GENERAL   ACCOUNT    OF   THE   PRE-CAMBRIAN    ROCKS.  59 
in  Canada  they  found  pre-Cambrian  rocks,  which  they  mapped  in 
detail.  The  two  areas  in  which  this  work  was  begun  were  the  north 
shore  of  Lake  Huron  and  the  Laurentide  Mountains.  With  scientific 
spirit  they  applied  to  the  rocks  of  these  areas  no  terms  which  involved 
any  theory  of  origin  or  equivalence,  but  gave  the  rocks  the  names  of 
the  localities,  in  this  following  one  of  the  fundamental  principles  of 
good  structural  work.  Having  no  fossils  for  guides,  they  built  up  a 
succession  on  the  north  shore  of  Lake  Huron  by  following  formations 
in  continuous  exposure,  by  lithological  likenesses  of  exposures  sepa- 
rated by  short  intervals,  by  a  like  order  of  formations  in  different 
localities,  and  by  the  use  of  an  unconformity,  which  was  held  to  occur 
between  the  Huronian  sediments  and  the  underlying  crystalline  rocks. 
In  Logan's  work  on  the  "  Laurentian  "  the  same  methods  were  used 
as  far  as  practicable,  but  on  account  of  the  complicated  structure  of 
the  region  his  success  was  here  much  less  conspicuous.  The  difficulty 
of  the  district  drove  Logan  to  take  the  one  characteristic  formation, 
the  limestones,  as  horizons  to  follow  and  to  serve  as  planes  of  refer- 
ence in  working  out  the  structure.  But  even  this  guide  was  not  a  cer- 
tain one,  as  Logan  never  became  quite  sure  as  to  the  number  of  lime- 
stones present.  As  the  study  of  the  Laurentides  continued  the  rocks 
were  divided  into  two  divisions,  a  "Lower  Laurentian'1  free  from 
limestone  and  an  "Upper  Laurentian"  containing  the  limestones. 
The  two  were  held  by  Vennor,  and  by  Selwyn  for  a  time,  to  be  un- 
conformable. As  the  area  studied  in  the  Laurentide  Mountains 
widened,  a  new  formation  was  found,  a  laminated  gabbro.  It  was 
recognized  as  being  largely  composed  of  labradorite  or  anorthite  and 
so  was  first  called  "  Anorthosite  "  or  "  Labradorian,"  and  afterward 
"  Norian."  The  contacts  of  this  formation  with  the  other  formations 
of  the  "  Laurentian  "  were  recognized  as  not  those  of  conformity.  In 
these  early  days  it  was  naturally  supposed  that  all  laminated  rocks, 
whatever  their  character,  were  sedimentary,  and  as  in  certain  places 
the  "Labradorian"  appeared  to  cut  across  or  overlap  the  old 
"  Laurentian  "  it  was  designated  "  Upper  Laurentian,"  and  what  had 
before  been  called  "Upper  Laurentian"  was  designated  "Middle 
Laurentian."  When  the  eruptive  character  of  the  "  Labradorian  " 
was  shown,  the  Canadian  Survey  returned  to  the  first  uses  of  the 
terms  "Upper  Laurentian"  and  "Lower  Laurentian." 
In  comparing  the  "Huronian"  and  "Laurentian"  it  appears  that 
the  principle  used  in  reaching  the  conclusion  that  the  original  "  Upper 
Laurentian,"  separated  by  a  great  distance  from  the  original  Hu- 
ronian and  nowhere  in  contact  with  it,  is  the  older,  was  the  meta- 
morphic  character  of  the  former  as  compared  with  the  latter,  which 
in  the  early  work  of  Logan  and  Murray  was  called  n  nonmetamorphic 
series.  The  lithological  likeness  of  the  gneisses  and  granites  of  the 
original   "Lower  Laurentian"  to   the  granites   and    gneisses   called 
