366  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
quently  called  jaspers — and  the  amygdaloids  were  by  many  of  the 
earlier  authors  supposed  to  be  metamorphosed  sandstones.  This  posi- 
tion is,  we  believe,  for  the  acidic  rocks,  held  by  no  writer  at  the  pres- 
ent time,  with  perhaps  one  exception,  and  for  the  amygdaloids  by 
none.  The  work  of  Wadsworth,  Pumpelly,  and  Irving  has  demon- 
strated beyond  all  doubt  that  these  rocks  are  original  eruptives.  The 
Keweenawan  is  now  generally  recognized  as  a  series  many  thousands 
of  feet  thick,  consisting  of  interbedded  lava  flows  and  water-deposited 
detrital  material,  derived  chiefly  from  the  contemporaneous  igneous 
rocks.  The  volcanics  are  predominant  in  the  lower  part  of  the  series, 
the  interstratifications  of  the  two  are  most  frequent  in  the  middle  por- 
tion, and  the  upper  part  of  the  series  is  free  from  volcanics. 
The  last  point  to  be  considered  in  this  connection  is  the  reality  of 
the  existence  and  the  position  of  the  so-called  crowning  overflow  of 
the  northwest  shore.  This  was  described  by  Logan,  Bell,  Selwyn, 
and  others;  some  were  inclined  to  place  it  with  the  Animikie  and 
others  with  the  Keweenawan.  Irving,  in  his  general  treatise  on  the 
copper-bearing  rocks,  does  not  recognize  this  overflow  as  a  general 
formation,  but  places  the  more  important  flows  to  which  this  term 
has  been  applied  at  the  base  of  the  Keweenawan.  Later  work  in 
northeastern  Minnesota  shows  that  at  the  base  of  the  Keweenawan 
is  a  great  mass  of  gabbro,  the  thickness  and  magnitude  of  which  are 
incomparably  greater  than  the  so-called  crowning  overflow  of  Thun- 
der Bay.  This  gabbro  extends  from  Duluth  northeastward  to  the 
National  boundary,  a  distance  of  100  miles  or  more,  and  is  at  its 
maximum  outcrop  more  than  20  miles  in  width.  It  lies  near  the  base 
of  the  Keweenawan,  for  it  comes  in  contact  now  with  one  member 
of  the  Animikie  and  now  with  another,  and  in  the  Mesabi  district 
with  Giants  Range  granite;  at  other  places  it  is  in  contact  with  the 
crystalline  schists  of  the  lower-middle  Huronian,  and  again  with  the 
granite  and  gneiss  of  the  Laurentian;  so  it  is  evident  that  if  this 
gabbro  is  a  part  of  the  regular  succession  all  these  rocks  were  deeply 
eroded  before  its  appearance.  It  was  long  a  matter  of  doubt  whether 
it  is  a  great  surface  flow  or  succession  of  flows,  or,  as  suggested  by 
Irving,  an  immense  reservoir  in  the  nature  of  an  early  laccolith  or 
batholith  which  furnished  material  for  the  subsequent  diabase  dikes 
and  sheets  of  the  Animikie  and  for  basic  surface  flows  of  the 
Keweenawan.  The  view  now  prevailing  is  that  the  gabbro  is  a  great 
intrusive  laccolith.  The  evidence,  first  stated  by  Grant,  is  princi- 
pally the  coarse  texture  of  the  mass,  its  shape,  and  its  profound 
metamorphic  effect  upon  the  adjacent  rocks. 
Van  Hise  and  Clements  found  that  the  diabase  of  Irving's  Beaver 
Bay  group  and  red  rock  are  not  surface  flows,  but  intrusives  which 
cut  the  lavas  of  the  Keweenawan. 
