610  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
to  that  which  characterized  Keweenawan  time  in  the  Lake  Superior 
region. 
Kemp,4'-  in  181)0,  in  connection  with  the  description  of  the  mag- 
netite deposits  of  the  Adirondacks,  briefly  describes  the  general  fea- 
tures of  the  geology  of  the  gabbro  and  gneiss  of  Westport,  Elizabeth- 
town,  and  Newcomb  townships  in  Essex  County,  N.  Y.,  and  presents 
a  geological  map  of  Westport  and  Elizabethtown  townships. 
Gushing,43  in  1899,  maps  the  boundary  between  the  Potsdam  and 
pre-Cambrian  rocks  north  of  the  Adirondacks,  from  the  line  between 
Clinton  and  Franklin  counties,  westward  across  Franklin  County 
into  St.  Lawrence  County,  to  a  few  miles  west  of  Potsdam. 
The  sequence  of  rocks  in  the  region  is  believed  to  be  as  follows : 
1.  A  series  of  gneisses  of  great  variety  of  structure  and  composi- 
tion, in  which  all  original  structures  are  lost,  of  igneous  origin,  and 
in  part  at  least  of  Archean  age.  They  seem  to  grade  into  the  basic 
gabbros  of  the  region ;  at  least  the  gabbros  present  phases  not  to  be 
distinguished  from  the  gneisses. 
2.  The  Grenville  series  (Oswegatchie  series),  comprising  quartzose 
gneisses  and  schists,  quartz-feldspar-biotite  gneisses,  dioritic  and  gab- 
broic  gneisses,  and  occasional  bands  of  coarsely  crystalline  limestone. 
These  rocks  are  accompanied  by  belts  of  gneiss,  similar  to  the  older 
gneiss,  which  seem  to  be  interstratified  with  the  other  rocks  of  this 
series,  but  whose  relationships  are  doubtful.  The  gneiss  of  the  Gren- 
ville series  differs  in  appearance  from  the  older  gneiss,  and  a  consid- 
erable portion  seems  to  be  unquestionably  of  sedimentary  origin, 
although  dynamic  metamorphism  has  obscured  all  traces  of  clastic 
structure  and  has  given  the  gneiss  a  foliation  in  common  with  the 
older  gneisses,  rendering  the  field  relations  obscure.  From  Parish- 
ville  westward  to  Potsdam  the  Grenville  series  is  more  widely  dis- 
tributed, less  faulted,  and  less  completely  metamorphosed  (and  hence 
its  sedimentary  character  is  less  disguised)  than  the  Grenville  series 
farther  east,  probably  because  of  its  greater  distance  from  the  anor- 
thosite  intrusion.  However,  it  seems  to  be  beyond  question  that  the 
eastern  and  western  series  are  equivalent. 
3.  The  anorthosite  intrusion. 
4.  Later  gabbros. 
5.  Granitic  intrusions.  The  region  was  then  subjected  to  intense 
dynamic  metamorphism,  after  which  occurred  the  intrusion  of 
G.  Diabase  and  trachyte  dikes. 
7.  Paleozoic  rocks  overlying  unconformably  all  the  preceding. 
Smyth,44  in  1899,  summarizes  his  ideas  to  date  on  the  geology  of 
the  Adirondacks.  Gneisses,  limestone,  and  gabbro  are  the  principal 
rocks.  From  studies  in  the  western  Adirondacks  it  is  certain  that 
some  of  the  gneisses  are  of  igneous  origin,  being  granites,  syenites, 
gabbros,  etc.,  which  have  been  modified  by  metamorphism;  while 
