620  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
syenite  on  the  other.  In  the  eastern  part  of  the  district  especially  the 
gabbro  occurs  in  great  batholithic  intrusions.  Later  diabases  are  sub- 
ordinate igneous  rocks. 
Much  attention  has  been  given  to  the  study  of  the  nature  and  rela- 
tions of  the  various  gneisses  of  the  district.  Some  of  the  gneisses  are 
sedimentary  and  some  of  them  are  igneous,  but  the  origin  of  the  great 
body  of  them  is  in  doubt.  Statements  of  the  relative  abundance  of 
these  classes  of  gneisses  vary  for  different  writers,  for  different  parts 
of  the  district,  and  for  different  stages  in  the  progress  of  the  mapping. 
But  there  has  been  recently  a  marked  tendency  toward  increasing 
emphasis  on  the  abundance  of  igneous  gneisses,  although  the  presence 
of  sedimentary  gneisses  interbedded  with  the  limestones  is  conceded 
by  all. 
Recent  workers  have  concluded  that  the  igneous  gneisses  are  mainly 
intrusive  in  the  sediments.  Thus  Cushing  states,  for  the  northern 
part  of  the  district,  that  while  the  relations  of  most  of  the  gneisses 
are  unknown,  wherever  units  have  been  separated  from  the  gneissic 
complex  they  have  been  found  to  be  intrusive  into  the  sedimentary 
series;  that  most  of  the  gneisses  will  be  found  to  be  intrusive  into  the 
sediments;  and  that  no  rocks  have  been  found  in  the  northern 
Adirondacks  which  can  be  shown  to  be  older  than  the  sedimentary 
series.  Smyth,  who  worked  on  the  Avestern  and  northwestern  side 
of  the  Adirondacks,  concludes  that  some  gneisses  are  certainly,  and 
most  of  them  probably,  of  igneous  origin  and  intrusive  into  the  lime- 
stones. He  also  thinks  that  near  Natural  Bridge  certain  of  the  horn- 
blende gneisses  result  from  the  deformation  and  alteration  of  the 
augite  syenite.  Kemp,  principally  from  his  studies  on  the  eastern 
and  southeastern  sides  of  the  district,  concludes  that  the  gneisses, 
where  separated,  are  intrusive  into  the  sedimentary  series  and  that 
there  is  no  certain  evidence  of  the  existence  of  basal  gneisses  in  the 
area  studied  by  him. 
In  general,  then,  the  origin  and  relations  of  a  large  body  of  the 
igneous  gneiss  are  unknown.  Where  the  relations  are  clear  it  is  found 
that  gneisses  are  intrusive  into  the  sedimentary  series,  and  as  yet  no 
decisive  evidence  of  the  existence  of  basal  gneisses  has  been  presented, 
although  the  possibility  of  its  presence  is  not  denied  by  any  of  the 
men  who  have  studied  the  region. 
Some  of  the  gneissic  beds  are  interlaminated  with  the  limestone; 
these  are  garnetiferous  and  micaceous,  or  pyroxenic  and  hornblendic. 
Near  Hague  is  a  bed  of  strongly  graphitic  gneiss  between  beds  of  ordi- 
nary gneiss,  which,  viewed  at  a  distance,  has  the  appearance  of  a 
layer  of  coal  in  an  ordinary  bedded  succession.  The  gneisses  inter- 
laminated with  the  limestone  are  everywhere  completely  crystalline. 
Neither  macroscopically  nor  microscopically  do  any  of  them  show 
clastic  structure,  but  the  original  beds  were  not  obliterated.     The 
