NEW   ENGLAND.  591 
where  least  affected  by  dynamic  metamorphism  is  a  coarse  gneiss 
composed  of  white  microcline  and  blue  quartz,  with  accessory  biotite, 
zircon,  apatite,  and  magnetite.  This  gneiss  is  believed  to  be  intrusive 
in  the  gneissic  complex.  Later  intrusives  known  to  be  of  pre-Cam- 
brian  age  occur  in  narrow  dikes,  cutting  across  the  foliation  of  the 
gneisses,  and  appear  to  represent  metamorphosed  diabase,  while  an- 
other intrusive;  probably  of  pre-Cambrian  age,  is  a  peridotite. 
Farther  south  along  the  Green  Mountain  range  Emerson  has  de- 
termined the  pre-Cambrian  sequence  with  considerable  detail.  Four 
formations  are  distinguished;  these  are,  in  ascending  sequence,  the 
Hinsdale  gneiss,  the  Colesbrook  limestone,  the  Washington  gneiss, 
and  the  Becket  gneiss.  Two  of  these,  the  limestone  and  the  Wash- 
ington gneiss,  are  plainly  of  sedimentary  origin:  the  first  may  in- 
clude some  igneous  material,  and  the  last  is  an  igneous  rock  intrusive 
in  the  others.  The  Hinsdale  gneiss  is  in  places  an  evenly  banded 
hornblende  gneiss,  but  more  commonly  is  a  highly  contorted  black 
or  rusty  mica  schist,  and  the  presence  of  graphite  in  flakes  is  char- 
acteristic for  this  rock.  The  Colesbrook  limestone  is  in  part  a  very 
pure,  coarse,  white  marble,  usually  containing  scattered  flakes  of 
graphite.  In  places  this  rock  is  full  of  lime  silicates,  and  these  in- 
crease in  abundance  until  in  some  localities  the  marble  is  largely  rep- 
resented by  these  metamorphic  minerals.  The  Washington  gneiss 
approaches  a  quartzite  in  composition,  but  differs  somewhat  in  tex- 
ture and  general  appearance.  This  formation  is  largely  made  up 
of  stringers  and  beds  of  blue  opalescent  quartz. 
In  the  earlier  study  of  this  region  the  Becket  gneiss  was  believed 
to  include  some  conglomeratic  beds,  and  these  together  with  the  re- 
semblance of  much  of  the  gneiss  to  the  white  gneiss  forming  the  base 
of  the  Cambrian  on  Hoosac  Mountain  were  taken  as  indicative  of  the 
metamorphic  origin  of  this  granitic  gneiss.  On  this  basis  the  gneiss 
was  correlated  with  the  Vermont  quartzite  and  regarded  as  of  Cam- 
brian age.  Later  it  was  found  possible  to  discriminate  between  the 
metamorphosed  sedimentary  rock  and  the  older  granite,  so  that  the 
Becket  gneiss  is  now  believed  to  be  a  granite  intrusive  in  the  other 
pre-Cambrian  rocks,  although  it  is  itself  more  or  less  gneissoid.  It 
is  medium  to  fine  grained  and  contains  quartz,  oligoclase,  microcline, 
muscovite,  and  biotite.  The  Tvringham  and  Lee  gneisses,  earlier  de- 
scribed as  separate  pre-Cambrian  rocks,  are  now  considered  facie-  of 
the  Becket  gneiss;  the  Tvringham  gneiss  is  characterized  by  a  fibrous 
or  "  pencil  "  texture  and  represents  an  exceptional  phase  of  the  meta- 
morphism of  the  Becket  gneiss,  while  the  Lee  gneiss  is  the  border 
facies  or  contact  phase  of  the  granite.  The  Beckel  granite  has  in- 
cluded large  masses  of  the  Colesbrook,  as  well  as  of  the  other  pre- 
Cambrian  format  ions. 
