564  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
tened  and  have  a  shriveled  appearance,  like  dried  fruit.  In  some 
cases  the  concretions  occupy  more  than  half  the  mass.  These  concre- 
tions have  sometimes  been  called  petrified  butternuts.  The  rock  is 
regarded  as  produced  by  the  metamorphism  of  a  stratified  rock. 
Veins  of  granite  are  found  to  cut  syenite,  schist,  gneiss,  and  lime- 
stone, in  a  most  intricate  manner  at  many  localities.  Sometimes 
there  are  several  generations  of  granite  veins.  The  foregoing  facts 
lead  to  the  conviction  that  the  granite  acts  essentially  like  a  liquid 
mass,  but  it  is  regarded  as  the  product  of  aqueo-igneous  fusion  rather 
than  dry  fusion.  Also  it  is  believed  that  for  the  most  part  the  gran- 
ites have  formed  in  situ,  their  material  being  furnished  by  the  sedi- 
mentary rocks ;  hence  they  are  called  metamorphic.  As  evidence  that 
they  are  metamorphic  is  cited  the  fact  that  it  is  often  difficult  to  tell 
where  a  gneiss  ceases  and  a  granite  begins.  It  is  sometimes  found, 
where  granite  masses  come  in  contact  wTith  stratified  rocks,  that  the 
latter  have  been  more  or  less  disturbed  and  broken,  but  only  to  a 
limited  extent,  and  often  not  at  all.  In  many  cases  also  the  adjoin- 
ing strata  have  suffered  mechanical  displacement,  such  as  the  forcible 
injection  of  melted  matter  would  produce.  For  a  considerable  dis- 
tance around  the  granitic  masses,  also,  the  strata  are  frequently  in- 
durated and  metamorphosed  as  if  by  heat,  a  fact  that  seems  to  decide 
the  question  of  the  emanation  of  much  heat  from  granitic  foci.  The 
granites  are  more  abundant  in  the  crystalline  than  in  the  fossiliferous 
series ;  in  fact,  it  is  uncertain  whether  any  occur  in  rocks  which  bear 
fossils,  although  they  are  found  in  those  which,  are  regarded  as  the 
equivalent  of  the  Devonian.  The  granite  is  most  common  in  gneiss 
and  mica  schist,  less  so,  especially  in  the  form  of  veins,  in  clay  slates, 
and  least  of  all  in  talcose  schist.  In  the  stratified  rocks  cleavage  and 
foliation  occur,  while  joints  are  found  in  both  the  stratified  and  the 
unstratified  rocks. 
Thompson,26  in  1861,  describes  the  greenstone  or  trap  dikes  of  parts 
of  Vermont.  These  are  found  to  cut  all  the  other  rocks  at  many  local- 
ities. Besides  these  there  occur  calcareous  dikes,  veins  and  dikes  of 
quartz,  and  metallic  veins. 
Hitchcock  (C.  IT.),27  in  1861,  gives  detailed  descriptions  of  sections 
in  Vermont.  The  relations  of  the  Ascutney  syenite  to  the  stratified 
rocks  are  again  described.  In  the  ledges  immediately  contiguous  to 
the  granite  most  powerful  marks  of  alteration  by  heat  are  found.  The 
syenite  is  surrounded,  to  a  distance  of  a  quarter  to  a  half  mile,  with 
indurated  schists  that  ring  like  pot  metal  when  touched  with  a  ham- 
mer, and  the  lime  rock  that  is  usually  arranged  in  separate  strata 
seems  to  have  become  a  constituent  part  of  each  stratum,  the  whole 
rock  resembling  the  compact  vitrified  quartz  west  of  the  Green  Moun- 
tains. Often  crystals  of  staurotide,  and  perhaps  scapolite,  are  formed 
in  the  schist  by  the  heat.     In  the  west  part  of  the  larger  mountain 
