SUMMARY   OF    GENERAL   LITERATURE.  81 
Marquette  district,  which  form  a  constituent  part  of  the  so-called 
Huronian,  are  overlain  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-continued  investigations  in  regions  where  Pocks  of  this  kind 
occur.  Not  that  all  gneisses  are  of  this  character ;  but  those  are  ordi- 
narily so  which  with  granite  make  up  the  axial  masses  of  disturbed 
regions.  That  the  parallel  structure  of  the  materials  forming  gneiss 
is  not  necessarily  the  result  of  sedimentation  seems  clearly  to  result 
from  that  which  has  been  done  in  both  experimental  and  field  geology 
within  the  last  few  years.  It  can  not  be  denied  that  a  foliated  ar- 
rangement 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  meta- 
morphosed, because  formed  at  a  period  of  great  chemical  and  mechan- 
ical activity.  With  these  stratified  and  highly  altered  masses  are 
associated  eruptive  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  volcanic  activity  more  and  more  predominating  with 
the  progress  of  geological  time.  Since  these  eruptive  materials  come 
from  a  gradually  increasing  depth  below  the  surface  of  the  original 
55721— Bull.  3G0— 00 6 
