NEW   ENGLAND.  569 
solidated  the  rocks  heat  is  predominant,  although  chemical  action  has 
played  an  important  part.  The  mica  slates  have  been  mechanically 
deposited  in  water,  and  subsequently  subjected  to  such  a  degree  of 
heat  as  to  enable  their  materials  to  enter  into  a  crystalline  arrange- 
ment without  destroying  their  structure.  The  granite  is  supposed  to 
have  resulted  from  the  melting  down  of  other  rocks.  Where  it  is 
completely  melted,  granite  results;  where  partially  fused,  granite 
gneiss  is  found ;  while  another  portion  might  be  converted  into  por- 
phyritic  gneiss  and  another  into  schistose  rock.  This  theory  explains 
the  gradation  of  gneiss  into  granite  and  the  crystalline  and  por- 
phyritic  structure  of  the  gneiss.  The  unstratified  rocks  are  all 
igneous.  They  occur  in  irregular  protruding  masses,  in  the  form  of 
veins  of  various  sizes,  and  as  overlying  masses.  In  cases  in  which 
they  exist  interstratified  with  other  rocks,  an  examination  shows  that 
such  interlaminated  masses  are  always  connected  with  an  unstratified 
mass,  and  are  merely  veins  which  for  a  time  coincide  in  direction  with 
the  strata.  The  syenite  quarries  of  Sandy  Bay,  Cape  Ann,  have  a 
parallel  lamination,  but  as  these  grade  into  an  unstratified  syenite 
they  are  considered  as  examples  of  concretionary  structure  on  a  large 
scale  rather  than  as  a  result  of  real  stratification. 
Hitchcock  (Edward),43  in  1841,  in  a  systematic  account  of  the 
geology  of  Massachusetts,  divides  the  rocks  below  the  New  Red  sand- 
stone into  the  following  classes:  Graj'wacke,  metamorphic  slates, 
argillaceous  slate,  limestone,  quartz  rock,  mica  slate,  talcose  slate, 
serpentine,  hornblende  slate,  gneiss,  greenstone,  porphyry,  syenite, 
and  granite.  From  the  graywacke  to  the  gneiss  this  is  the  order  of 
occurrence.  The  greenstone,  porphyry,  syenite,  and  granite  are 
regarded  as  eruptive.  At  Bellingham  is  a  remarkable  metamorphic 
rock,  which  is  a  distinct  mica  slate  and  a  no  less  distinct  conglomerate. 
In  this  formation  are  also  placed  aggregates  of  porphyry,  which  is  a 
coarse  breccia  or  conglomerate,  chiefly  made  up  of  fragments  of  por- 
phyry reunited  by  a  cement  of  the  same  material,  and  sometimes 
almost  reconverted  into  a  compact  porphyry.  Flinty  slate,  chert, 
and  jasper  are  simply  the  ordinary  slate  changed  to  an  unusual  con- 
dition by  the  proximity  of  granite,  porphyry,  or  trap.  The  clay 
slate  is  entirely  destitute  of  organic  remains,  as  is  also  the  gray- 
wacke. The  limestones  are  water-deposited  stratified  rocks  meta- 
morphosed by  heat.  The  mica  slate  is  generally  associated  with 
gneiss,  but  also  occurs  associated  with  all  other  rocks  at  least  as  high 
as  the  argillaceous  slate.  Hornblende  schist  and  greenstone  slate,  a 
single  formation,  are  tentatively  regarded  as  metamorphosed  from 
ordinary  argillaceous  rock  by  the  action  of  heat.  Gneiss  i^  often 
much  folded  and  curved,  and  is  cut  by  veins  of  other  locks,  hut  is 
regarded  as  in  general  regularly  strati  lied. 
