436  PKE-C AMEBIAN    GEOLOGY   OF    NORTH    AMERICA. 
Great  indisputable  results  were  reached  by  the  early  Canadian 
geologists,  Logan  and  Murray.  This  district  north  of  Lake  Huron 
was  the  first  in  which  it  was  shown  that  an  unmistakable  detrital  and 
little  metamorphosed  series  of  rocks  rests  unconformably  under  the 
upper  Cambrian.  Also  it  was  shown  that  this  series  is  of  such  a  char- 
acter that  the  ordinary  stratigraphic  methods  apply,  and  Logan  and 
Murray  were  able  to  subdivide  it  into  formations  upon  a  lithological 
basis  in  the  same  manner  as  is  done  in  fossiliferous  series.  This  is  so 
evident  that  it  would  not  be  emphasized  if  it  had  not  been  denied. 
Far  more  to  the  credit  of  Logan  and  Murray  is  the  recognition  of  the 
character  of  the  amygdaloids  and  the  interbedded  greenstones.  No 
extreme  metamorphic  theory  was  applied  to  them,  and  they  were  dis- 
tinctly regarded  as  an  exception  to  the  ordinary  stratigraphic  laws 
and  were  separated  both  in  descriptions  and  in  mapping.  This  is  the 
more  creditable  because  for  many  years  afterward  similar  rocks  were 
supposed  by  many  other  writers  to  be  parts  of  the  stratified  succes- 
sions in  a  completely  metamorphosed  condition  that  had  caused  them 
to  become  crystalline.  Finally  it  was  recognized  that  this  Huronian 
series  rests  unconformably  upon  an  older  gneissic  and  granitic  crystal- 
line series,  which  has  yielded  abundant  fragments  to  the  overlying 
rocks. 
The  only  rocks  in  the  original  TTuronian  area  which  Logan  cor- 
related with  those  of  the  original  Laurentian  area  are  the  unconform- 
able underlying  granitic  and  gneissic  series.  These  were  called 
"  Lower  Laurentian,"  the  idea  being  evidently  to  correlate  them  with 
the  lower  noncalcareous  division  of  the  original  Laurentian.  This 
correlation  was  plainly  made  on  the  ground  of  lithological  likeness. 
That  the  Huronian  is  more  recent  than  the  "  Upper  Laurentian  "  was 
stated  only  as  a  belief.  This  belief  appears  to  have  been  based  upon 
the  "  nonmetamorphic  "  character  of  the  Huronian  as  compared  with 
the  "  Upper  Laurentian."  It  is  also  possible  that  the  fact  that  there 
is  a  structural  break  on  the  north  shore  of  Lake  Huron  between  the 
Huronian  and  the  gneissic  series,  whereas  no  such  break  was  found 
between  the  upper  and  lower  divisions  of  the  original  Laurentian, 
had  an  influence  in  leading  to  this  conclusion. 
As  to  the  relations  of  the  "  Huronian  "  and  "  Laurentian  "  north  of 
Lake  Huron,  Murray,  in  1857,  made  the  distinction  to  rest  upon  age 
and  upon  lithological  characteristics,  the  "Laurentian  "  being  older 
and  more  completely  crystalline  than  the  "Huronian."  While  the 
"  Huronian  "  and  "  Laurentian  "  of  Logan  and  Murray  are  not  de- 
scribed at  any  definite  locality  as  having  unconformable  relations, 
Logan  states  that  the  "  Huronian  "  is  a  stratified  series  and  reposes 
discordantly  upon  the  "Laurentian  system,"  and  in  1858  he  again 
clearly  indicates  the  same  thing  by  the  statement  that  in  the  slate  con- 
