LAKE    SUPERIOR   REGION.  343 
feet  thick.  These  beds  of  sediments  are,  however,  important  as  indi- 
cating that  very  generally  for  the  Keweenawan  the  first  deposits  of 
the  period  were  sedimentary  and  antedated  the  outbreak  of  regional 
igneous  activity. 
MIDDLE   KEWEENAWAN. 
The  middle  Keweenawan  is  the  great  epoch  of  combined  igneous 
and  aqueous  activities.  Thus  there  are  two  divisions  of  rocks — orig- 
inal igneous  and  derived  sedimentary. 
Igneous  rocks. — The  igneous  rocks  constitute  a  province  of  rather 
remarkable  uniformity.  The  different  kinds  of  igneous  rocks  and 
their  relations  are  substantially  the  same  in  each  of  the  important 
districts.  From  a  chemical  point  of  view  the  igneous  rocks  include 
basic,  acidic,  and  intermediate  varieties.  However,  the  basic  ma- 
terials overwhelmingly  dominate.  The  acidic  rocks  are  important  in 
quantity,  and  the  intermediate  rocks  are  subordinate.  As  to  structure, 
each  variety  of  rocks  includes  both  intrusive  and  extrusive  facies,  so 
that  the  basic,  acidic,  and  intermediate  groups  each  have  textures 
characteristic  for  plutonic  and  volcanic  rocks. 
The  basic  plutonic  igneous  rocks  comprise  gabbros,  both  olivinitic 
and  nonolivinitic,  diabases,  olivinitic  and  nonolivinitic,  and  anortho- 
site.  The  surface  varieties  include  melaphyres,  porphyrites,  and 
amygdaloids.  The  coarse-grained  melaphyres  have  often  been  called 
dolerites,  diabases,  or  ophites,  depending  upon  their  structure.  The 
deep-seated  phase  of  the  acidic  rocks  is  granite,  augitic  or  horn- 
blendic,  and  the  extrusive  phase  is  porphyry,  quartziferous  and  non- 
quartziferous,  and  felsite.  The  most  important  intrusive  phases  of 
the  intermediate  rocks  are  described  by  Irving  as  augite  syenites  and 
orthoclase  gabbros,  and  the  extrusive  varieties  as  porphyrites.  Trap 
is  used  in  its  usual  sense  to  cover  both  basic  and  intermediate  vari- 
eties of  rock. 
The  extrusive  rocks  are  almost  altogether  a  mass  of  lava  beds,  one 
piled  upon  another,  the  volcanic  elastics  being  insignificant  in  quan- 
tity. The  total  volume  of  the  extrusives  is  vast,  but  probably  not  so 
great  as  the  volume  of  the  intrusive  rocks.  The  textures  and  struc- 
tures exhibited  by  these  ancient  lavas  are  in  all  respects  like  those  of 
modern  lavas,  although  of  course  they  have  undergone  extensive 
metasomatic  changes.  The  basic  lavas  greatly  predominate.  The 
beds  vary  from  those  less  than  2  feet  in  thickness  to  those  100  feet 
or  more  in  thickness.  The  thin  flows  have  a  very  moderate  extent, 
and  even  the  thicker  flows  have  usually  not  been  traced  any  great 
distance,  although  it  is  said  of  certain  definite  flows  that  they  have 
been  identified  for  distances  varying  from  10  to  30  miles.  Groups 
of  lava  beds  have  been  traced  to  great  distances — up  to  a  maximum 
