ADIRONDACK    MOUNTAINS.  019 
The  following  succession  in  this  region  is  therefore  recognized  and 
adopted  by  the  committee: 
Cambrian:  Potsdam  sandstone,  etc. 
Unconformity. 
Pre-Cambrian :  Grenville  series. 
Intrusive  contact. 
Laurentian. 
The  committee  considers  that  it  is  inadvisable  in  the  present  state 
of  our  knowledge  to  attempt  any  correlation  of  the  Grenville  series 
with  the  Huronian  or  Keewatin,  so  extensively  developed  in  the  re- 
gion of  the  Great  Lakes.  The  Grenville  series  has  not  as  yet  been 
found  in  contact  with  either  of  these,  and  until  this  has  been  done  and 
the  relations  of  the  several  series  have  been  carefully  studied  their 
relative  stratigraphic  position  must  remain  a  mere  matter  of  con- 
jecture. 
Newland  and  Kemp,  (!la  in  1908,  discuss  (he  geology  of  the  Adiron- 
dack magnetic  iron  ores.  Their  paper  includes  a  good  general  account 
of  the  geology  of  the  Adirondacks  and  a  detailed  discussion  of  the 
geology  about  certain  magnetite  deposits,  but  the  essential  features 
for  purposes  of  this  bulletin  are  covered  in  preceding  summaries. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
The  Adirondack  region  constitutes  a  great  pre-Cambrian  outlier 
surrounded  by  Paleozoic  sediments.  It  may  be  said  at  the  outset 
that  the  rocks  and  their  relations  are  very  similar  to  those  of  the 
original  Laurentian  area.  In  the  region  there  are  two  great  classes 
of  rocks — metamorphosed  sediments  and  igneous  rocks. 
The  sediments  consist  of  three  classes  of  rocks — gneisses,  lime- 
stones, and  quartzites.  The  gneisses,  which  are  almost  certainly  of 
sedimentary  origin,  are  usually  graphitic,  garnet  i  I'erons,  micaceous, 
pyroxenic,  hornblendic,  or  some  combination  of  these.  Moreover, 
the  sedimentary  gneisses  are  closely  associated  and  interstratitied 
with  the  limestones  and  quartzites.  Within  the  sedimentary  series 
are  workable  beds  of  magnetic  iron  ore.  The  limestones  are  pretty 
well  scattered  throughout  the  Adirondack  district.  In  the  eastern 
and  central  portions  of  the  district  they  usually  appear  in  rather 
small  and  separated  exposures.  In  the  northwestern  part  of  the  dis- 
trict they  are  much  more  prevalent.  Here  the  largest  belt  i<  20  miles 
long  and  G  miles  broad.  The  quartzite  and  quartz  schists  arc  less 
abundant  than  either  the  sedimentary  gneisses  or  the  limestones. 
They  are  apparently  more  prevalent  in  the  eastern  than  in  the  western 
portion  of  the  district. 
The  igneous  rocks  have  the  greatest  variety.  The  mo<t  abundant 
is  gneiss,  which  includes  granite  gneiss,  syenite  gneiss,  diorite  gneiss. 
gabbro  gneiss,  etc.,  with  intermediate  varieties.  The  mosl  abundant 
massive  rocks  is  gabbro,  winch  varies  into  anorthosite  on  one  side  and 
