82  PEE-CAMBEIAN    GEOLOGY   OF    NORTH   AMERICA. 
crust  they  are  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  are  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  that  form  the  core  or  axis  of 
the  disturbed  region.  With  these  slaty  rocks  are  also  associated  lime- 
stone masses,  which,  so  far  as  our  observations  go,  are  not  ordinarily 
interstratified  with  the  slates,  but  are  of  the  nature  of  segregated 
deposits,  having  been  formed  posterior  to  the  formation  of  the  sedi- 
mentary beds  with  which  they  are  associated,  while  the  metamor- 
phosing agencies  were  at  work  making  over  the  beds  into  the  crystal- 
line form  in  which  we  now  see  them. 
In  the  division  of  the  rocks  into  Laurentian,  Huronian,  Norian, 
Montalban,  Taconian,  and  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  No- 
rian ;  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  Lau- 
rentian or  Norian,  which  has  been  separated  from  the  Lower  Lauren- 
tian by  the  predominance  of  plagioclase  feldspar.  These  rocks  occur 
in  detached  areas  in  the  Laurentian  districts  and  are  similar  to  the 
gabbro  and  gabbro-diorites  of  Scandinavia.  At  least  nine  areas  are 
known  to  exist  in  Canada,  and  one  in  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 
