ADIRONDACK    MOUNTAINS.  617 
intermingling  is  so  great  as  to  render  attempts  at  detailed  mapping  of 
the  several  elements  futile,  and  to  require  their  designation  as  belts 
of  mixed  rocks.  The  great  metamorphism  which  has  destroyed  the 
old  rock  structures  and  given  them  a  common  foliation,  the  inextri- 
cable intermingling  of  igneous  rocks  with  the  Grenville  sediments,  and 
the  later  great  igneous  invasions  from  beneath  have  so  disguised  the 
rock  relationships  as  to  make  it  very  likely  that  the  base  of  the  Gren- 
ville will  never  be  satisfactorily  made  out  in  the  region. 
Gushing,00  in  1907,  maps  and  describes  the  geology  of  the  Long 
Lake  quadrangle,  in  the  heart  of  the  Adirondacks.  With  the  excep- 
tion of  the  recent  unconsolidated  surface  deposits  all  the  rocks  are 
of  pre-Cambrian  age,  and  they  may  be  grouped  as  follows : 
1.  A  series  of  old  sedimentary  rocks,  the  Grenville  series,  much 
involved  with  igneous  rocks,  some  of  which  seem  of  approximately 
the  same  age. 
2.  A  series  of  gneisses  which  seem  to  be  mainly  or  wholly  of  igne- 
ous origin ;  they  may  be  in  part  older  than  the  Grenville  rocks,  though 
no  certain  evidence  of  this  has  yet  been  forthcoming  in  the  Adiron- 
dack region.  If  there  are  in  the  region  any  exposed  rocks  more 
ancient  than  the  Grenville  rocks,  they  are  these. 
3.  A  series  of  igneous  rocks,  usually  in  great  masses  (batholiths), 
which  are  demonstrably  younger  than  both  the  preceding,  and  which 
are  not  so  profoundly  changed  in  character,  in  many  places  retaining 
traces  of  their  original  textures  and  structures. 
4.  A  series  of  very  much  younger  igneous  rocks  which  have  under- 
gone little  change  since  their  intrusion. 
All  but  the  last  have  an  extensive  representation,  the  quadrangle 
being  rather  unusual  in  this  respect. 
The  Grenville  series  consists  of  well-banded  gneisses  and  schists, 
some  of  them  grading  into  quartzites,  with  bands  of  coarsely  crystal- 
line limestone  of  varying  thickness.  There  is  apparently  a  great 
thickness  of  these  rocks,  but  neither  their  base  nor  their  summit  is 
known,  and  they  are  so  disturbed  and  usually  so  poorly  exposed  that 
our  ideas  concerning  their  thickness  are  extremely  vague.  They  must 
have  been  deposited  upon  a  floor  of  older  rocks,  but  we  are  at  present 
ignorant  as  to  what  these  rocks  were,  and  whether  or  not  they  are  any- 
where exposed  in  the  district. 
Adams,  Barlow,  Coleman,  Cushing,  Kemp,  and  Van  Hise,01  form- 
ing a  special  committee  on  the  correlation  of  the  pre-Cambrian  rocks 
of  the  Adirondack  Mountains,  the  "Original  Laurentian  area"  of 
Canada,  and  eastern  Ontario,  in  1907,  consider  the  pre-Cambrian 
sedimentary  development  in  the  area  examined  by  them  as  repre- 
sented by  one  great  series.  This  series  is  essentially  identical  in  petro- 
graphical  character  throughout  the  whole  region. 
