580  PKE-CAMBEIAN    GEOLOGY   OF    NORTH   AMERICA. 
well-recognized,  conspicuous  limestone  formation,  below  which  fre- 
quently is  also  a  quartzite  of  varying  thickness.  This  limestone  is 
a  good  horizon  marker,  which  makes  the  determination  of  an  age 
succession  for  western  Massachusetts,  Connecticut,  and  New  York 
comparatively  easy.  The  limestone  appears  on  the  west  side  of 
Hoosac  Mountain,  but  is  not  found  east  of  that  mountain.  In  this 
eastern  area  lenses  of  limestone  occur  sporadically,  but  it  is  not  a 
continuous  formation.  The  difficulties  are  further  complicated  by 
the  fact  that  in  the  eastern  area  great  masses  of  intrusive  granite 
and  other  rocks  of  Paleozoic  age  are  present,  whereas  in  the  western 
area  few  or  no  acidic  Paleozoic  intrusives  are  found.  The  absence  of 
the  key  limestone  horizons  and  the  presence  of  the  great  masses  of 
intrusives  which  have  profoundly  metamorphosed  the  rocks  they 
entered  make  the  discrimination  between  the  Paleozoic  and  the  pre- 
Paleozoic  for  the  area  under  discussion  a  matter  of  extreme  difficulty. 
In  the  Avestern  part  of  the  area  under  discussion — the  immediate 
vicinity  of  the  area  where  the  base  of  the  Cambrian  is  determined — 
there  is  a  continuation  of  the  gneisses  which  have  been  called  Becket 
and  Fordham  and  which  are  certainly  pre-Cambrian.  While  this 
basal  complex  is  represented  by  many  widely  differing  types  of 
gneiss,  it  has  not  been  found  possible  to  delimit  separate  formations 
within  it.  It  has  been  definitely  established  that  areas  of  schist  and 
gneiss  of  Ordovician  or  later  age  occur  in  connection  with  the  areas 
of  the  Becket  gneiss  without  being  separated  by  intervening  Paleo- 
zoic quartzite  or  dolomite.  In  the  northwestern  portion  of  this 
area,  or  near  the  areas  of  Stockbridge  (Cambro-Ordovician)  dolo- 
mite, these  belts  of  schist  and  gneiss  more  closely  resemble  the  Berk- 
shire or  Hudson  formation,  and  are  believed  to  represent  a  phase  of 
that  formation.  To  the  east  of  the  principal  core  of  pre-Cambrian, 
which  occupies  the  northwestern  half  of  Litchfield  County,  Conn., 
a  different  type  of  schist  is  encountered,  which  is  characterized  by 
filmy  foliation  planes  covered  by  muscovite  and  bearing  such  meta- 
morphic  minerals  as  kyanite  or  garnet  and  staurolite  of  large  dimen- 
sions. This  formation,  which  occupies  large  areas  to  the  east  and 
southeast  of  the  pre-Cambrian  core,  has  not  had  its  age  definitely  de- 
termined, but  has  been  called  Hartland  schist,  and  it  is  believed  to 
be  of  Paleozoic  age. 
The  Becket  gneiss  is  doubtless  in  large  part  igneous — a  gray  to 
white  biotite  granite  gneiss — in  which  are  found  lenses,  dikes,  bosses, 
and  irregular-shaped  masses  of  amphibolite  having  the  characteris- 
tics of  an  older  intrusive,  and  regarded  as  such.  The  name  Corn- 
wall amphibolite  has  been  applied  to  them.  Within  the  pre-Cam- 
brian core,  also,  are  considerable  areas  of  more  recent  intrusive 
granite,  to  which  the  name  Thomaston  granite  has  been  applied.  The 
same  intrusive  is  found  even  more  abundantly  developed  in  the  Hart- 
