LAKE   SUPERIOR  REGION.  345 
from  all  varieties  of  the  basic,  intermediate,  and  acidic  rocks.  The 
ease  of  recognizing  the  fragments  of  considerable  size  has  led  to  a 
closer  study  of  the  conglomerates  than  of  the  sandstones  and  shales. 
However,  the  detritus  of  the  conglomerates  and  sandstones  is 
largely,  or  even  dominantly,  from  the  acidic  group  of  lavas,  felsites, 
porphyries,  and  granites,  and  often  also  from  the  intermediate  rock, 
augite  syenite,  even  when  the  sedimentary  beds  are  between  basic 
lavas. 
These  sediments  are  regarded  as  largely  terrestial  deposits. 
As  to  the  extent  of  the  sediments  interstratified  with  the  lavas, 
the  same  statements  may  be  made  as  with  reference  to  the  lavas — 
none  of  them  are  regional.  Indeed,  the  detrital  beds  between  the 
lavas  are  usually  extremely  local;  at  least,  as  yet,  they  have  for  the 
most  part  not  been  traced  any  great  distance.  In  proportion  as  these 
are  thick  they  naturally  have  a  greater  lateral  extent.  The  thickest 
of  these  formations,  the  great  conglomerate  of  Keweenaw  Point, 
which  has  a  maximum  thickness  of  2,300  feet,  has  been  traced  more 
than  100  miles,  and  one  of  the  comparatively  thin  conglomerates 
lying  immediately  under  the  greenstone  of  Keweenaw  Point  has 
been  traced  50  miles.  A  given  conglomerate  bed  may  vary  greatly 
along  the  strike  in  the  proportion  of  the  constituents  from  a  given 
source;  it  frequently  varies  also  in  thickness  and  coarseness.  When 
conglomerate  beds  thin  they  often  run  laterally  into  sandstones  or 
shales,  the  coarser  fragments  failing  altogether.  Finally,  a  single 
sedimentary  bed  along  the  strike  may  be  split  into  more  than  one 
bed  by  interleaved  lavas. 
As  to  the  order  of  events  for  the  volcanic  epoch,  it  is  to  be  strongly 
emphasized  that  no  part  of  it  was  a  time  exclusively  of  igneous  or  of 
sedimentary  action.  While  water-deposited  sediments  are  thus  char- 
acteristic of  the  entire  middle  Keweenawan,  it  is  to  be  said  that  their 
mass  is  extremely  subordinate  for  all  except  the  upper  part  of  this 
series.  Thus  in  Minnesota,  where  the  lower  and  middle  portion  of 
the  middle  division  is  exposed,  Lawson  estimates  the  sediments  to  be 
less  than  one-half  of  1  per  cent.  On  Keweenaw  Point  also,  at  the 
Eagle  River  and  Portage  Lake  sections,  in  the  major  portions  of  the 
sections  sediments  are  extremely  subordinate.  As  the  upper  part  of 
the  division  is  reached,  however,  there  is  a  change  in  the  importance 
of  the  conglomerates.  The  interstratified  beds  of  sandstone  and  con- 
glomerate are  rather  numerous.  Commonly  they  are  of  moderate 
thickness,  but  at  the  upper  part  of  the  series  these  conglomerates 
occasionally  are  of  sufficient  thickness  to  become  important  forma- 
tions. By  far  the  thickest  of  these  sedimentary  beds  is  the  "  Great 
conglomerate,"  which  has  a  maximum  thickness  of  more  than  2,000 
feet.  Another  important  sedimentary  bed  of  this  later  time  is  the 
so-called  middle  conglomerate  of  Keweenaw  Point.  After  the  vol- 
canic period  had  been  inaugurated  and  considerable  masses  of  the 
