362  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA. 
conglomerate.  Thus  we  have  a  really  conglomeratic  syenite.  The  primi- 
tive greenstones  are  distinguished  from  the  secondary  because  the  lat- 
ter are  more  coarse  and  crystalline. 
Dewey,34  in  1824,  in  a  sketch  of  the  geology  of  western  Massachu- 
setts, divides  the  principal  rocks  into  granite,  gneiss,  mica-slate,  granu- 
lar limestone,  argillaceous  slate,  quartz-rock,  transition  limestone,  and 
graywacke,  mica-slate  being  the  more  abundant  rock.  The  granite  is 
not  stratified,  and  must  he  considered  as  beds  or  veins  rather  than  a 
continuous  rock  like  the  mica-slate.  In  the  town  of  Windsor  is  a  con 
gkmieratic  mica-slate. 
Nash,35  in  1827,  finds  the  rocks  of  Hampshire  county  to  include  gran 
ite,  mica-slate,  micaceous  limestone,  hornblende-rock,  talcose  slate,  Olc 
Red  sandstone,  etc.  The  limestone  often  alternates  with  mica-slate,  am 
frequently  passes  into  it  by  insensible  gradations.  It  is  oftentimes  gar 
netiferous.  The  granite  veins  are  of  all  sizes  up  to  3  or  4  feet,  cut  the 
rocks  in  every  possible  direction,  and  intrude  granite  as  well  as  the 
mica -si  ate. 
Hitchcock  (Edward),36  in  1833,  divides  the  rocks  of  Massachusetts 
into  stratified  and  unstratified.     Below  the  New  Eed  sandstone  in  the 
former   are   graywacke,  argillaceous   slate,   limestone,  scapolite-rocl 
quartz-rock,  mica-slate,  talcose  slate,  serpentine,  hornblende-slate,  aid 
gneiss.    The  unstratified  are  greenstone,  porphyry,  syenite,  and  trap 
each  of  which  is  discussed.     Among  the  agents  which  have  consolidate! 
the  rocks  heat  is  the  predominant  one,  although  chemical  action  hi 
played  an  important  part.     The  mica-slates  have  been  mechanically} 
deposited  in  water,  and  subsequently  subjected  to  such  a  degree  i 
heat  as  to  enable  their  materials  to  enter  into  a  crystalline  arrangemem 
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  porphyritic  gneiss  am 
another  into  schistose  rock.     This   theory  explains  the  gradation  o 
gneiss  into  granite  and  the  crystalline  and  porphyritic  structure  of  the 
gneiss.     The  unstratified  rocks  are  all  igneous.     They  occur  in  irregu 
lar  protruding  masses  in  the  forms  of  veins  of  various  sizes  and  as 
overlying  masses.     In   cases  in  which    they  exist  interstratified  witlK 
other  rocks,  an  examination  shows  that  such  interlaminated  inassei 
are  always  connected  with  an  unstratified  mass,  and  are  merely  vein! 
which  tor  a  time  coincide  in  direction  with  the  strata.     The  syenite 
quarries  of  Sandy  bay,  cape  Ann,  have  a  jmrallel  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  oi 
real  stratification. 
Hitchcock  (Edward),37  in  1841,  in  a  systematic  account  of  the 
Geology  of  Massachusetts,  divides  the  rocks  below  the  New  Red  sand- 
stone into  the  following  classes:  Graywacke,  lnetamorphic  slates,  argil 
