632  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
practically  impossible  to  state  whether  or  not  sections  of  the  pre- 
Cambrian  core  are  here  present,  although  it  is  agreed  that  the  positive 
evidence  is  in  favor  of  the  whole  being  post-Cambrian.  As  bearing 
on  this  question,  it  is  observed  that  where  the  undoubted  Manhattan 
schist  occurs  both  garnet  and  fibrolite  are  prevalent,  and  these  are 
minerals  which  would  be  expected  to  develop  during  the  metamor- 
phism  of  an  argillaceous  sediment  such  as  the  Manhattan  schist  is 
believed  to  have  originally  been.  On  the  other  hand,  where  the 
Manhattan  aspect  is  lacking,  although  garnet  is  present,  the  alumi- 
num-silicate minerals  are  not  found. 
Merrill,90  in  1902,  describes  the  metamorphic  crystalline  rocks  of 
the  New  York  quadrangle.  Of  these  only  one,  the  Fordham,  is  of 
pre- Cambrian  age.  At  the  type  locality  this  is  a  gray  banded  gneiss. 
The  bands,  which  rarely  exceed  2  inches  in  thickness,  vary  much  in 
composition.  Some  are  highly  quartzose,  some  are  composed  largely 
of  biotite,  and  some  consist  of  pegmatite  or  granite  which  seems  to 
have  been  injected  parallel  to  the  regular  banding  of  the  gneiss. 
Eckel,91  in  1902,  summarizes  the  geology  of  the  entire  southeastern 
district,  and  advances  a  new  view  concerning  the  pre-Cambrian 
gneisses  of  the  New  York  Highlands,  separating  them  into  four  divi- 
sions, founded  on  lithology  or  composition,  rather  than  on  super- 
position. 
Berkey,92  in  1907,  discusses  the  structural  and  stratigraphic  fea- 
tures of  the  basal  gneisses  of  the  New  York  Highlands,  and  especially 
those  of  the  Tarrytown  and  West  Point  quadrangles.  These  are  not 
different  in  general  characteristics  from  those  described  under  the 
head  of  Fordham  gneiss  in  the  New  York  City  folio  (No.  83)  of  the 
United  States  Geological  Survey,  except  that  sediments  apparently 
become  more  abundant  northward.  Broadly,  the  formation  includes 
banded  granitic,  hornblendic,  micaceous,  and  quartzose  gneisses ;  mica, 
hornblende,  chlorite,  quartz,  and  epidote  schists;  garnetiferous, 
pyritiferous,  graphitic,  pyroxenic,  tremolitic,  and  magnetic  schists 
and  gneisses;  crystalline  limestone,  serpentinous  limestones,  ophical- 
cites,  serpentine,  tremolitic  limestone,  and  quartzite;  pyrite  and  mag- 
netite deposits ;  granite  and  diorite  gneisses ;  true  granite,  cliorite,  and 
gabbro  bosses;  numerous  dikes,  stringers,  and  lenses  of  pegmatite; 
and  occasional  basaltic,  diabasic,  and  andesitic  dikes.  All  of  these 
occur  with  many  variations  and  gradations,  such  as  can  be  seen  only 
in  an  area  of  extreme  metamorphism  and  many  dynamic  disturb- 
ances. Many  of  the  occurrences  of  gneisses,  a  few  of  the  schists,  and 
all  of  the  granites,  diorites,  and  gabbros  are  of  igneous  origin,  but  all 
occur  as  intrusions  or  injections — as  sills,  dikes,  or  bosses  cutting  the 
metamorphosed  sedimentary  members  of  the  formation.  No  sub- 
division of  the  gneiss  formation  at  present  seems  possible.  There  is 
no  natural  stratigraphic  break.     Because  of  the  abundance  and  regu- 
