456  PRE-CAMBRIAN    ROOKS    OF    NORTH    AMERICA.  [bull.  86. 
so  that  in  a  mixed  series  of  eruptive  and  detrital  rocks  nothing  is  more 
common  than  to  have  the  debris  of  one  inclosed  in  another,  without 
that  inclosure  proving  that  the  rocks  differ  in  geological  age.  This  is 
well  known  to  be  the  case  with  the  copper-bearing  rocks  of  Keweenaw 
point,  and  it  has  been  shown  that  the  iron  ores  of  the  Marquette  dis- 
trict, which  form  a  constituent  part  of  the  so-called  Huronian,  are  over- 
lain by  a  conglomerate  containing  the  debris  of  the  former ;  yet  both 
are  by  every  geologist  placed  in  the  same  series. 
The  basis  of  fact  which  forms  the  main  support  of  the  twofold  divi- 
sion of  the  Archean — including  under  that  designation  all  rocks  lying 
below  the  lowest  fossiliferous  series — is  this:  the  axial  or  eruptive 
portions  of  disturbed  and  mountain  regions  are  largely  granitic  and 
gneissoid  in  character.  These  granitic,  granitoid,  and  gneissic  masses 
are  brought  to  light  in  the  cores  of  great  mountain  chains,  where  long- 
continued  uplift  of  the  original  crust  of  the  earth  has  through  a  suc- 
cession of  geological  ages  been  furnishing  the  material  from  which  the 
sedimentary  formations  were  built  up.  That  the  gneissic  or  gneissoid 
rocks  are  closely  allied  to  the  distinctly  granitic  and  not  necessarily 
metamorphosed  stratified  deposits  is  clear,  as  the  result  of  long  con- 
tinued invescigations  in  regions  where  rocks  of  this  kind  occur.  Not 
that  all  gneisses  are  of  this  character;  but  those  are  ordinarily  so 
which  with  granite  make  up  the  axial  masses  of  disturbed  regions. 
That  the  parallel  structure  of  the  materials  forming  gneiss  is  not  neces- 
sarily the  result  of  sedimentation  seems  clearly  to  result  from  that 
which  has  been  done  both  in  experimental  and  field  geology  within 
the  last  few  years.  It  can  not  be  denied  that  a  foliated  arrangement 
or  a  parallel  disposition  of  the  mineral  elements  of  various  sedimentary 
rocks  can  be,  and  often  has  been,  induced  in  them  after  their  deposition, 
and  that  this  parallel  arrangement  is  not  by  any  means  necessarily 
coincident  with  the  planes  of  stratification.  This  fact  alone  is  absolutely 
conclusive  in  favor  of  the  idea  that  parallel  arrangement  of  the  mineral 
constituents  of  a  rock — in  other  words,  a  gneissic  structure,  in  rocks 
of  the  granite  family — is  not  proof  of  sedimentation. 
Overlying  the  granitic  and  gneissic  axial  rocks  we  are  likely  to  find, 
and  in  many  cases  do  find,  the  stratified  masses  which  were  formed 
from  the  preexisting  crust  themselves  usually  highly  metamorphosed, 
because  formed  at  a  period  of  great  chemical  and  mechanical  activity. 
With  these  stratified  and  highly  altered  masses  are  associated  erup- 
tive materials — both  interbedded  and  injected  in  dike  form — these  also 
often  greatly  metamorphosed,  and  to  such  an  extent  that  their  original 
character  is  only  with  difficulty,  and  with  the  aid  of  the  microscope,  to 
be  recognized.  This  protrusion  or  forcing  out  of  eruptive  materials 
seems  to  have  followed  the  preceding  uplift  of  the  original  crust,  if  not 
as  a  necessity  at  least  as  something  extremely  likely  to  occur,  as  is 
shown  by  the  fact  that  in  so  many  great  mountain  chains  we  find  vol- 
canic activity  more  and   more  predominating   with  the  progress  of 
