572  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Hunt,52  in  1872.  states  that  the  rocks  seen  in  the  vicinity  of  Boston 
consist  of  three  classes — crystalline  stratified  rocks,  eruptive  granites, 
and  unaltered  slates,  sandstones,  and  conglomerates.  The  crystalline 
stratified  rocks  include  felsite  porphyries,  nonporphyritic  and  jasper- 
like varieties,  and  porphyritic  syenite;  while  the  second  division  in- 
cludes dioritic  and  chloride  rocks,  sometimes  schistose,  and  frequently 
amygdaloidal.  These  rocks  are  penetrated  by  intrusive  granites, 
generally  more  or  less  hornblendic — the  syenites  of  Hitchcock  and 
others.  At  several  places  the  phenomena  of  disruption  and  inclosuiv 
of  broken  fragments  of  rock  in  the  granite  are  well  seen,  the  lines  of 
contact  being  always  sharp  and  Avell  defined.  The  third  class,  con- 
sisting of  the  unaltered  argillites  of  Braintree,  containing  the  Primor- 
dial fauna,  were  found  to  rest  directly  upon  the  hard  porphyritic 
felsite  of  the  ancient  series,  the  line  of  demarcation  being  very  dis- 
tinct. At  other  places  reddish  granulites  directly  underlie  the  black 
argillites,  and  in  several  places  quartzites  with  conglomerates  are 
observed  in  contact  with  the  old  dioritic  and  epidotic  rocks.  The 
Roxbury  conglomerate  contains  pebbles  of  the  felsite  porphyries, 
diorites,  and  intrusive  granites  of  the  older  series,  besides  fragments 
of  argillaceous  slate. 
Burbank,53  in  1872,  states  that  the  bands  of  crystalline  limestone 
that  occur  in  the  granitic  gneiss  which  extends  in  a  southwesterly 
direction  from  near  the  mouth  of  Merrimac  River,  supposed  to  con- 
tain Eozoon,  are  not  true  stratified  rocks  but  subsequent  deposits  of  a 
veinlike  character.  As  evidence  of  this  are  cited  the  following  facts: 
The  principal  deposits  occur  along  the  line  of  an  anticline,  filling 
cavities  produced  by  the  folding  and  falling  down  of  portions  of  the 
included  strata  of  the  gneiss.  The  deposits  are  all  of  very  limited 
extent,  the  largest  appearing  at  the  surface  being  not  more  than  220 
feet  in  length  and  its  widest  part  about  60  feet.  The  aggregate 
length  of  all  the  limestone  deposits  occurring  in  a  line  some  25  miles 
in  length  is  probably  less  than  1,000  feet.  The  principal  masses  are 
coarsely  crystalline  magnesian  limestones,  homogeneous  in  texture 
and  showing  no  traces  of  stratification.  The  various  silicates  occur 
attached  to  or  near  the  inclosing  walls  of  the  cavities. 
Dodge,54  in  1875,  divides  the  rocks  of  eastern  Massachusetts  into 
two  groups,  the  crystallines  and  the  more  clearly  stratified  rocks 
among  them.  In  the  crystallines  are  placed  the  syenite  and  green- 
stone. These  rocks  have  a  dip  to  the  west  or  northwest,  and  they 
unconformably  underlie  strata  holding  Paradoxides.  For  the  most 
part  metamorphism  has  been  so  complete  that  the  rocks  have  entirely 
lost  their  original  character.  Eruptive  rocks  have  often  an  appear- 
ance of  schistose  structure ;  in  metamorphic  syenites  and  diorites,  on 
the  other  hand,  the  original  stratification  is  often  completely  lost. 
Throughout  the  crystalline  area  there  are  immense  masses  of  horn- 
