78  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
unavoidably  limited  and  local,  study  in  the  laboratory  and  theoretical 
deductions  therefrom  have  been  unlimited  and  widespread,  but,  as 
might  have  been  expected,  have  not  only  afforded  no  satisfactory  solu- 
tion of  the  intricacies  of  Appalachian  geological  structure,  but  have, 
on  the  contrary,  involved  it  in  deeper  mystery  and  complication.  It 
is  now  evident  that  an  entirely  different  system  of  procedure  must  be 
adopted  before  there  will  be  any  hope  of  definitely  and  satisfactorily 
solving  the  problems  which  have  been  presenting  themselves  to  suc- 
cessive observers  in  this  difficult  field. 
Selwvn  and  Dawson,14  in  1884,  state  that  recent  investigation  has 
greatly  enlarged  the  area  over  which  the  Archean  rocks  are  known 
to  extend,  though  it  has  not  yet  afforded  any  more  satisfactory  evi- 
dence of  the  relations  of  the  ITuronian  rocks  to  the  Laurentian.  In 
all  cases  the  supposed  junction  of  the  strata  of  the  two  systems 
either  shows  them  vertically  side  by  side  or  the  ITuronian  strata 
apparently  dipping  under  the  Laurentian,  while  both  present  a  very 
constant  northeasterly  strike.  Notwithstanding  these  facts,  their 
exceedingly  different  mineralogical  characters  and  general  appear- 
ance, broadly  viewed,  render  it  almost  impossible  to  suppose  that 
the  superposition,  as  indicated  by  these  dips,  is  the  true  one  or  that 
the  ITuronian  is  not  newer  than  the  Laurentian.  If  so,  we  must 
admit  that  both  systems  are  presented  in  a  constant  succession  of 
enormously  thick  overturned  folds,  with  perhaps  many  dislocations 
and  slips  on  the  lines  of  the  anticlinal  axes. 
As  regards  the  so-called  Norian  or  Upper  Laurentian  formation, 
the  writers  have  no  hesitation  in  asserting  that  it  has,  as  such,  no  ex- 
istence in  Canada,  its  theoretical  birthplace.  Wherever  these  Norian 
rocks  have  been  observed  either  they  are  intimately  and  conformably 
associated  with  the  ordinary  orthoclase  or  pyroxenic  gneisses  or 
they  occur  as  intrusive  masses,  when  they  present  no  gneissic  or 
bedded  structure.  They  clearly  cut  the  surrounding  gneiss,  and  are 
probably  clue  to  volcanic  or  other  igneous  agency  in  the  Laurentian 
age.  Such  masses  may  not  unreasonably  be  supposed  to  mark  the 
sites  of  the  Laurentian  volcanoes,  while  the  bedded  labradoritic 
gneisses  and  other  associated  strata  may  with  equal  probability 
represent  the  eruptive  rocks — lava  flows,  etc. — which  emanated  from 
them  and  wen1  locally  interbedded  with  the  ordinary  sediments  of 
the  period,  as  rocks  of  similar  origin  and  composition  certainly  were 
in  the  ITuronian  and  in  all  later  geological  ages,  a  fact  which  has 
been  singularly  overlooked  or  ignored  by  most  writers  on  American 
geology. 
At  present  we  have  in  Canada  no  evidence  which  would  warrant  us 
in  making  more  than  two  great  divisions  in  the  Archean  crystalline 
rocks.  In  many  parts,  especially  in  the  eastern  provinces,  it  has  been 
found  impossible  to  define  even  these  clearly.     Rocks  of  typical  Lau- 
