646  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
The  weight  of  opinion  in  former  years  has  been  in  favor  of  the 
sedimentary  origin  of  the  gneissic  series.  The  development  of  this 
hypothesis  reached  its  culmination  in  the  work  of  Britton  (1885  and 
1887),  who  describes  the  more  massive  and  the  markedly  foliated 
rocks,  and,  dividing  the  whole  series  into  three  groups,  states  their 
order  of  superposition  as  follows:  (1)  Massive  group,  (2)  iron-bear- 
ing group,  (3)  gneissic  and  schistose  group.  Nason  shows  (1890) 
that  Britton's  groups  are  not  mutually  exclusive  and  negatives  his 
suggestion  of  superposition.  Agreeing,  however,  that  Britton  has 
demonstrated  the  possibility  of  dividing  and  mapping  the  foliated 
Archean  rocks,  Nason  selects  four  lithologic  types  believed  to  be  so 
constant  in  general  appearance  that  they  can  be  recognized  in  widely 
separated  areas.  These  types  are  not  to  be  taken  in  any  sense  as 
dividing  the  Archean  into  groups.  The  author  believed  that,  having 
fixed  these  types,  other  rocks  with  less  marked  characteristics  would 
eventually  be  separated.  The  avowed  purpose  of  Nason's  method  was 
to  approach  the  problem  without  bias,  leaving  hypotheses  of  origin 
aside  until  the  facts  of  composition  and  distribution  should  be  ascer- 
tained. 
Wolff  regards  the  gneisses  which  inclose  the  bed  of  iron  ore  at 
Hibernia  as  sedimentary  because  they  contain  graphite,  and  espe- 
cially because  they  show  a  folded  structure.  From  this  structure  he 
argues  that  the  gneisses  in  the  vicinity  are  definitely  bedded,  with 
top  and  bottom,  and  that  the  ore  bed  lies  in  conformable  position 
with  the  layers  of  gneiss.  On  the  other  hand,  an  igneous  origin  for 
the  gneisses  of  Sussex  County,  in  the  New  Jersey  Highlands,  is  sup- 
ported by  Spencer.  Recent  work  by  Bayley  a  and  Spencer  (Raritan, 
Passaic,  and  Franklin  Furnace  quadrangles)  leads  them  to  the  conclu- 
sion that  the  gneisses  are  mainly  igneous,  later  than  the  white  crystal- 
line limestones,  and  that  the  gneissic  structure  is  essentially  not  due  to 
secondary  dynamic  pressure,  but  is  a  primary  feature  of  the  rock, 
acquired  during  intrusion  and  consolidation.  Nevertheless,  large  but 
indeterminate  amounts  of  gneiss  are  supposed  to  represent  sediments 
which  were  very  completely  granitized  during  the  invasion  of  the 
igneous  material,  and  the  bulk  of  the  magnetite  iron  ore  is  thought 
to  have  been  formed  mainly  as  metasomatic  replacements  of  sedi- 
mentary rocks  brought  about  during  the  general  metamorphism.  A 
minor  part  of  the  iron  ore  is  connected  in  origin  with  later  intru- 
sions of  pegmatite.  The  several  types  of  gneiss  recognized  are  not 
correlated  with  those  described  by  Nason,  though  their  separation  is 
made  upon  similar  grounds.  They  can  not  be  mapped  in  an  entirely 
consistent  manner,  because  of  intimate  interlamination  and  a  certain 
amount  of  gradation ;  but  mapping  is  possible  upon  the  basis  of  pre- 
ponderance of  the  several  types  in  different  areas.     This  work,  if 
a  Manuscript  summary  of  unpublished  work  furnished  by  Arthur  C.    Spencer,   1906. 
