630  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
granitoid  hornblende  gneiss,  which  is  followed  by  a  second  member, 
the  iron-bearing  group,  and  this  in  turn  by  the  schistose  group.  The 
thickness  of  the  pie-Cambrian  rocks  in  the  Hudson  River  Valley  is 
between  2,300  and  2,800  feet.  They  are  unconformable  below  the 
Cambrian  quartzite,  and  nothing  more  definite  can  be  predicted  as  to 
their  age.  These  rocks  display  a  number  of  anticlines,  two  of  which 
are  those  at  Fishkill  and  the  Storm  King.  In  the  synclinal  trough  be- 
tween are  the  rocks  of  the  iron-bearing  group.  The  metamorphic 
strata  of  New  York  and  Westchester  counties,  called  the  Manhattan 
group,  are  classified  in  several  divisions  as  follows,  from  the  base  up- 
ward:  (1)  Yonkers  gneiss,  which  is  an  arkose  gneiss;  (2)  Fordham 
gneiss,  a  quartzite  gneiss;  (3)  Inwood  limestone,  and  (4)  Manhattan 
mica  schists.  The  age  of  the  Manhattan  group  has  not  been  deter- 
mined, but  it  is  thought  to  be  pre- Cambrian.  This  group  and  the 
Lower  Cambrian  sandstone  are  both  found  to  lie  on  the  second  or 
iron-bearing  member  of  the  pre-Cambrian  formation,  and  no  uncon- 
formity has  been  found  between  the  Manhattan  group  and  the  under- 
lying pre-Cambrian  \hh\<.  Of  equal  significance  is  the  lack  of  uncon- 
formity between  the  Lower  Silurian  strata  of  Peekskill  Hollow, 
Tompkins  Cove,  and  Verplanks  Point  and  the  partially  metamorphic 
beds  of  the  Manhattan  group. 
Kemp  and  Hollick,85  in  1894,  find  the  granite  at  Mounts  Adam  and 
Eve,  New  York,  to  be  intrusive  within  the  limestone.  Adjacent  to  the 
granites  the  limestone  is  white  and  crystalline  and  is  charged  with 
peculiar  contact  minerals.  This  white  limestone  grades  into  blue 
limestone  with  transitional  graphitic  forms.  This  limestone  in  New 
Jersey  contains  Cambrian  fossils. 
Kemp,86  in  1895,  describes  the  East  River  and  Blackwells  Island 
section  made  by  an  underground  tunnel  at  Seventieth  street,  New 
York  City.  Under  the  west  channel  is  a  fine-grained  mica  gneiss  con- 
taining pegmatite  seams.  Under  Blackwells  Island  and  the  adjacent 
waters  is  a  gray  gneiss.  In  the  center  of  the  east  channel  is  a  dolo- 
mite, which  is  flanked  on  the  east  side  by  mica  schist  locally  pegma- 
tized.  Beyond  the  mica  schist  on  the  Ravenswood  shore  is  a  massive 
hornblende  gneiss  or  granite,  which  is  thought  to  be  intrusive. 
Merrill,87  in  1898,  gives  a  general  account  of  the  geology  of  the 
crystalline  rocks  of  southeastern  New  York. 
The  crystalline  rocks  lie  east  of  Hudson  River,  in  New  York,  West- 
chester, Putnam,  and  Dutchess  counties,  whence  they  extend  into  Con- 
necticut; and  west  of  the  river,  in  Orange  and  Rockland  counties, 
whence  they  extend  southwesterly  into  New  Jersey.  The  lowest  mem- 
ber is  a  coarse  hornblende  granite,  which  forms  the  central  mass  of 
the  range  of  mountains  known  as  the  Highlands  of  the  Hudson,  and, 
in  their  highest  peak,  Breakneck  Mountain,  is  exposed  through  a  ver- 
tical height  of  nearly  1,200  feet.     Other  granites,  nearly  free  from 
