278  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
are  numerous  places  where  Keweenian  strata  are  capped  by  thick 
sheets  of  trap,  identical  with  those  which  cap  the  Animikie,  but, 
though  these  sheets  can  not  be  traced  in  absolute  continuity  in  the 
interval,  there  are  many  outlying  patches  which  fill  the  gap.  The 
same  trap  sheets  are  found  in  several  instances  to  pass  from  the 
Animikie  to  the  Keweenian,  and  there  are  the  same  evidences  of  in- 
trusion of  independent  trap  sheets  in  the  Keweenian  that  there  are  in 
the  Animikie.  These  rocks  are,  therefore,  of  post-Keweenian  age, 
and,  to  discriminate  them  from  the  Keweenian  and  Animikie,  they 
are  designated  the  Logan  sills. 
Lawson,260  in  1894,  describes  a  multiple  diabase  dike  near  the 
mouth  of  White  Gravel  River,  on  the  northeast  coast  of  Lake  Su- 
perior, where  occur  in  a  breadth  of  14  feet  no  fewer  than  twenty- 
eight  vertically  intrusive  sheets  of  diabase,  ranging  in  thickness  from 
1  inch  to  6J  inches,  separated  by  twenty-seven  sheets  of  granite,  rang- 
ing in  thickness  from  one- fourth  inch  to  8  inches.  The  dikes  anasto- 
mose and  are  connected  at  various  places,  showing  that  they  are  due 
to  a  single  intrusion.  The  granite  is  seemingly  homogeneous,  there 
being  no  differentiation  of  structure  or  of  mineral  composition.  It  is 
believed  that  the  splitting  of  the  granite  was  due  directly  to  the  in- 
vasion of  the  diabase  magma.  This  occurrence  is  comparable  to  the 
complex  parallel  invasion  of  the  schistose  rocks  of  the  Ontarian 
system  by  granite. 
Grant,261  in  1894,  describes  the  lowest  beds  of  Grand  Portage 
Island,  north  coast  of  Lake  Superior,  as  consisting  .of  arenaceous 
slates,  sandstones,  and  conglomerates,  the  fragments  of  the  latter  being 
quartz,  quartzite,  siliceous  slate,  a  dark  flinty  rock,  red  quartz  por- 
phyry, and  red  granite.  These  are  in  part  clearly  waterworn.  All 
of  the  pebbles  of  the  conglomerate  can  be  matched  in  the  Animikie 
strata  near  by.  These  beds  are  regarded  as  the  lowest  of  the  Ke- 
weenawan in  this  locality,  and  the  material  in  the  conglomerate  shows 
that  the  Animikie  elastics  had  been  subjected  to  metamorphosing 
forces  before  Keweenawan  time  and,  as  agreed  by  all  Lake  Superior 
geologists,  that  there  was  an  erosion  interval  between  the  two.  As  the 
red  quartz  porphyry  and  the  granite  have  been  shown  to  be  intrusive 
in  the  Animikie,  and  also  in  the  gabbro  and  diabase  of  Pigeon  Point 
and  Grand  Portage,  it  is  concluded  that  these  intrusions  occurred  at  a 
date  earlier  than  the  Keweenawan. 
Coleman,262  in  1895,  gives  a  summary  of  the  geology  of  the  Rainy 
Lake  region.     Following  Lawson,  he  classifies  the  rocks  as  follows: 
Archean • 
u 
r  Tipper  division,  f  a.     Keewatin    (Hnronian?) 
Coutchiching. 
Lower  division.  Lanrentian. 
