642  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
hornblendic  and  micaceous  gneisses,  and  the  presence  of  magnetic 
iron  ore,  suggest  a  detrital  origin  for  at  least  a  part  of  the  gneisses, 
and  consequently  their  reference  to  the  Algonkian.  A  part  of  the 
hornblende  gneisses  associated  with  the  limestones  is  also  recognized 
as  igneous,  and  attention  is  called  to  the  similarity  to  hornblende 
gneisses  occurring  within  the  light-colored  rock  making  up  the  main 
mass  of  the  mountain.  There  may  be  really  two  series  of  rocks:  (1) 
A  series  of  limestone  and  associated  interbedded  rocks,  of  sedimentary 
origin,  and  (2)  a  series  of  more  massive  granitoid  gneisses,  probably 
older,  and  of  unknown  origin.  This  supposition  is  based  only  on  the 
fact  that  the  limestones  are  persistently  associated  with  the  horn- 
blendic and  micaceous  gneisses  and  quartz-pyroxene  rock,  and  are 
not  found  associated  or  in  contact  with  the  light-colored  granitoid 
gneisses  which  constitute  the  main  mass  of  the  mountain.  However, 
there  is  not  sufficient  evidence  to  refer  a  part  of  the  gneisses  to  the 
Algonkian,  and  all  are  therefore  classed  as  pre-Cambrian. 
The  crystalline  limestones  are  believed  to  be  distinct  from  and 
older  than  the  blue  magnesian  limestone  of  Cambrian  age  which 
occurs  along  the  northwestern  side  of  the  New  Jersey  Highlands  and 
which  outcrops  in  isolated  areas  in  the  valleys  adjacent  to  Jenny 
Jump  Mountain,  for  the  following  reasons:  (1)  They  differ  lith- 
ologically  from  the  blue  limestone  in  being  thoroughly  crystalline 
and  in  containing  large  amounts  of  accessory  metamorphic  minerals, 
showing  that  they  have  been  subjected  to  general  metamorphic  forces 
of  which  the  neighboring  blue  limestone  shows  no  trace.  (2)  They 
occur  in  intimate  association  with  the  gneisses,  which  are  of  admitted 
pre-Cambrian  age.  (3)  They  show  no  intimate  association  in  areal 
distribution  with  the  blue  limestone,  nor  any  tendency  to  grade  into 
it.  (4)  The  metamorphic  changes  to  which  the  white  limestones  have 
been  subjected  are  general  in  their  nature,  and  not  due  to  the  action 
of  eruptives  by  which  they  are  cut;  so  that  no  sufficient  agent  is  at 
hand  to  account  for  the  supposed  change  from  blue  into  white  lime- 
stone. The  white  crystalline  limestones  are  therefore  believed  to  be 
of  pre-Cambrian  age. 
Wolff  and  Brooks,115  in  1898,  present  a  final  discussion  of  the  age 
of  the  Franklin  white  limestone,  of  Sussex  County,  N.  J.  The  pre- 
Cambrian  age  of  the  white  limestone  is  believed  to  be  shown  by  the 
following  facts : 
The  supposed  cases  of  interbedding  of  the  white  limestone  and  the 
Cambrian  quartzite  are  found  to  be  due  to  faulting  or  to  peculiar 
conditions  of  deposition.  On  the  other  hand,  while  it  is  difficult  to 
prove  that  the  white  limestone  and  the  pre-Cambrian  gneiss  are 
actually  interbedded,  narrow  bands  of  the  true  gneiss  do  occur  within 
the  white  limestone  belt,  and  seem  to  be  an  integral  part  of  the  series. 
