HIGHLANDS    OF    NEW   JERSEY.  645 
are  gneisses  and  limestones  of  Grenville  type,  dipping  monoclinally 
southeastward.  Straight  or  gently  curving  structural  features  are 
the  rule,  but  in  many  places  individual  layers  or  sets  of  layers,  if 
followed  along  the  strike  or  along  the  dip,  exhibit  at  intervals  sharp, 
troughlike  corrugations.  These  corrugations  range  in  size  from  mere 
wrinkles  to  folds  of  considerable  span.  Two  of  the  lithological  units 
are  called  the  Losee  gneiss  and  Byram  gneiss.  They  are  acidic 
igneous  rocks  intrusive  into  the  rocks  with  which  they  are  associated, 
and  occupy  a  larger  area  than  the  other  rocks  of  the  group.  Less 
abundant  than  the  granitoid  rocks,  but  still  of  considerable  impor- 
tance in  the  field  at  large,  is  the  dark  Pochuck  gneiss.  The  rocks 
embraced  under  this  term  have  the  composition  of  igneous  diorites  or 
gabbros,  but  whether  they  have  been  derived  from  igneous  or  sedi- 
mentary originals,  or,  as  is  thought,  in  part  from  both,  their  present 
characteristics  have  in  most  places  been  acquire* I  by  metamorphism, 
involving  secondary  crystallization. 
These  rocks  are  invaded  by  irregular  dikelike  masses  of  pegmatite 
which  have  a  genetic  relationship  with  the  magnetite  deposits. 
SUMMARY    OF    PRESENT    KNOWLEDGE. 
The  following  summary  has  been  prepared  for  this  bulletin  by 
Arthur  C.  Spencer: 
Occupying  most  of  the  New  Jersey  Highlands  are  crystalline  rocks 
resembling  those  of  the  adjacent  part  of  New  York  and  bearing  such 
relations  to  the  Cambrian  sandstone  as  to  indicate  a  great  structural 
break  between  them.  The  rocks  of  the  area  consist  largely  of  grani- 
toid gneisses  showing  several  mineralogic  types.  Though  in  many 
places  they  appear  to  be  nearly  massive,  there  is  usually  a  laminated 
arrangement  of  the  mineral  constituents.  The  strike  of  the  lamina- 
tion and  or*  the  bands  of  different  gneisses  conforms  to  the  trend 
of  the  area  as  a  whole,  being  east  of  north  and  south  of  west.  Inter- 
layered  with  the  gneisses  are  tabular  masses  of  limestone  and  bed- 
like bodies  of  magnetic  iron  ore.  Certain  bands  of  the  gneisses  are 
graphitic.  All  of  these  are  cut  by  various  acidic  and  basic  eruptives, 
the  former  being  of  pre-Cambrian  age  and  the  latter  mainly  but  not 
entirely  post-Cambrian.  In  several  places  the  white  limestones  occur 
in  proximity  to  the  blue  limestones  of  .Paleozoic  age,  and  there  has 
been  much  discussion  as  to  the  relations  of  the  two.  Wolff  and 
Brooks  have,  however,  definitely  proved  the  pre-Cambrian  age  of  the 
white  limestone  occurring  in  the  Franklin  Furnace  belt,  and  the  same 
conclusion  is  ably  argued  by  Westgate  for  the  similar  Limestones  on 
Jenny  Jump  Mountain.  The  pre-Cambrian  age  of  the  while  lime- 
stone in  this  area  must  be  accepted  for  the  northern  end  of  the  same 
belt,  which  extends  into  New  York  State. 
