  their  trend  and  present  numerous 
outcrops.  While  some  of  the  beds  present  lithological  characters  so 
constant  that  they  can  be  identified  wherever  seen,  others  undergo 
great  changes.  Marble  passes  into  quartzite,  which  in  turn  graduates 
into  novaculite. 
Near  the  junction  of  the  Huronian  and  Laurentian  systems,  in  the 
Marquette  region,  are  several  varieties  of  gneissic  rocks,  composed 
in  the  main  of  crystalline  feldspar,  with  glassy  quartz  and  much 
chlorite.  Intersecting  these  are  beds  of  hornblendic  schist,  argillite, 
and  sometimes  chloritic  schist.  These  rocks  are  entirely  beneath  all 
of  the  iron  beds,  seem  to  contain  no  useful  mineral  ores,  and  are 
