612  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
Lawrence  counties,  is  an  augite  syenite  of  igneous  origin,  although  it 
passes  into  a  hornblende  gneiss  which  is  unquestionably  a  result  of 
dynamic  action.  The  origin  of  other  gneisses  has  been  inferred  to 
be  igneous  from  their  similarity  to  this  gneiss,  which  has  been  par- 
ticularly studied,  and  it  is  evidence  of  this  kind  which  serves  as  a 
basis  for  Smyth's  conclusions,  previously  published,  that  some  of  the 
gneisses  on  the  western  Adirondacks  are  certainly,  and  most  of  them 
probably,  of  igneous  origin. 
With  the  view  of  exploring  the  central  and  little  known  portion  of 
the  Adirondacks,  a  reconnaissance  was  made  through  the  area  con- 
tiguous to  the  Fulton  chain  of  lakes  and  Kaquette  Lake,  in  the  coun- 
ties of  Hamilton  and  Herkimer.  It  was  found  that  the  heart  of  the 
Adirondacks  is  made  up  essentially  of  gneiss,  with  minor  quantities 
of  crystalline  limestone  and  its  associated  sedimentary  gneisses  and 
schists.  This  is  precisely  analogous  to  what  was  found  by  the  writer 
in  St.  Lawrence,  Jefferson,  northern  Lewis,  and  southwestern  Hamil- 
ton counties,  and  by  Kemp  and  Cushing  in  the  eastern  Adirondacks. 
These  facts  lead  to  the  conclusion  that  the  Adirondack  region,  in- 
stead of  consisting  of  a  great  central  mass  of  gabbro,  surrounded  by  a 
narrow  fringe  of  gneisses  and  limestones  Avith  quaquaversal  dip,  is 
essentially  composed  of  gneisses,  with  numerous  limestone  belts,  hav- 
ing northeast  strike  and  northward  dip,  and  cut  through  on  the  east 
by  immense  intrusions  of  gabbro.  It  is  still  possible,  of  course,  that 
some  areas  of  gabbro  may  be  found  in  the  unexplored  portions  of 
the  western  half,  but  even  should  this  be  so  it  would  not  materially 
modify  the  above  conclusion,  as  such  masses  must  necessarily  be  iso- 
lated intrusions  of  no  great  extent,  rather  than  parts  of  a  large  area. 
Kemp,48  in  1900,  summarizes  the  present  knowledge  of  the  pre- 
Cambrian  rocks  of  the  Adirondacks.  Most  of  the  features  have  been 
covered  in  previous  articles.  xVttention  is  called  to  the  distribution 
of  the  sedimentary  crystalline  rocks,  the  Oswegatchie  series  (equiva- 
lent to  the  Grenville  series  of  Adams  and  perhaps  the  Huronian). 
These  consist  of  limestones,  sedimentary  gneisses,  and  quartzites. 
They  occupy  a  greater  area  than  has  been  supposed.  The  limestones 
are  found  chiefly  in  the  northwestern  and  southeastern  or  eastern 
portions  of  the  Adirondack  area  of  crystalline  rocks.  They  are  in 
small  quantity  or  altogether  absent  in  the  northern  portion,  in  the 
broad  belt  running  from  northeast  to  southwest  across  the  area,  and 
along  the  southern  and  southwestern  border.  On  the  northwest  they 
are  in  extended  and  comparatively  broad  belts,  but  in  the  eastern  por- 
tion they  appear  in  many  small  and  separated  exposures,  associated 
with  some  quartzites  and  much  greater  amounts  of  characteristic 
gneisses,  but  greatly  broken  up  by  igneous  intrusions.  The  quartzites 
thus  far  known  are  in  small  quantity,  and  are  found  principally  in 
the  eastern  portions  of  the  area,  where  the  limestones  are  thinnest 
