ADIRONDACK    MOUNTAINS.  609 
Theresa,  across  Rossie  and  Macomb,  into  De  Peyster.  This  belt  is 
perhaps  separated  from  the  first  by  narrow  strips  of  gneiss,  along 
the  northern  boundary  of  Gouverneur,  although  the  precise  extent 
of  the  gneiss  belts  is  undetermined.  The  third,  the  Edwards  belt,  to 
the  south  of  the  Gouverneur  belt,  and  separated  from  it  by  a  belt  of 
gneiss,  begins  in  Fowler,  crosses  Edwards,  and  runs  out  in  the  western 
part  of  Russell.  The  fourth,  the  Diana  belt,  south  of  the  Edwards 
belt,  and  separated  from  it  by  gneiss,  crosses  the  towns  of  Pitcairn 
and  Diana.  In  general,  the  limestones  have  their  greatest  develop- 
ment in  the  northwestern  part  of  the  region,  decreasing  as  the  eastern 
and  southern  parts  of  the  district  are  approached. 
The  limestone  is  everywhere  thoroughly  crystalline,  ranges  in 
color  from  white  to  dark  bluish-gray,  and  often  contains  disseminated 
and  aggregated  silicates,  of  which  the  more  important  are  serpentine 
and  tremolite. 
The  term  gneiss  is  used  to  include  rocks  ranging  from  acidic  to 
basic,  from  fine  to  coarse  grained,  and  from  distinctly  gneissoid,  or 
even  schistose,  to  entirely  massive.  They  constitute  a  complex  series 
of  rocks,  differing  somewhat  in  age,  and  largely,  if  not  almost  wholly, 
of  igneous  origin.  Parts  of  this  series  are  clearly  younger  than  the 
limestones;  other  parts  may  be  older  than  the  latter  formation,  but 
there  is  nothing  as  yet  to  prove  that  such  is  the  case.  A  probable 
exception  to  the  last  statement  is  afforded  by  certain  laminated 
gneisses,  of  limited  extent,  which  appear  to  underlie  the  limestone, 
perhaps  marking  the  base  of  the  series. 
Many  of  the  gneisses  have  heretofore  been  believed  to  be  sedimen- 
tary, and  the  evidence  leading  to  the  conclusion  that  they  are  largely 
igneous  may  be  briefly  summarized:  The  negative  evidence  of  the 
absence  of  all  structures  pointing  to  sedimentary  origin;  the  uni- 
formity of  composition  and  structure  over  wide  areas,  with  changes 
by  gradual  transition ;  a  common  occurrence  of  massive  cores,  in  every 
way  identical  with  plutonic  rocks;  the  presence  of  structures  in  the 
gneiss  that  would  result  from  the  application  of  pressure  to  igneous 
rocks;  eruptive  contacts  between  the  abundant  light-colored  gneiss 
and  the  less  common  and  older  dark  gneiss,  together  with  widespread 
instances  of  inclusions  of  the  dark  gneiss  in  the  light;  the  identity 
of  the  gneiss  near  Natural  Bridge  with  the  plutonic  gabbro  intrusive 
in  the  limestone ;  eruptive  contacts  of  the  gneisses  with  the  limestone 
at  a  number  of  places. 
Cushing,41  in  1808,  describes  syenite  porphyry  dikes  in  (lie  north- 
ern Adirondacks.  They  are  shown  to  he  of  pre-Cambrian  age,  but 
later  than  the  gabbros  and  granites  of  the  region.  The  syenite  por- 
phyries constitute  the  complementary  rocks  to  the  diabases  of  the 
region,  and  together  with  them  form  an  eruptive  assemblage  similar 
55721— Bull.  3G0— 09 30 
