1.62  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [hill.) 
corisin  geologists  corroborated  Brooks's  and  Pu'mpelly's  results.  Whe 
tbe  relations  of  the  series  on  the  north  shore  were  closely  examine 
actual  evidence  of  the  erosion  interval  was  found  by  Irving  at  Thuuch 
bay.  The  same  was  seen  by  McKellar,  and  upon  mapping  the  t\\ 
series  in  northeastern  Minnesota,  Irving  and  Merriam  found  that  tl 
same  discordance  which  was  found  on  the  south  shore  appeared,  that 
to  say,  the  base  of  the  Keweenawan  is  now  in  contact  with  one  membt 
of  the  underlying  series  and  now  with  another.  Selwyn  finds  at  Thin 
der  bay  no  evidence  of  this  erosion  interval,  but  this  testimony  is  nej 
ative  and  stands  alone.  The  great  mass  of  evidence  from  many  local 
ties  agrees  that  there  is%  physical  break  at  the  base  of  the  Kewei 
nawan. 
The  last  point  to  consider  in  this  connection  is  the  reality  of  the  e: 
istence  and  the  position  of  the  so-called  crowning  overflow  of  the  nortl 
west  shore.  This  was  described  by  Logan,  Bell,  Selwyn  and  other; 
and  some  were  inclined  to  place  it  with  the  Animikie  and  others  wit 
the  Keweenawan.  Irving,  in  his  general. treatise  on  the  copper-bearin 
rocks,  does  not  recognize  this  overflow  as  a  general  formation,  but  placj 
the  more  important  flows  to  which  this  term  has  been  applied  at  the  has 
of  the  Keweenawan.  Later  work  in  northeastern  Minnesota  shows  ths 
at  the  base  of  the  Keweenawan  is  a  great  area  of  gabbro,  the  thicknee 
and  magnitude  of  which  is  incomparably  greater  than  the  so-called  crowi 
ing  overflow  of  Thunder  bay.  This  great  mass  of  gabbro  extends  IVoi 
Duluth  northeast  to  the  National  boundary,  a  distance  of  100  miles  c 
more,  and  is  at  its  maximum  outcrop  more  than  20  miles  in  widtl 
There  is  no  question,  unless  it  be  considered  a  subsequent  intrusiJ 
that  this  great  gabbro  is  the  base  of  the  Keweenawan,  for  it  now  come 
in  contact  with  one  member  of  the  Animikie  and  now  with  anothi 
At  other  times  it  is  in  contact  with  the  crystalline  schists  of  the  Lowe 
Huronian,  and  again  with  the  granite  and  gneiss  of  the  Laurent i an,  s 
that  it  is  evident,  if  it  is  a  part  of  the  regular  succession,  that  all  i 
these  rocks  have  been  deeply  eroded  before  its  appearance.  This  gat 
bro  has  been,  however,  too  little  studied  to  venture  an  opinion  as  t 
whether  it  is  a  great  surface  flow  or  succession  of  flows,  or,  as  su£ 
gested  by  Bayley,  an  immense  reservoir  in  the  nature  of  an  early  la| 
colite  or  batholite  which  furnished  material  for  the  subsequent  diabas 
dikes  and  sheets  of  the  Animikie  and  for  basic  surface  flows  of  th 
Keweenawan.  Recently  N.  H.  Winchell,  contrary  to  all  previous  worl 
places  this  gabbro  as  older  than  the  Animikie.  Little  evidence  is  give] 
to  support  this  change  of  view.  No  section  is  given  illustrating  the  snjj 
posed  structure.  To  the  writer  there  appears  to  be  great  if  not  insupei 
able  difficulties  in  the  way  of  the  correctness  of  this  conclusion. 
GENERAL   SUCCESSION   ACCORDING  TO   DIFFERENT   WRITERS. 
In  taking  thenext  step  downward,  we  conic  to  the  complex  about  whid 
there  has  been  great  diversity  of  opinion  and  about  which  it  is  difficul 
