31 '2  PRE-CAMBRIAN    KOCKS    OF    NORTH    AMERICA.  [bill. 86. 
ite  is  found  the  Olenellus  fauna,  hence  the  only  pre-Cambrian  rock  is 
tlie  granitoid  gneiss.  In  structure  Greylock  is  a  complex  synclinal, 
while  Hoosac  mountain  is  an  anticlinal  overturned  toward  the  west. 
At  the  ends  of  the  Hoosac  ridge  the  anticlinal  bends  nearly  to  an  east 
and  west  direction.  This  is  explained  by  regarding  the  granite-gneiss 
as  a  rigid  mass  which  resisted  the  lateral  thrust,  and  the  abnormal 
overfbldings  as  the  result  of  compensatory  movements. 
Wolff.67  in  1880,  gives  a  systematic  account  of  the  geology  of  Hoosac 
mountain.  The  rocks  of  the  region  are  thoroughly  crystalline,  but 
little  trace  remains  of  their  original  elements,  whether  of  detrital  or 
of  eruptive  origin;  but  the  bedding  corresponding  to  their  original 
planes  of  deposit  is  well  marked,  and  under  proper  conditions  the  order 
of  succession  can  be  determined.  The  basement  rock  is  a  coarse 
granitoid  banded  gneiss,  which  forms  the  base  of  the  Hoosac  mountain 
proper.  Crushing  and  development  of  new  minerals  makes  it  perhaps 
impossible  to  say  certainly  what  is  the  origin  of  this  rock.  It  could 
perfectly  well  be  an  eruptive  granite  modified  by  inetamorphism, 
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  between  coarse 
gneisses  similar  to  the  basement  gneiss;  finer  grained  banded  gneisses ; 
gneisses  with  but  a  small  amount  of  mica;  metamorphic  gneiss-con- 
glomerate: ordinary  quartzite-conglomerate  and  quartzites.  These 
phases  pass  into  each  other  along  the  strike.  In  the  metamorphic  eon- 
glomerate  it  is  difficult  or  impossible  to  separate  the  old  quartz  and 
feldspar  from  that  formed  in  situ.  The  rock  is  considered  metamorphic 
because  of  the  shape  and  distribution  of  the  pebbles  in  alternations  of 
coarse  and  fine  materials,  because  of  the  diverse  nature  of  the  pebbles, 
including  blue  quartz,  white  quartz,  granulite  rock,  and  granite,  and  be- 
cause of  frequent  transitions  into  quartzite  and  quartzite-conglomerate. 
The  next  member  of  the  series  is  the  Hoosac  schist,  which  conform 
ably  overlies  the  Vermont  formation.     In  this  schist  no  recognizable 
clastic  element  is  found.     The  minerals  appear  to  have,  formed  con- 
temporaneously.    This  schist  is  often  coarsely  crystalline,  yet  it  is  very 
similar  to  the  albite  phyllites  of  Germany. 
The  next  rock  found  is  tin1  stratified  limestone,  which  occurs  in 
the  Hoosac  valley,  where  it  is  in  contact  with  the  Vermont  forma- 
tion and  the  Hoosac  and  Berkshire  schists.  The  rock  is  generally  an 
impure,  coarsely  crystalline  white  marble.  Layers  of  quartzite  are 
frequent  in  the  limestone  and  the  change  from  one  to  the  other  is 
gradual.  Also,  there  is  in  some  sense  a  transition  between  the  Stock- 
bridge  limestone  and  Vermont  gneiss,  and  also  a  transition  between  the 
limestone  and   Hoosac  schist,     There  also  occur  certain  dark  colored 
