474  PRE-CAMBRTAN   ROCKS    OF   NORTH   AMERICA.  [bull.  86. 
are  not  really  so.  This  was  used  to  a  considerable  extent  in  the  sixties 
and  seventies,  and  retains  a  place  in  literature  to  the  present  time. 
This  term  implies  a  theory  just  the  opposite  of  Azoic. 
As  already  said,  the  theory  involved  in  referring  all  pre-Cambrian 
rocks  to  the  Laurentian  and  Huronian  is  that  there  was  in  pre-Cam- 
brian  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  separated  by 
unconformities.  These  are,  from  the  base  upward:  Laurentian,  Norian, 
Arvonian,  Huronian,  Montalban ,  Taconian.  Of  these  terms  Norian  was 
devised  to  include  the  laminated  gabbros,  the  so-called  Upper  Lauren- 
tian of  Logan.  Arvonian  was  imported  from  Wales,  where  it  was  ap- 
plied by  Hicks  to  a  series  of  acid  volcanics.  Montalban  came  from  the 
White  mountain  region  in  New  Hampshire,  where  a  series  of  gneisses 
was  thought  to  be  of  different  lithological  character  from  the  Lauren- 
tian 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  Hypo- 
zoic,  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  metamor- 
phic, assuming  that  they  are  all  produced  by  the  alteration  of  sediments 
of  some  kind.  This  went  so  far  as  to  include  perfectly  massive  rocks, 
such  as  diabases,  gabbros,  granites,  etc.,  among  the  metainorphics. 
Eecently  the  term  has  also  been  applied  to  rocks  recognized  as  lami- 
nated eruptives,  but  this  is  not  the  use  referred  to.  This  term  meta- 
morphic 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  have  been  applied  to  formations  or  series 
in  order  to  avoid  any  theory  of  age  or  correlation.  The  most  conspicu- 
ous example  of  this  kind  is  that  of  the  Keweenaw  series  of  lake  Superior. 
More  recently  Lawson  has  proposed  the  terms  Keewatin  and  Coutchi- 
ching  for  certain  series  northwest  of  lake  Superior,  and  to  include  these 
two  he  proposes  the  Ontarian  system.  In  the  Grand  canyon  the  local 
names  Chuar,  Grand  canyon,  and  Vishnu  have  been  applied  to  pre- 
Cambrian  series  which  there  occur.  Comstoek  has  proposed  the  terms 
Burnetian,  Fernandian,  and  Texian  for  series  which  are  found  in  Texas. 
This  tendency  to  return  to  the  use  of  local  names  in  recent  years  is. 
plainly  a  reversion  to  scientific  methods  which  were  never  departed 
from  by  certain  geologists.  This  class  has  declined  to  use  any  term  for 
the  ancient  rocks  which  involves  a  theory  of  origin  or  succession,  but 
have  divided  the  rocks  which  they  found  in  their  respective  districts 
into  lithological  divisions  or  into  local  formations.  Conspicuous  among 
early  geologists  of  this  class  are  Jukes,  Percival,  and  Lieber. 
