400  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
dikes  or  veins  penetrating  the  strata.  The  massive  granitoid  gneisses 
of  the  Highlands  are  in  striking  contrast  with  the  gneiss  belt  of  New 
York  and  Staten  island,  which  reappears  at  Trenton  and  ranges  through 
Pennsylvania  and  Maryland,  which  is  distinguished  by  the  prevalence 
of  mica  and  other  thinly  laminated  minerals,  imparting  to  the  rock  a 
schistose  structure  or  the  thinly  bedded  character  of  ordinary  gneiss. 
The  massive  strata  are,  upon  the  whole,  decidedly  less  than  in  the 
Philadelphia  belt.  They  are  usually  highly  inclined,  the  average  dip 
exceeding  45°.  In  many  of  the  principal  ridges  an  anticlinal  arrange- 
ment is  plainly  visible. 
There  are  three  main  axes  of  elevation  in  the  granitic  area  rising  above 
the  secondary  sandstones  and  limestones.  The  metalliferous  veins 
generally  coincide  with  the  direction  of  the  strata  in  strike  and  dip, 
but  they  exhibit  many  minor  irregularities,  such  as  frequent  change  in 
thickness  and  deviation  from  the  direction  of  the  strata,  and  are 
regarded  as  unchanged  matter.  The  gneiss  formation  of  Trenton  has 
a  steep  inclination,  about  70°  to  the  southeast,  and  rests  unconform- 
ably  under  the  more  recent  formations,  and  is  regarded  as  the  equiva- 
lent of  the  gneiss  of  Manhattan  island. 
The  blue  limestone  belonging  to  the  older  secondary  strata  has  often 
a  secondary  cleavage  corresponding  with  the  slate  to  which  it  is  adja- 
cent. Associated  with  these  limestones  are  various  igneous  rocks 
which  have  locally  caused  it  to  become  crystalline  and  have  developed 
within  it  plumbago  and  various  silicates.  Often  these  crystalline  forms 
of  limestones  are  associated  with  the  metalliferous  veins,  which  are 
regarded  as  the  cause  of  its  crystalline  character. 
Jackson,134  in  1854,  maintains  that  the  New  Jersey  crystalline  lime- 
stones are  of  igneous  origin. 
Kitchell,135  in  1856,  places  the  formations  of  the  Highlands  in  the 
Azoic  system.  These  include  gneiss,  hornblendic,  micaceous,  feldspathic, 
and  quartzose  schists,  and  white  crystalline  limestone  interstratified 
with  seams  or  layers  of  magnetic  iron  ore.  These  rocks  are  traversed 
by  numerous  intrusive  dikes  of  granite  and  syenite ;  the  strata  are 
highly  metamorphic ;  exhibit  violent  dislocations ;  their  general  strike 
is  northeast  and  southwest,  the  same  as  the  intrusive  dikes,  and  their 
dips  southeast.  In  addition  to  their  distinct  stratification  they  exhibit  ' 
planes  of  cleavage  frequently  at  right  angles  to  the  former  and  gen-  . 
erally  inclining  toward  the  northeast  at  an  angle  varying  from  the 
horizontal  to  45°.  At  one  place  limestone  rests  unconformably  upon 
the  gneiss. 
Cook,136  in  1868,  places  under  the  Azoic  rocks  the  gneisses,  crystal- 
line limestone,  and  beds  of  magnetic  iron  ore.  The  crystalline  lime- 
stone iu  every  case  is  conformable  to  the  gneiss  and  interstratified 
with  it.  It  is  not,  as  supposed  by  Rogers,  the  metamorphosed  blue  lime- 
stone. The  iron  ores,  instead  of  being  igneous,  are  believed  to  be  true 
beds  which  were  deposited  as  sediments  in  the  same  way  as  the  material 
