vanhise.]  .  DISCUSSIONS    OF    PRINCIPLES.  453 
New  England  and  in  the  adjacent  provinces  of  Canada,  where  these 
rocks  are  chiefly  developed,  renders  almost  impossible,  or  at  any  rate 
has  never  yet  been  attempted.  Hence  the  maps  hitherto  published 
representing  the  geological  structure  of  these  regions,  have  necessarily 
been  based  almost  entirely,  so  far  as  the  crystalline  groups  are  con- 
cerned, on  lithological  and  mineralogical  comparisons  and  considerations, 
producing  petrological  rather  than  geological  maps,  and  as  a  conse- 
quence, though  important  and  valuable  aids  to  future  investigation, 
they  afford  a  very  incorrect  and  imperfect  idea  of  the  true  geological 
structure  and  the  sequence  and  distribution  of  the  several  formations. 
Unfortunately,  while  careful,  patient,  and  minute  observation  in  the 
field  has  been  unavoidably  limited  and  local,  stduy  in  the  laboratory 
and  theoretical  deductions  therefrom  have  been  unlimited  and  wide- 
spread, but,  as  might  have  been  expected,  have  not  only  afforded  no 
satisfactory  solution  of  the  intricacies  of  Appalachian  geological  struc- 
ture, but  have  on  the  contrary  involved  it  in  deeper  mystery  and  com- 
plication. 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  successive  observers  in  this  difficult  field. 
Selwyn,14  in  1884,  states  that  recent  investigation  has  greatly  en- 
larged the  area  over  which  the  Archean  rocks  are  known  to  extend, 
though  it  has  not  yet  afforded  any  more  satisfactory  evidence  of  the  re- 
lations of  the  Huronian  rocks  to  the  Laurentian.  In  all  cases  the  sup- 
posed junction  of  the  strata  of  the  two  systems  either  shows  them  ver- 
tically side  by  side  or  the  Huronian  strata  apparently  dipping  under 
the  Laurentian,  while  both  present  a  very  constant  northeasterly  strike. 
Notwithstanding  these  facts,  their  exceedingly  different  mineralogical 
characters  and  general  appearance,  broadly  viewed,  render  it  almost 
impossible  to  suppose  that  the  superposition,  as  indicated  by  these  dips, 
is  the  true  one,  or  that  the  Huronian  is  not  newer  than  the  Laurentian. 
If  so,  then  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 
writer  has  no  hesitation  in  asserting  that  it  has,  as  such,  no  existence 
in  Canada,  its  theoretical  birthplace.  Wherever  these  Norian  rocks 
have  been  observed  they  are  either  intimately  and  conformably  associ- 
ated 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  due  to  vol- 
canic 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  were  locally  interbedded 
