402  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
it  does  not  necessitate  any  correlation  or  theory,  as  would  the  use  of 
Laurentian  or  Azoic. 
Britton,141  in  1885,  states  that  few,  if  any,  of  the  ridges  are  simple 
anticlinal  folds,  the  southeast  dips  being  generally  as  prevalent  on  one 
side  of  the  mountain  as  on  the  other,  though  often  differing  percepti- 
bly in  degree.  The  crystalline  limestones  do  not  represent  the  blue 
magnesian  limestone  metamorphosed  by  granite  and  syenite.  The  sup- 
posed dikes  of  granite  are  strata  conformable  to  the  white  limestone, 
as  are  the  iron  and  zinc  ore  beds  contained  in  it,  all  geologically  older 
than  the  blue  limestone  with  the  quartzites  and  slates  composing  the 
Lower  Silurian  system.  The  conclusion  is  now  reached  that  the  un- 
stratified  rock  masses  underlie  the  bedded  crystalline  rocks,  although 
the  line  of  separation  is  but  poorly  defined,  as  the  stratified  rocks  of 
the  same  mineral  composition  commonly  occur  on  the  sides  of  the  mas- 
sive area  with  an  apj>arent  gradual  passage  between  the  two,  and  at 
no  point  was  any  actual  uneonformability  found,  although  at  some 
places  abrupt  changes  in  the  lamination  have  been  observed  within 
short  distances.  This  leads  to  the  conclusion  that  the  massive  beds 
are  only  so  because  stratification  has  been  wholly  destroyed  through 
greater  metamorphism.  The  schistose  series  commonly  have  a  steeper 
dip  along  their  southern  margins  than  along  their  opposite  sides;  thus 
the  axial  planes  of  the  folds  are  often  inclined  toward  the  southeast. 
While  the  Potsdam  and  Paleozoic  rocks  are  unconformable  upon  the 
Archean,  the  newer  rocks  are  tilted  in  such  a  way  as  to  show  that  fold- 
ing has  occurred  since  they  were  deposited.  At  only  a  few  places  are 
actual  junctions  found,  the  two  more  important  being  in  Owens  island, 
in  Sussex  county,  and  at  Franklin  furnace.  At  several  localities  the 
relations  are  perplexing,  for  the  quartzites  and  conglomerates  are  so 
heavily  feldspathic  that  near  the  junction  they  appear  to  grade  gradually 
into  the  older  rocks,  fragments  and  masses  of  which  are  included  in  them. 
Along  the  southeastern  margin  of  the  Highlands  the  Silurian  is  crys- 
talline, including  crystalline  limestones  and  hydromica- slates,  and  here 
the  unconformity  is  much  less  pronounced,  no  satisfactory  contacts 
being  known  in  New  Jersey.  At  Pomp  ton  the  slate  ledges  have  nearly 
the  same  dip  and  strike  as  the  nearest  Archean  outcrops.  At  Peeks- 
kill  hollow  and  Annsville  cove,  in  New  York,  the  slates  and  quartzites 
and  crystalline  rocks  appear  to  be  directly  conformable,  the  strata  hav- 
ing been  subjected  to  an  overturn  and  causing  the  quartzite  to  dip 
under  the  older  rocks,  and  it  is  difficult  to  say  where  the  line  of  sepa- 
ration is. 
Britton,  142  in  1887,  divides  the  Archean  rocks  into  a  massive  group 
an  iron-bearing  group,  and  a  gneissic  and  schistose  group,  which  is  also 
believed  to  be  the  order  of  superposition,  although  there  is  a  gradual 
change  from  one  sedimentary  rock  into  the  other.  While  the  massive 
rocks  are  but  faintly  laminated,  there  is  no  evidence  adduciblein  favor 
of  an  igneous  origin  for  them,  but  all  indications  point  to  their  deposi- 
