vax  hise.]  LAKE    SUPERIOR    REGION.  167 
If  the  objection  of  great  thickness  for  tbe  Keewatin  and  Coutchi- 
ching  has  weight,  the  enormous  thickness  of  over  120,000  feet  given 
for  the  conformable  system  in  northeastern  Minnesota,  on  the  theory 
that  the  Keewatin  and  Coutch idling  are  conformable  and  grade  into 
the  gneissic  series,  and  that  all  are  of  sedimentary  origin,  now  folded 
into  a  simple  synclinal  structure,  will  be  still  more  weighty  evidence 
against  the  correctness  of  the  conclusion  reached.  It  is  notable  that 
Willis's  explanation  of  the  structure  at  the  center  of  this  supposed 
syncline,  Vermilion  lake,  reverses  this  and  makes  it  an  anticline. 
Lawson  does  not  have  to  meet  this  difficulty  because  he  regards  the 
granite  and  gneiss  as  irruptive  and  consequently  a  series  to  which  the 
principles  of  ordinary  stratigraphical  geology  do  not  apply. 
From  the  foregoing  statements  it  might  be  concluded  that  lake 
Superior  stratigraphy  below  the  Keweenawan  is  in  a  greater  state  of 
confusion  than  is  really  the  case,  for  a  closer  examination  of  the  vari- 
ous successions  shows  that  many  apparent  discrepancies  are  not  real, 
if  conclusions  are  not  extended  beyond  the  held  studied  in  each  case. 
Confusion  has  resulted  because  the  conclusions  built  up  from  a  study 
of  a  small  part  of  the  region  have  been  assumed  to  apply  to  the  whole, 
and  because  different  names  are  used  for  the  same  thing.  The  lake 
Superior  region  is  so  large  that  no  one  has  had  or  can  have  a  detailed 
personal  knowledge  of  more  than  a  small  part  of  it. 
LITHOEOGICAL  CHARACTERS  OF  AZOIC,  LACREXTIAN,   HUROXIAN,  ETC. 
Before  going  farther  it  will  be  well  to  summarize  the  lithological 
characters  of  the  pre-Keweenawan  rocks  included  by  the  various  writers 
under  the  terms  applied  to  the  different  series  in  different  districts, 
although  to  a  certain  extent  this  will  repeat  the  preceding  paragraphs. 
It  is  here  much  less  easy  to  make  definite  statements  than  in  the  case  of 
the  lake  Superior  sandstone  and  Keweenawan. 
Azoic,  as  used  by  Foster  and  Whitney  and  those  who  followed  these 
authors,  was  made  to  include  everything  below  the  Keweenawan,  with 
the  exception  of  the  rocks  which  are  plainly  igneous.  It  covered  con- 
glomerates, (juartzites,  slates  and  marble,  as  well  as  the  gneisses, 
mica  schists,  hornblende-schists,  etc.;  that  is,  rocks  which  vary  in 
their  character  from  those  which  are  plainly  clastic,  as  conglomerates, 
to  those  which  are  completely  crystalline.  The  granites,  syenites, 
greenstones,  greenstone  slates,  iron  ore,  jasper,  etc,  were  regarded  as 
igneous  rocks,  in  part  contemporaneous  with  and  in  part  newer  than 
the  rocks  of  the  Azoic  system,  all  of  which  were  supposed  to  be  of 
detrital  origin,  but  in  age  earlier  than  the  Keweenawan. 
Laurentian,  as  used  by  most  of  the  earlier  Canadian  geologists,  covers 
the  most  of  the  light  colored  granites  and  coarse  grained  gneisses.  It 
was  recoguized  that  these  rocks  are  cut  by  basic  eruptives.  This  usage 
was  followed  by  many  of  the  American  geologists  up  to  the  time  of 
Brooks,  who  excluded  a  large  part  of  the  granite  and  gneiss  from  the 
