616  PRE-CAMBRIAN    GEOLOGY   OP    NORTH   AMERICA. 
The  main  area  on  the  northeast  consists  of  syenite,  usually  gneiss- 
oid,  of  post-Grenville  age.  Associated  with  this  are  smaller  areas  of 
Grenville  rocks.  These  comprise  a  series  of  light-colored,  often  white, 
gneisses,  very  rich  in  quartz,  interbanded  with  less  quartzose  rocks 
of  darker  color,  and  often  with  a  very  respectable  percentage  of  black 
minerals — hornblende,  black  mica,  and  magnetite.  The  argument  for 
their  sedimentary  origin  rests  on  their  composition,  mineralogical  and 
chemical,  and  on  their  frequent  variations  in  composition,  beds  of 
different  original  character  having  produced  differing  metamorphic 
rocks,  whose  comparatively  sharp  junctions  look  like  old  bedding 
planes.  At  most  of  the  Grenville  exposures  of  any  extent  rocks  which 
are  regarded  as  igneous  are  found  mingled  with  them.  They  are 
always  thoroughly  gneissoicl  and  are  interbanded  with  the  old  sedi- 
ments. They  are  thought  to  represent  old  dikes  and  sheets  of  igneous 
rock,  possibly  surface  flows  also,  which  were  formed  during  or  not 
long  after  the  deposition  of  the  sediments. 
At  Little  Falls  and  Middleville  the  rocks  are  mainly  syenite,  mostly 
augite  syenite,  sometimes  porphyritic,  usually  gneissoid,  of  post- 
Grenville  age,  but  much  older  than  the  diabase.  At  the  outlier  north- 
east of  Little  Falls  is  gray  gneiss  which  is  provisionally  classed  with 
the  green  gneiss  associated  with  the  Grenville  rocks. 
Gushing,59  in  1905,  summarizes  the  geology  of  the  northern  Ad- 
irondack region,  and  concludes  that  while  in  many  parts  of  the 
Adirondacks  areas  of  varying  size  are  found  in  which  the  rocks  that 
occur  may  be  unhesitatingly  classed  as  Grenville  sediments  or  as 
later  igneous  intrusions,  over  much  of  the  district  this  is  not  the  case, 
but  an  intimate  admixture  of  various  rocks  is  found,  in  apparently 
hopeless  confusion.  Thus  we  find  Grenville  sediments  elaborately 
interbanded  with  other  rocks,  apparently  igneous,  yet  seemingly  con- 
formable with  them  as  an  integral  part  of  the  series.  We  also  find 
rocks  which  are  not  to  be  distinguished  in  appearance  or  in  composi- 
tion from  the  rocks  of  the  great  intrusions,  except  for  perhaps  a 
more  thoroughly  gneissoid  character,  and  yet  so  interwoven  with  other 
rocks,  so  far  as  yet  known  not  represented  in  the  great  intrusions, 
that  it  hardly  seems  possible  that  the  two  can  belong  together.  There 
are  also  considerable  areas  of  gneisses  which  are  quite  like  the  un- 
certain gneisses  involved  often  with  the  Grenville  rocks,  yet  without 
any  Grenville  admixture,  and  the  relationship  of  such  rocks  forms  a 
very  difficult  problem.  The  Grenville  belts  and  patches  and  the  areas 
occupied  by  the  later  igneous  intrusions  have  been  in  the  main  dis- 
covered and  mapped.  There  yet  remains  the  exceedingly  difficult 
problem  of  the  separation  of  these  mixed  belts  into  their  several  ele- 
ments and  the  working  out  of  their  affiliations.  This  is  likely  in 
many  cases  to  prove  impossible,  and  in  nearly  all  cases  the  amount  of 
