606  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
This  limestone  series  has  a  marked  resemblance  to  the  Grenville 
series,  but  because  it  is  difficult  to  establish  such  an  equivalency  it  is 
suggested  that  it  be  called  the  Oswegatchie  series.  The  areas  between 
the  belts  of  limestone  are  occupied  by  gneiss,  whose  origin  and  rela- 
tions to  the  limestone  series  are  doubtful.  The  limestone  series  can 
hardly  be  regarded  as  of  other  than  sedimentary  origin.  In  many 
cases  these  gneisses  adjacent  to  the  limestone  closely  resemble  the  in- 
terbedded  garnetiferous  gneisses,  and  doubtless  should  be  regarded 
as  members  of  the  limestone  series.  These  varieties  pass  gradually 
into  more  nearly  massive  gneisses  of  feldspathic  aspect,  and  in  a 
number  of  cases  these  are  in  direct  contact  with  the  limestone.  A 
part  of  these  gneisses  at  least  are  of  igneous  origin,  as  is  shown  by 
their  contact  relations,  but  whether  this  explanation  is  applicable 
to  all  of  them  it  is  impossible  to  show.  Intrusive  in  the  limestone 
series  are  granite,  diorite,  gabbro,  and  diabase.  Their  intrusive  na- 
ture is  shown  by  all  the  usual  phenomena  characteristic  of  such 
relations. 
The  gabbro  is  most  variable  in  its  petrographical  character.  At 
one  place  it  is  in  sharp  contact  with  the  granite.  The  relations  of 
the  gabbro  to  the  gneiss  are  difficult  to  unravel.  At  Natural  Bridge 
is  found  the  normal  gabbro,  and  in  passing  toward  the  red  gneiss  it 
appears  to  grade  into  it,  and  the  two  may  be  different  facies  of  the 
same  eruptive  mass.  The  contact  zones  between  the  limestone  and 
the  intrusive  gabbro  are  narrow  and  sharply  defined,  and  this  fact, 
combined  with  the  great  mechanical  disturbances  of  the  limestone 
series,  justifies  the  conclusion  that  its  metamorphism  is  largely 
dynamic. 
Kemp,33  in  1895,  describes  the  titaniferous  iron  ores  of  the  Adiron- 
dack^. These  occur  in  the  gabbros.  The  ores  are  regarded  as  segre- 
gations from  the  igneous  magma  formed  during  the  process  of  cool- 
ing and  crystallization. 
Kemp,34  in  1895,  describes  the  crystalline  limestones,  ophicalcites, 
and  associated  schists  of  the  eastern  Adirondacks.  Study  of  the 
region  seems  to  corroborate  the  conception  of  the  Adirondack  Moun- 
tains which  has  been  sketched  by  Van  Hise — a  central  intrusion  of 
igneous  rocks,  with  a  fringing  rim  of  older  gneisses,  schists,  and  lime- 
stones. A  closer  approximation  would  be  to  regard  the  intrusions  as 
in  several  more  or  less  parallel  ranges,  with  remnants  of  the  other 
rocks  in  the  valleys  between  them  and  on  the  flanks. 
The  limestones  and  the  associated  rocks  always  occur  in  depressions, 
the  resistant  ridges  consisting  of  the  harder  gneiss  or  anorthosite. 
The  former  constitute  sections  as  broad  as  1,000  feet,  in  which  the 
limestone  strata  are,  however,  less  than  half,  and  the  true  thickness  of 
which  is  difficult  to  determine  because  of  the  varying  dips,  the  schis- 
tosity,    and   the   possibility    of    faults.     The    white    limestones    are 
