576  PKE-C AMEBIAN   GEOLOGY   OF   NORTH   AMERICA. 
valley  between  these.  The  mountain  rocks  are  composed  of  crystal- 
line schists,  which  are  found  to  be  of  Cambrian  and  Lower  Silurian 
age,  resting  on  pre-Cambrian  rocks.  The  valley  has  a  floor  of  crystal- 
line limestone  or  saccharoidal  marble,  on  which  are  ridges  of  schists, 
both  being  of  Lower  Silurian  age.  The  Taconic  Range  is  a  syncline 
in  the  Lower  Silurian  schists,  the  limestone  foundation  appearing 
only  at  its  base.  At  Hoosac  Mountain  the  succession  is  (1)  grani- 
toid gneiss,  (2)  quartzite  conglomerate  and  white  gneiss,  (3)  Hoosac 
phyllite,  and  (4)  Rowe  schist.  On  Greylock  the  succession  is  (1) 
Stockbridge  limestone,  (2)  Berkshire  phyllite,  (3)  Bellowspipe  lime- 
stone, and  (4)  Greylock  phyllite.  At  Hoosac  Mountain  the  quartzite 
conglomerate  and  white  gneiss  appear  to  grade  down  into  the  grani- 
toid gneiss  in  perfect  conformity.  At  Stamford  a  basic  dike  was 
discovered  which  cuts  the  granitoid  gneiss,  but  stops  abruptly  at 
the  quartzite.  Indeed,  the  quartzite  sags  down  at  this  place,  its  layers 
thickening  and  filling  the  hollow.  These  relations  are  considered 
definite  proof  of  an  unconformity  between  the  granitoid  gneiss  and 
the  quartzite.  The  general  transition  between  the  two  is  explained 
by  considering  the  granitoid  gneiss  as  disintegrated  at  the  time  of 
the  transgression  which  formed  the  quartzite.  The  quartzite  con- 
glomerate and  the  white  gneiss  are  traced  into  each  other  laterally, 
and  are  therefore  but  different  forms  of  a  sediment  of  the  same  age, 
unequally  metamorphosed.  Across  the  valley  the  Hoosac  phyllite 
was  traced  by  gradual  transition  into  the  limestone;  and  the  Stock- 
bridge  limestone,  Berkshire  phyllite,  Bellowspipe  limestone,  and  Grey- 
lock phyllite  are  all  correlated  with  the  Hoosac  phyllite.  In  the 
quartzite  is  found  the  Olenellus  fauna,  hence  the  only  pre-Cambrian 
rock  is  the  granitoid  gneiss.  In  structure  Greylock  is  a  complex  syn- 
cline, while  Hoosac  Mountain  is  an  anticline  overturned  toward  the 
west.  At  the  ends  of  the  Hoosac  Ridge  the  anticline  bends  nearly  to 
an  east-west  direction.  This  is  explained  by  regarding  the  granite 
gneiss  as  a  rigid  mass  which  resisted  the  lateral  thrust,  and  the  ab- 
normal overfoldings  as  the  result  of  compensatory  movements. 
Wolff,62  in  1889,  gives  a  systematic  account  of  the  geology  of 
Hoosac  Mountain.  The  basement  rock  is  a  coarse  granitoid  banded 
gneiss,  which  forms  the  base  of  Hoosac  Mountain  proper.  Crushing 
and  development  of  new  minerals  make  it  perhaps  impossible  to  say 
certainly  what  is  the  origin  of  this  rock.  It  could  perfectly  well  be 
an  eruptive  granite  modified  by  metamorphism,  while  on  the  other 
hand  its  field  relations  show  its  close  association  with  and  frequent 
transition  into  coarse  gneisses  which  seem  to  form  a  part  of  the 
detrital  series. 
Overlying  the  granitoid  gneiss  is  a  series  of  rocks  called  the  Ver- 
mont formation.  At  one  place,  where  perhaps  folded,  it  is  600  or 
700  feet  thick.     This  formation  contains  numerous  gradations  from 
