vanhise.]  DISCUSSIONS    OF    PRINCIPLES.  457 
geological  time.  Since  these  eruptive  materials  conic  from  a  gradually 
increasing  depth  below  the  surface  of  the  original  crust  they  arc  more 
basic  than  this,  and,  since  as  a  rule  they  contain  more  iron  than  that 
crust,  are  darker  colored  than  the  masses  by  which  they  arc  directly 
underlain.  Hence  the  detrital  beds  formed  from  the  debris  of  these 
more  basic  materials  are  themselves  of  a  dark  color,  and  as  a  result  of 
their  metamorphism  we  have  the  various  slates,  argillaceous,  talcose, 
and  chloritic,  which  so  commonly  rest  upon  the  granitic  and  gneissoid 
rocks  which  form  the  core  or  axis  of  the  disturbed  region.  With  these 
slaty  rocks  are  also  associated  limestone  masses,  which,  so  far  as  our 
observations  go,  are  not  ordinarily  inter  stratified  with  the  slates,  but 
are  of  the  nature  of  segregated  deposits,  having  been  formed  posterior 
to  the  formation  of  the  sedimentary  beds  with  which  they  are  associ- 
ated, while  the  metamorphosing  agencies  were  at  work  making  over 
the  beds  into  the  crystalline  form  in  which  we  now  see  them. 
In  the  division  of  the  rocks  into  Laurentian,  Huronian,  dorian, 
Montalban,  Taconian,  Arvonian,  only  lithological  principles  are  now 
used,  and  every  fact  pertaining  to  the  origin  and  relations  of  these 
rocks  is  ignored;  and  since,  while  it  is  assumed  that  all  these  rocks 
are  sedimentary,  they  are  found  to  occur  in  dikes  and  other  eruptive 
forms,  it  becomes  necessary  to  hold  that  all  eruptive  (including  vol- 
canic) rocks  were  the  products  of  a  metamorphic  (aqueo-igneous) 
action.  Hence  it  is  claimed  that  all  these  rocks  had  been  deeply 
buried  and  then  denuded,  and  most  extravagant  views  have  become 
current  regarding  denudation. 
It  thus  came  about  that  the  coarser  grained  granitoid  and  gneissic 
rocks  were  set  apart  as  Laurentian,;  the  gabbros  and  some  of  the  more 
coarsely  crystalline  diabases  and  diorites  were  erected  into  the  Norian ; 
the  felsites  and  quartz-porphyries  were  placed  as  the  Arvonian;  the 
finer  grained  diorites,  diabases,  melaphyres,  and  chlorite-schists  were 
formed  into  the  Huronian;  the  more  friable  granitic  and  gneissic  rocks 
with  the  mica-schists  were  classed  as  Montalban,  and  the  quartzites, 
limestones,  and  argillites  were  united  into  the  Taconian.  Of  course,  in 
each  case  the  metamorphic  fragmental  forms  of  each  rock  were  placed 
with  the  rocks  they  resembled,  while  the  other  forms  of  crystalline 
rocks  were  distributed  through  the  groups. 
Adams,16  in  1887,  gives  a  general  consideration  of  the  Upper  Lauren- 
tian or  Korian,  which  has  been  separated  from  the  Lower  Laurentian 
by  the  predominance  of  plagioclase  feldspar.  These  rocks  occur  in 
detached  areas  in  the  Laurentian  districts  and  are  similar  to  the  gab- 
bro  and  gabbro-diorites  of  Scandinavia.  At  least  nine  areas  are  known 
to  exist  in  Canada,  and  one  in  the  state  of  New  York.  Besides  pyroxene 
and  plagioclase,  many  other  minerals  are  found. 
The  rocks  show  much  variation  in  structure.  They  are  rarely  quite 
massive,  frequently  well  foliated,  but  usually  consist  of  a  rather 
coarsely  crystalline   groundmass  through  which  are  scattered  irregular 
