344  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
of  150  miles.  However,  no  group  of  lava  beds  has  been  traced 
through  the  entire  Lake  Superior  region. 
The  acidic  flows  differ  materially  from  the  basic  flows.  In  general 
they  appear  to  have  been  much  less  fluid  and  therefore  have  a  much 
shorter  lateral  extent  in  proportion  to  their  thickness.  In  fact,  a 
bunchy  or  lenticular  form  is  characteristic  of  them.  This  is  illus- 
trated by  Mounts  Houghton  and  Bohemia,  on  Keweenaw  Point. 
Amygdaloidal  textures  are  not  common  with  them,  as  in  the  basic 
lavas.  A  flowage  structure,  on  the  other  hand,  is  much  more  fre- 
quent in  them  than  in  the  basic  lavas,  and  glassy  textures  are  exceed- 
ingly common. 
Chemically  the  intrusives  include  the  basic,  acidic,  and  intermediate 
varieties  of  rocks.  Structurally  the  intrusive  masses  comprise  every 
known  form  of  that  class  of  rocks  with  the  exception  of  batholiths. 
There  are  great  laccoliths,  many  large  bosses,  numerous  and  extensive 
sills,  and  very  abundant  dikes,  from  those  of  small  size  to  those  hun- 
dreds of  feet  across.  Many  of  the  dikes  and  sills  beautifully  show  a 
columnar  structure.  In  many  of  the  earlier  studies  the  sills  were  not 
separated  from  the  lava  flows.  As  to  magnitude,  the  masses  vary 
from  the  "  St.  Louis  River  gabbro  "  of  Minnesota,  which  has  an  area 
of  2,400  square  miles,  to  emanations  so  small  as  to  be  lost  in  the  in- 
truded rocks.  It  appears  probable  that  the  volume  of  the  intrusives 
within  the  previously  formed  extrusive  lavas  and  conglomerates  is 
really  greater  than  the  volume  of  the  lavas  themselves. 
The  greatest  of  the  intrusions  of  late  Keweenawan  time  are  basic. 
These  are  represented  by  the  gabbro  laccoliths  of  Minnesota  and  Wis- 
consin. The  acidic  masses  are  also  sometimes  large,  but  they  are 
likely  to  occur  in  bosslike  forms. 
No  general  order  of  eruption  for  the  Keweenawan  lavas  can  be 
announced,  but  locally  an  order  has  been  determined.  Thus  near  the 
mouth  of  Montreal  River,  on  Keweenaw  Point,  Hubbard  finds  that 
melaphyres  have  first  been  ^extruded,  then  porphyrites,  then  felsites, 
and  following  these  the  great  period  of  basic  activity. 
Sedimentary  rocks. — The  sedimentary  rocks  are  dominantly  con- 
glomerates and  sandstones.  Shales  are  subordinate.  A  light-red  to 
dark-red  color  is  very  characteristic  for  the  Keweenawan  detritus 
interstratified  with  the  lava  beds.  Among  them  gray  sandstones  are 
unknown.  The  conglomerates  vary  in  coarseness  from  great  bowlder 
conglomerates  to  fine  conglomerates,  and  these  grade  into  the  sand- 
stones, and  the  latter  often  into  shales.  All  the  sediments  belonging 
to  the  division  under  consideration  are  interstratified  with  the  lava 
beds.  The  detritus  of  the  sandstones  and  conglomerates  is  dominantly 
derived  from  the  Keweenawan  igneous  rocks  themselves. 
The  detritus,  comprising  bowlders,  pebbles,  and  grains  of  sand, 
being  derived  from  the  igneous  rocks  of  the  group,  includes  materials 
