SOUTHEASTERN    NEW    YORK.  633 
larity  of  the  igneous  injections  and  the  close  folding  and  frequent 
faulting  even  the  order  of  superposition  of  the  constituent  members 
is  not  clear.  At  a  few  places,  in  what  seems  to  be  an  upper  member, 
because  of  its  connection  with  overlying  formations,  the  banded 
black  and  white  gneiss  most  characteristic  of  the  series  passes  grad- 
ually and  normally  into  a  mica  quartz  schist-,  and  this  in  turn  into  a 
rather  pure  quartzite,  a  few  feet  thick.  The  series  has  all  the  phys- 
ical characters  of  the  Grenville  series  of  the  Adirondacks  and  Canada 
and  is  so  called. 
Possibly  unconformably  upon  but  closely  infolded  with  the  Gren- 
ville series  is  the  Inwood  limestone.  In  the  New  York  City  folio  this 
is  regarded  as  conformable  with  the  underlying  series.  The  condi- 
tions may  be  those  of  overlap.  This  in  turn  is  overlain  conformably 
by  the  Manhattan  mica  schists. 
Intrusive  into  the  Manhattan  schists  and  Inwood  limestone  are 
basaltic,  diabasic,  granitic,  and  other  intrusiyes.  Both  are  believed 
to  be  overlain  unconformably  by  the  Poughquag  quartzite.  of  Cam- 
brian age.  They  have  before  been  correlated  with  the  Wappinger 
limestone  and  Hudson  River  slates.  While  no  unconformable  con- 
tacts have  been  actually  observed,  the  Inwood  limestone  is  coarsely 
crystalline,  whereas  the  Wappinger  is  fine;  it  is  very  impure  with 
silicates  and  pegmatites,  and  has  occasional  dike  intrusions  and 
strong  epidotic  development,  whereas  the  other  has  none:  there  is  no 
quartzite  on  either  margin  of  the  Sprout  Brook  (Inwood)  limestone 
and  none  beneath  it,  as  may  be  seen  by  following  up  the  brook  to  the 
point  where  the  limestone  disappears,  whereas  the  other  has  at  least 
500  feet  of  quartzite  conformably  beneath.  For  more  than  a  mile  this 
crystalline  limestone  occupies  the  valley  for  a  width  of  at  least  one- 
fourth  mile.  It  is  well  developed  for  a  distance  of  more  than  6  miles. 
It  must  be,  at  the  lowest  estimate,  several  hundred  feet  thick. 
Farther  up  the  valley,  where  the  limestone  disappears,  only  gneisses 
of  typical  Highland  types  remain.  It  is  clear  that  the  limestones  of 
these  two  adjacent  valleys  can  not  be  the  same.  The  Sprout  Brook 
representative  is  much  the  older. 
Stewart,9221  in  1908,  discusses  the  magnetite  belts  of  Putnam 
County,  N.  Y.,  and  shows  their  similarity  to  magnetites  of  the  High- 
lands of  New  Jersey  described  by  Spencer,  Bayley,  and  others.  De- 
tails of  Grenville  geology  are  discussed. 
SUMMARY    OF   PRESENT   KNOWLEDGE. 
Contrasting  views  of  the  classification  of  the  crystalline  rocks  of 
southeastern  New  York  are  held  by  Spencer,  Eckel,  Bayley,  and 
Merrill  on  the  one  hand,  based  on  work  of  Eckel  and  Merrill  in  the 
New  York  City  quadrangle  and  of  Spencer  and  Bayley  in  New  Jersey, 
and  on  the  other  hand  by  C.  P.  Berkev,  who  has  been  mapping  the 
