62  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Cambrian  time  an  invariable  succession.  This  theory  was  carried  to 
the  extreme  by  Hunt  and  his  school,  who  held  that  before  Cambrian 
time  there  are  six  rock  "  systems,"  which  are  universal  and  are  sep- 
arated by  unconformities.  These  are,  from  the  base  upward :  Lau- 
rentian,  Norian,  Arvonian,  Huronian,  Montalban,  Taconian.  Of 
these  terms  "  Norian  "  was  devised  to  include  the  laminated  gabbros, 
the  so-called  Upper  Laurentian  of  Logan.  "Arvonian "  was  im- 
ported from  Wales,  where  it  was  applied  by  Hicks  to  a  series  of 
acidic  volcanics.  "  Montalban  "  came  from  the  White  Mountain  re- 
gion in  New  Hampshire,  where  a  series  of  gneisses  was  thought  to 
be  of  different  lithological  character  from  the  "Laurentian"  and 
"  Huronian  "  and  to  overlie  them.  "  Taconian  "  was  introduced  by 
Ebenezer  Emmons  to  cover  a  series  of  fossiliferous  rocks  which  was 
supposed  to  be  earlier  than  the  base  of  the  "  Silurian." 
Besides  the  terms  given,  others  have  been  used  to  some  extent,  but 
they  are  of  little  importance.  Among  these  may  be  mentioned  "  Hy- 
pozoic,  "  "  Prozoic,  "  and  "  Pyrocrystalline." 
As  the  metamorphic  theory  gained  force  it  became  the  habit  of 
many  geologists  to  refer  to  old  crystalline  or  semicrystalline  rocks 
as  metamorphic,  assuming  that  they  are  all  produced  by  the  altera- 
tion of  sediments  of  some  kind.  This  went  so  far  as  to  include  among 
the  metamorphic  sediments  perfectly  massive  rocks,  such  as  diabases, 
gabbros,  granites,  etc.  Recently  the  term  has  also  been  applied 
to  rocks  recognized  as  laminated  eruptives,  but  this  is  not  the  use 
referred  to.  The  term  metamorphic  had  the  advantage  of  saying 
nothing  as  to  age  or  correlation,  but  in  escaping  this  difficulty  another 
theory  was  accepted  which,  so  far  as  its  assumption  is  concerned,  was 
quite  as  bad. 
In  many  cases  local  names  were  applied  to  formations  or  groups 
in  order  to  avoid  any  theory  of  age  or  correlation.  The  most  con- 
spicuous example  of  this  kind  is  that  of  the  Keweenawan  series  of 
Lake  Superior. 
This  tendency  to  return  to  the  use  of  local  names  was  plainly  a 
reversion  to  scientific  methods,  which  were  never  departed  from  by 
certain  geologists,  who  declined  to  use  any  term  for  the  ancient  rocks 
which  involves  a  theory  of  origin  or  succession,  but  divided  the  rocks 
which  they  found  in  their  respective  districts  into  lithological  di- 
visons  or  local  formations.  Conspicuous  among  early  geologists  of 
this  class  are  Jukes,  Percival,  and  Lieber. 
In  the  late  eighties  Irving  proposed  that  there  be  placed  below  the 
Paleozoic  another  system  of  coordinate  value,  for  which  the  term 
"Agnotozoic  "  or  "  Eparchean  "  was  suggested.  This  term  cut  out  of 
the  Archean  a  large  class  of  rocks  which  had  before  been  included  in 
that  system.  In  1889  the  name  Algonkian  was  brought  forward  by 
the  United  States  Geological  Survey  for  the  systemic  place  for  which 
