640  PKE-CAMBKIAN    GEOLOGY    OF    NORTH   AMERICA. 
with  later  granitic  eruptions.  In  passing  away  from  a  boss  or  dike 
the  limestone  is  white,  but  changes  steadily  with  rapid  gradations 
into  the  blue  limestone.  Sandstones  and  quartzites  of  identical  char- 
acter underlie  both  the  white  and  the  blue  limestone  and  bind  them 
together.  The  one  distinguishing  fact  which  separates  the  white 
from  the  blue  limestone  is  the  presence  of  eruptive  rocks.  It  is  there- 
fore concluded  that  the  two  are  identical.  As  the  blue  limestone 
belongs  to  the  Cambrian,  it  is  concluded,  contrary  to  what  has  been 
supposed,  that  in  this  region  there  are  no  Archean  limestones. 
Kemp,100  in  1903,  describes  the  ore  deposits  at  Franklin  Furnace 
and  Ogdensburg,  N.  J.,  and  briefly  sketches  the  general  geology  of  the 
area.  The  ore  deposits  occur  in  white  crystalline  limestone,  which  is 
cut  in  numerous  places  by  dikes  of  granite,  trap,  and  a  rock  taken  to 
be  altered  gabbro.  The  white  limestone  is  closely  involved  through- 
out its  extent  with  a  blue  limestone  of  Cambrian  or  Cambro-Silurian 
age. 
Nason,110  in  1894,  finds  as  a  result  of  analyses  that  the  white  and 
blue  limestones  of  Sussex  County,  N.  J.,  are  essentially  the  same  in 
composition,  both  being  magnesian  limestones  or  true  dolomites.  The 
coarsely  crystalline  white  limestone  near  its  contact  with  granite  is 
generally  nonmagnesian. 
Nason,111  in  1894,  gives  a  summary  of  facts  showing  that  the  white 
limestone  of  Sussex  County,  N.  J.,  is  of  Cambrian  age,  as  follows :  (1) 
The  white  limestones  are  continuous  with  the  blue  limestones  (now 
accepted  as  of  Cambrian  age),  and  every  degree  of  transition  may  be 
found  between  them.  (2)  Both  have  the  same  strike  and  dip.  (3) 
Both  are  conformable  with  a  quartzite  also  containing  Cambrian  fos- 
sils. (4)  Both  are  unconformable  with  the  gneiss  upon  which  they 
rest.  (5)  Both  have  in  sum  total  the  same  chemical  composition  and 
are  magnesian.  (6)  The  altered  crystalline  condition  of  the  white 
limestone  is  due  to  the  intrusion  of  igneous  masses  and  to  regional 
metamorphism,  while  the  blue  limestone  never  contains  such  igneous 
injections.  (7)  The  presence  of  certain  minerals,  especially  chondro- 
dite,  is  not  indicative  of  geological  age,  as  this  mineral  is  known  to 
occur  in  modern  volcanic  rocks. 
Westgate,112  in  1894,  holds  that  the  crystalline  limestones  of  War- 
ren County,  N.  J.,  are  distinct  from  and  older  than  the  blue  mag- 
nesian limestone  and  are  of  pre-Cambrian  age,  for  the  following 
reasons :  They  have  a  well-developed  crystalline  character,  and  hold 
large  quantities  of  accessory  metamorphic  minerals;  they  show  no 
intimate  association  with  the  blue  Cambrian  limestones;  they  show 
no  tendency  to  grade  into  them ;  they  have  been  subjected  to  general 
metamorphic  forces,  of  which  the  neighboring  blue  limestone  shows 
no  trace;  they  occur  in  intimate  association  with  the  granitoid 
gneisses,  and  in  some  cases  appear  to  be  interbedded  with  them. 
