vanhise.]  LAKE    SUPERIOR    REGION.  143 
rock.  These  green  schists  are  regarded  as  the  equivalent  of  those  cut 
by  granite  in  the  Penokee-Gogebic  district.  On  this  view  the  slate 
series  of  the  Marquette  district,  consisting  in  the  main  of  little  altered 
rocks,  was  built  up  on  a  basement  composed  of  granite  and  gneiss 
and  greenish  schist,  and  subsequently  pushed  into  trough-like  forms. 
En  support  of  this  view  is  cited  the  failure  of  the  granite  to  penetrate 
the  slates  and  quartzites  associated  with  the  iron,  and  the  occurrence 
in  the  higher  series  of  fragments  from  the  lower,  recomposed  rocks 
occurring  at  points  where  the  quartzites  come  in  contact  with  the 
basement  rocks. 
The  Archean  in  these  regions  is  then  divisible  unless  the  upper  series 
ire  called  Cambrian,  for  which  there  is  no  ground  until  in  them  fossils 
tiave  been  discovered.  These  upper  series  are  compared  with  the  Hu- 
ronian of  lake  Huron,  and  are  found  to*  be  lithologically  like  them,  and 
bo  bear  the  same  relations  to  the  underlying  rocks,  and  to  them  the 
term  Huronian  is  applied,  while  the  underlying  complex  is  regarded  as 
Lauren  tian. 
Irving  196,  in  1885,  gives  a  preliminary  account  of  an  investigation  of 
the  Archean  formations  of  the  Northwestern  States.  The  problems  to 
je  solved  are  discussed.  An  examination  of  the  Original  Huronian  area 
)f  Murray  and  Logan  shows  that  it  is  a  series  of  rocks  which  is  bent 
nto  gentle  folds  and  which  is  composed  for  the  most  part,  excluding 
eruptive  material,  of  quartzites  and  graywackes,  with  a  subordinate 
proportion  of  limestone  and  chert.  The  rocks  as  a  whole  are  very  little 
iltered  and  resemble  more  the  fossiliferous  formations  than  the  crystal- 
line schists. 
The  Marquette  and  Menominee  iron-bearing  series  are  highly  folded 
and  the  metasomatic  changes  which  the  crystalline  members  of  the 
series  have  undergone  are  often  extreme.  Excluding  the  greenish 
schists  of  the  lower  part  of  the  series,  which  may  belong  to  an  older 
formation,  the  rocks  are  mainly  fragmental  slates  and  quartzites,  in- 
eluding  a  large  proportion  of  basic  eruptives,  and  also  iron  ores,  lime- 
stones, etc.,  the  whole  having  a  distinctly  Huronian  aspect.  The  vari- 
pus  greenstone  layers  of  Brooks'  scheme  are  regarded  as  eruptive,  either 
contemporaneous  or  subsequent,  as  are  also  many  of  the  greenish  schists 
which  by  gradation  pass  into  the  massive  greenstones.  The  iron  ore 
and  jasper  are  regarded  as  of  sedimentary  origin,  being  remarkably 
like  similar  material  in  the  Penokee-Gogebic  and  Vermilion  formations 
where  there  can  be  no  doubt  of  their  water-deposited  character.  In  the 
Marquette  district,  as  well  as  in  the  Vermilion  lake  district  in  Minne- 
sota, are  conglomerates  overlying  the  iron  belt  which  sometimes  con- 
tain fragments  of  the  underlying  formation  several  feet  in  length. 
These  fragments  prove  the  existence  of  the  thejaspery  and  chalcedonic 
material  in  its  present  condition  before  the  formation  of  the  overlying 
quartzite. 
The  Penokee-Gogebic  iron  belt  is  regarded  as  continuous  with  the 
