588  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
tions  at  such  places  is  extremely  difficult,  yet  as  a  whole  there  can  be 
distinguished  two  unconformable  series — an  older  completely  crys- 
talline basement,  upon  which  later  sediments  have  been  deposited. 
Thus  in  the  Hoosac  Mountain  portion  of  the  Taconic  quadrangle  it 
is  found  that  the  white  gneiss,  which  had  previously  been  considered 
as  belonging  with  the  Cambrian  Vermont  formation,  is  pre-Cam- 
brian,  that  there  is  here  no  transition  between  the  pre-Cambrian 
igneous  and  the  Cambrian  sedimentary  gneiss,  but  that  the  pre- 
Cambrian  white  gneiss  can  be  lithologically  distinguished  from  the 
sedimentary  series  above,  and  that  the  unconformity  between  the  two 
is  further  shown  by  the  frequent  presence  of  conglomerate  in  the 
base  of  the  overlying  Vermont  quartzite. 
It  was  further  found  that  much  of  the  series  which  had  previously 
been  mapped  as  Becket  gneiss  of  Cambrian  age,  consisting  of  schis- 
tose granite,  interlaminated  biotitic  and  amphibolitic  schist,  and 
locally  including  quartzite  and  quartz  schist,  should  be  regarded  as 
an  older  basement  of  pre-Cambrian  age,  upon  which  the  later  sedi- 
ments were  deposited,  and  that  where  the  Becket  has  been  mapped  as 
containing  rocks  of  sedimentary  origin  it  should  be  separated  into 
two  unconformable  series.  There  is  thus  present  in  this  region  a 
lower  series  of  pre-Cambrian  age,  comprising  the  complex  of  schis- 
tose granite  and  gneiss,  pegmatite,  and  biotitic-amphibolitic  schist, 
which  are  predominantly  of  igneous  origin,  and  unconformably 
above  this  an  upper  (Palezoic)  series  consisting  predominantly  of 
rocks  of  sedimentary  origin.  Both  series  have  been  injected  by  later 
basic  and  acidic  igneous  rocks  and  together  have  been  subjected  to 
severe  dynamic  movement.  The  upper  series  is  locally  so  metamor- 
phosed that  it  is  with  difficulty  discriminated  from  the  underlying 
pre-Cambrian. 
A  striking  instance  of  local  extreme  metamorphism  of  formations 
of  comparatively  late  age  is  found  at  Worcester,  Mass.  Here  there 
are  mica  schists  and  argillites  of  Carboniferous  age  which  by  deep- 
seated  metamorphism  have  been  altered  completely  to  crystalline 
schists  and  which  are  cut  in  a  most  intricate  manner  by  igneous  intru- 
sions, making  a  complex  very  similar  in  appearance  to  that  character- 
istic of  the  pre-Cambrian.  Another  instance  of  the  similarity  of  the 
metamorphosed  Paleozoic  formations  to  certain  phases  of  the  pre- 
Cambrian  is  found  at  Hoosac  Mountain,  Massachusetts.  The  gray- 
wacke  schist  member  of  the  Vermont  (Cambrian)  formation  is  here 
locally  so  homogeneous  in  character  that  further  metamorphism  of  it 
would  produce  a  gneiss  which  could  not  be  distinguished  from  a  gneiss 
of  igneous  origin,  while  the  quartzite  member  of  this  formation  has 
also  locally  the  thoroughly  crystalline  character  of  pre-Cambrian 
quartzites. 
