636  ■  PRE-CAMBPJAN    GEOLOGY   OF    NORTH   AMERICA. 
are  usually  highly  inclined,  the  average  dip  exceeding  45°.  In  many 
of  the  principal  ridges  an  anticlinal  arrangement  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°  SE.,  rests  unconformably 
under  the  more  recent  formations,  and  is  regarded  as  the  equivalent 
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  adjacent.  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  crys- 
talline forms  of  limestones  are  associated  with  the  metalliferous  veins, 
which  are  regarded  as  the  cause  of  their  crystalline  character. 
Jackson,90  in  1854,  maintains  that  the  Xew  Jersey  crystalline  lime- 
stones are  of  igneous  origin. 
Kitchell,97  in  1856,  places  the  formations  of  the  Highlands  in  the 
Azoic  system.  These  include  gneiss,  hornblendic,  micaceous,  felcl- 
spathic  and  quartzose  schists,  and  white  crystalline  limestone  inter- 
stratified  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;  they  exhibit  violent  dislocations; 
their  general  strike  is  northeast  and  southwest,  the  same  as  the  in- 
trusive dikes,  and  their  dip  southeast.  In  addition  to  their  distinct 
stratification  they  exhibit  planes  of  cleavage,  frequently  at  right 
angles  to  the  former  and  generally  inclining  toward  the  northeast  at 
all  angles  up,  to  45°.  At  one  place  limestone  rests  unconformably 
upon  the  gneiss. 
Cook,98  in  1868,  places  under  the  Azoic  rocks  the  gneisses,  crystal- 
line limestone,  and  beds  of  magnetic  iron  ore.  The  crystalline  lime- 
stone in  every  case  is  conformable  to  the  gneiss  and  interstratified 
with  it.  It  is  not,  as  supposed  by  Rogers,  the  metamorphosed  blue 
limestone.  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  of  the  gneiss  rocks.  The  gneiss  is  divided  into  four  principal 
belts.  The  Azoic  formations,  with  trifling  exceptions,  are  stratified. 
Usually  they  are  highly  inclined,  but  they  vary  between  horizontal  and 
vertical.  The  axes  of  the  folds  are  generallv  in  a  northeast-southwest 
direction.  Some  of  the  rocks  are  so  thin  bedded  as  to  be  schistose, 
while  other  portions  are  so  thick  bedded  that  for  long  distances  it 
is  almost  impossible  to  tell  which  way  the  rock  dips.  The  gneiss  is 
cut  by  veins  and  dikes  of  trap  and  granite.     The  Azoic  rocks  of 
