van  hise.]  DISCUSSIONS   OF   PRINCIPLES.  471 
places  iii  Canada  pre-Cambrain  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  ex- 
posure, by  lithological  likenesses  of  exposures  separated  by  short  in- 
tervals, 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  upon  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  lime- 
stones as  horizons  to  follow  and  to  serve  as  planes  of  reference  in  work- 
ing out  the  structure.  But  even  this  guide  was  not  a  certain  one,  as 
Logan  never  became  quite  sure  as  to  the  number  of  limestones  present. 
As  the  study  of  the  Laurentides  continued  the  rocks  were  divided  into 
two  divisions,  a  Lower  Laurentian  free  from  limestone  and  an  Upper 
Laurentian  containing  the  limestones.  The  two  were  held  by  Veimor, 
and  by  Selwyn  for  a  time,  to  be  unconformable.  As  the  area  studied  in 
the  Laurentide  mountains  widened,  anew  formation  was  found,  a  lami- 
nated gabbro.  It  was  recognized  as  being  largely  composed  of  labra do- 
rite  or  anorthite  and  so  was  first  called  Anorthosite  or  Labradorian,  and 
afterwards  Norian.  The  contacts  of  this  formation  with  the  other 
formations  of  the  Laurentian  were  recognized  as  not  those  of  con- 
formity. In  these  early  days  it  was  naturally  supposed  that  all  lami- 
nated rocks,  whatever  their  character,  were  sedimentary,  and  as  in  cer- 
tain places  the  Labradorian  appeared  to  cut  across  or  overlap  the  old 
Laurentian  it  was  designated  as  Upper  Laurentian,  and  what  had  be- 
fore been  called  Upper  Laurentian  was  designated  Middle  Laurentian. 
When  the  eruptive  character  of  the  Labradorian  was  shown,  the  Cana- 
dian Survey  returned  to  the  first  uses  of  the  terms  Upper  Laurentian 
and  Lower  Laurentian. 
In  comparing  the  Huronian  and  Laurentian,  it  appears  that  the  prin- 
ciple used  in  reaching  the  conclusion  that  the  original  Upper  Lauren- 
tian, separated  by  a  great  distance  from  the  original  Huronian  and  no- 
where in  contact  with  it,  is  the  older,  was  the  metamorphic  character 
of  the  former  as  compared  with  the  latter  which  in  the  early  work  of 
Logan  and  Murray  was  called  a  non metamorphic  series.  The  lithologi- 
cal likeness  of  the  gneisses  and  granites  of  the  original  Lower  Lauren- 
tian to  the  granites  and  gneisses  called  Lower  Laurentian  un conforma- 
bly underlying  the  Huronian  doubtless  was  the  reason  for  placing 
these  as  equivalents. 
