vanhisbI  DISCUSSIONS    OF    PRINCIPLES.  483 
powerful  dynamic  action  tAvo  unconformable  series,  the  one  of  which  is 
composed  of  material  from  the  other  and  therefore  resembles  it  in  com- 
position, may  have  developed  conformable  secondary  structures  and 
gradations,  the  latter  consequent  upon  the  induced  crystalline  charac- 
ter of  the  clastic  series,  the  original  structures  being  simultaneously 
obliterated.  Also  recently  Pumpelly  has  ascertained  that  snbaerial  dis- 
integration of  the  earlier  series  is  an  important  assistance  in  the 
production  of  such  gradations.  In  certain  areas  it  has  been  demon- 
strated that  perfect  conformity  and  complete  gradation  exist  between 
series  separated  originally  by  wide  unconformities,  the  earlier  of  which 
was  probably  of  igneous  origin  while  the  later  was  certainly  sedi- 
mentary. 
Whatever  the  origin  of  the  fundamental  complex,  it  is  plain  that  the 
parts  of  any  given  area  of  it  are  not  all  of  the  same  age.  The  dikes 
which  everywhere  cut  it  are  the  pipes  through  which  have  passed  the 
later  eruptives.  At  the  time  of  the  intrusion  of  these  eruptives,  large 
lakes  of  liquid  material  may  have  formed  which  crystallized  as  bosses, 
causing  the  Archean  to  contain  considerable  masses  of  rocks  of  really 
later  age.  Where  these  rocks  are  predominant  the  material  must  be 
classified  as  a  later  eruptive ;  where  they  are  subordinate  to  the  Archean 
material  they  are  often  difficult  to  separate  from  it  although  really  later 
in  age.  Between  the  areas  which  rank  as  eruptives  of  later  age  and 
the  genuine  Archean,  there  are  doubtless  gradations.  Along  the  zone 
of  contact,  if  the  mass  of  later  eruptive  be  great,  there  might  be  an 
area  which  could  equally  well  be  placed  with  the  fundamental  complex 
or  with  the  later  eruptive.  Between  the  Archean  and  later  eruptives, 
as  between  the  Archean  and  undoubted  sedimentaries,  there  are  gra- 
dations. 
The  problem  of  the  relations  of  the  Archean  as  a  whole  to  the  over- 
lying elastics  is  the  same  as  that  within  the  Archean  itself.  The  finely 
laminated  crystalline  schists  and  gneisses,  and  the  granite-gneisses  and 
granites  with  which  they  are  associated,  have  contacts  in  every  respect 
analogous  to  those  occasionally  found  between  the  Archean  complex 
and  the  clastic  series.  For  example,  it  has  been  seen  that  the  rocks 
heretofore  called  Archean  on  the  north  shore  of  lake  Huron  comprise 
pwo  parts.  One  part  is  older  than  and  lies  uncon  form  ably  below,  yield- 
ng  fragments  to  the  original  Huronian.  The  other  part  has  relations 
vith  the  elastics  of  the  character  just  considered  with  transition  phe- 
lomena.  If  this  material  is  an  extraneous  intrusive  it  is  a  post- Archean 
eruptive.  If  in  situ  it  represents  a  portion  of  the  pre-Huronian  floor 
jompletely  metamorphosed  by  selective  metamorphism,  or  by  aqueo- 
gneous  fusion,  it  can  fairly,  according  to  the  first  and  second  school,  be 
jailed  a  part  of  the  Archean. 
It  is  plain  from  the  great  diversity  of  opinion  as  to  the  origin  of  the 
Archean  rocks,  and  fromtthe  fact  that  many  of  the  opinions  are  beliefs 
ather  than  verified  conclusions,  that  we  have  no  definite  knowledge 
