624  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
gneiss.  In  the  second  district  the  limestones  are  changed  to  white  or 
red,  coarse-grained,  crystalline  limestone,  containing  various  crystal- 
lized minerals,  with  scales  of  plumbago,  and  rarely  show  any  traces  of 
stratification.  The  slate  is  changed  to  mica  slate,  micaceous  gneiss, 
or  hornblende  slate,  and  the  quartz  rock  is  changed  so  as  to  be  scarcely 
recognized  as  such.  In  the  first  class,  also,  the  intrusive  rocks  bear  but 
a  small  ratio  to  the  altered  rocks,  and  are  mostly  quartz  and  granite ; 
but  in  the  second  class  the  undoubted  plutonic  rocks  abound,  and  con- 
sist of  granite,  syenite,  greenstone,  augite,  serpentine,  diallage,  and 
intrusive  metalliferous  veins. 
The  metamorphic  rocks  east  of  the  Hudson  and  the  Highlands  are 
in  a  continuous  range  from  Bennington  in  Vermont  to  the  western 
part  of  Massachusetts  and  Connecticut  and  the  eastern  part  of  New 
York.  Between  the  Taconic  rocks  and  the  metamorphic  rocks  to  the 
east  no  well-marked  line  of  distinction  can  be  drawn,  as  they  blend 
into  each  other  by  insensible  shades  of  difference.  In  considering  the 
metamorphic  rocks  as  a  whole  the  descriptions  necessarily  include 
certain  of  the  Taconic  rocks.  The  strata  of  metamorphic  rocks  are 
very  much  broken,  so  that  no  stratum  has  been  traced  continuously 
more  than  a  few  miles.  The  only  beds  which  can  be  traced  with  any 
degree  of  success  are  the  limestones,  which  are  described  in  detail. 
The  limestones  of  Westchester  County  have  the  same  dip  and  line  of 
bearing  as  the  contiguous  gneiss,  and,  like  that,  are  distinctly  strati- 
fied. They  form  several  nearly  parallel  ranges  at  intervals  of  2,  3, 
or  4  miles.  They  all  dip  east-southeast,  with  local  exceptions,  at  a 
high  angle,  varying  from  45°  to  90°.  The  metamorphic  slates  of 
Dutchess,  Putnam,  Westchester,  and  New  York  counties  have  been 
traced  in  different  localities  through  different  modifications  and  tex- 
ture from  the  gray  and  semicrystalline  limestones  associated  with 
talcose  slate  and  the  sandstone  of  the  Taconic  system  to  the  perfect 
dolomites  and  white  and  gray  crystalline  marbles  associated  with 
mica  slate  and  granular  quartz  rock  north  of  the  Highlands,  and  to 
still  more  crystalline  limestones  associated  with  mica  slate,  micaceous 
gneiss,  hornblende  slate,  hornblendic  gneiss,  hornblende  rock,  syenite, 
and  granite  south  of  the  Highlands.  In  these  latter  limestones  are 
frequently  found  some  mineral  substances,  such  as  serpentine,  brown 
tourmaline,  copper  and  iron  pyrites,  magnetic  sulphuret  of  iron, 
mica,  and  magnesian  minerals,  particularly  where  near  to  undoubted 
plutonic  rocks.  It  is  believed  that  all  the  crystalline  limestones  of 
Vermont,  Massachusetts,  Connecticut,  and  the  eastern  part  of 
New  York  are  metamorphic  rocks;  that  they  were  originally  the 
Mohawk  limestone  and  Calciferous  limestone,  and  that  the  associated 
rocks  were  originally  the  Potsdam  sandstone  and  the  slate  rocks  of 
the  Hudson  Valley;  that  they  were,  in  fact,  the  rocks  of  the  Cham- 
plain  division,  but  much  more  altered  and  modified  by  metamorphic 
agency  than  the  Taconic  rocks. 
